From Tasmania to the world: long and strong traditions in seaweed use, research, and development

3.1 Seaweed diversity

Tasmania has a rich marine-algal flora that includes some of the earliest-named seaweeds in Australia. A search of AlgaeBase (Guiry and Guiry 2022) revealed ∼750 species of green, brown and red seaweeds recorded from Tasmania. Womersley (1990, 2004; Wright et al. 2022) documents 1137–1155 species for the whole of southern Australia, from the Perth region to southern New South Wales and around Tasmania, meaning that 65% of that total is reported from Tasmania. The order Fucales, which originated in the Australian region has a particularly high diversity, but they are understudied in terms of their physiology and ecological roles (Coleman and Wernberg 2017). In contrast there are very few members of the order Laminariales (Ecklonia radiata, Lessonia corrugata, Macrocystis pyrifera, and the invasive Undaria pinnatifida) but these are comparatively well studied. Of particular interest are the 184 species and 18 genera based on type specimens from Tasmania, of which 11 genera and 22 species are endemic. Table 1 catalogues the type specimens that are the focus of this section which synthesises how our knowledge of the Tasmanian macroalgal flora has evolved since the first taxonomic collections in 1791.

The Tasmanian Herbarium (HO, Tasmanian Museum and Art Gallery) holds some 16,000 specimens of algae including micro- and macroalgae sourced from terrestrial, freshwater, and marine habitats. Approximately 80% of these now have a digital record that is accessible through the Australasian Virtual Herbarium (AVH 2022). Of the algal specimens archived at HO, 8835 (∼55%) are Tasmanian; the remainder were sourced from (other) Australian and global localities. There is a strong history of renowned algal collectors who contributed to the HO archives. Of the early collectors, R.C. Gunn (1840s) archived 80 specimens, W.H. Harvey (1850s) 2, L.A. Meredith (1870s) 92, A.H.S. Lucas and F. Perrin (1920s–1930s) 3867, and H.B.S. Womersley (1940s–1990s) 667. With the advent of SCUBA, further noteworthy contributions were made by S.A. Shepherd (1970s) 142, J.C. Sanderson (1980s) 303, G.J. Edgar (1980s–1990s) 92, N.S. Barrett (1990s) 46, G.T. Kraft (1980s–2022) 2068, and F.J. Scott (1980s–2022) 4655. Here we highlight significant explorers and taxonomists who have contributed to our understanding of the Tasmanian flora, whose archived material will assist future research using molecularly assisted alpha taxonomy (MAAT).

The earliest taxonomic samples in Tasmania were collected by French naturalist Jacques-Julien Houtou de La Billardière, a member of the D’Entrecasteaux expedition of 1791–1794. From April to May 1792 one of its ships anchored in Recherche Bay, and La Billardière became the first person to collect and later (1807) publish on the specimens that he took to Europe. The next collector was Robert Brown, a botanist aboard Matthew Flinders’ ship H.M.S. Investigator that stopped at the Kent Islands, Bass Strait. He collected species of green (two Caulerpa spp.), brown (including Xiphophora, Sargassum and Cystophora), and red (including Callophyllis, Hymenocladia and Laurencia) seaweeds that all appeared as Fucus in Turner’s 1811 four volumes titled “Fuci” (Turner 1808, 1809, 1811, 1819). Large and common brown seaweeds featured prominently in most of the earliest Tasmanian collections. Irish botanist William H. Harvey, a leading seaweed taxonomist of the 19th century, named many Australian, including Tasmanian, seaweeds. In the 20th century, H.B.S. Womersley made a major contribution to knowledge of the Australian flora, producing six volumes of The Marine Benthic Flora of Southern Australia, Parts I (seagrasses and green algae), II (the browns) and IIIA, B, C and D (the reds) (Womersley 1984, 1987, 1994, 1996, 1998, 2003), and these remain a critically important taxonomic resource. The ‘Marine Plants of Tasmania’ (Scott 2017) is the first text to focus solely on the diversity of Tasmanian seaweeds.

There is an historical absence of seaweed taxonomists in Tasmania, and so more recent techniques in molecular taxonomy/phylogeny have yet to be as systematically applied to the seaweed flora as they have in other regions. Nevertheless, in taxonomic surveys of southeastern Australia, G.W. Saunders employed DNA-sequencing to accompany morphological features for the establishment of new species, genera, and families (Table 1). Examples include the endemic monotypic genus Acrothesaurum (#77), the new endemic species Mychodea pseudoaciculare (#74), the resurrection of M. membranacea (#72) and M. terminalis (#73), the merging of the Tasmanian species Epymenia halymenioides (#84) with Rhodymenia cuneata (#85) and their transfer to the genus Halopeltis. More recently, Weber et al. (2017) discovered Durvillaea amatheiae (#23) was previously synonymous with D. potatorum, adding to the growing number of species in this genus recorded in New Zealand and Chile (Fraser, C.I. et al. 2020; Fraser, K.M. et al. 2020; Velásquez et al. 2020). An excellent example of the undiscovered diversity of Tasmanian seaweeds is that of Scott et al. (2013), who found Entwisleia bella (#79), which represents a previously undescribed genus, family, and order. Even more recent examples include Huonia sandersonii (#183) and Tythomenia barrettii (#184) (Saunders et al. 2017).

The 750 species of seaweeds in Tasmania is likely a substantial underestimate, and if the flora is carefully assessed combining morpho-taxonomy with molecular methods it is likely that more genera and species will be identified (Scott 2017). For example, systematic surveys in neighbouring New Zealand have revealed a 50% increase in species diversity since the 735 documented in 1985 (Parsons 1985; W.A. Nelson pers. comm.). Specific examples include: the order Bangiales, which includes the commercially important genus Pyropia, for which species numbers have increased from 4 to 46 (W.A. Nelson pers. comm.); many new seaweed genera have been described, and since 1983 the number of known invasive seaweeds has increased from 14 to 61 (Nelson et al. 2021). The diversity of coralline algae in Tasmania is likely to increase greatly if molecular taxonomic methods are applied – there are just 37 documented species in Tasmania compared to 122+ in New Zealand (J.C. Sanderson pers. comm.; Twist et al. 2019). The lack of a similar systematic study of the Tasmanian seaweed flora represents a major knowledge gap that is critical to fill if we are to understand how seaweed communities around Tasmania, and the ecological functioning of coastal ecosystems that they support, will respond to ongoing climate change, and the potential of the flora for natural products and pharmaceuticals.

3.2 Role of culture collections in seaweed taxonomy and applied research

Tasmania is home to the Australian National Algae Culture Collection, (Australian National Algal 2023) (CSIRO, Hobart), which, in line with several other global culture collections such as CCAP in the UK, NIES in Japan and NCMA in the USA, has recently expanded its scope to support the collection of living seaweeds. Historically, seaweeds were preserved as herbarium specimens whereas their smaller counterparts, the microalgae, have been maintained as unialgal strains in serial sub-culture in living culture collections, or dried samples of voucher specimens. At present, of the 41 registered algal collections on the Culture Collections Information or CCINFO (wdcm.org), most hold marine and/or freshwater microalgae and it is not evident what number maintain macroalgae (database search 26 June 2022).

Herbarium specimens, particularly Type specimens, as held at the Tasmanian Herbarium are fundamentally important for taxonomic studies and may even be used as a proxy for studying historical environmental conditions (Miller et al. 2020). The rationale for a macroalgal bioresource is similar to that for microalgae through provisioning living material for eco-physiological and life history studies under controlled conditions, the study of genetic and multi-omics variability within and between species, and for selecting phenotypes that may be best suited for bio-applications ranging from restorative kelp reseeding to multiple bioproducts documented in upcoming sections. As not all taxonomic groups of seaweed are amenable to long term collection logistics, the initial focus at ANACC has been on red algae and, through collaborations with the Institute for Marine and Antarctic Studies (IMAS), University of Tasmania, the gametophytes of targeted brown algae such as Macrocystis pyrifera. Future research will examine the cryopreservation of desired phenotypes that will alleviate the risks of genetic drift which is liable to occur in serial subculture. An important role of culture collections is not only to acquire and preserve viable living strains (e.g. from the research community) but in the digitization (e.g. provenance, biogeography, image, genetic, trait data) of those strains so that they can be resupplied for further research and industry use. In the case of ANACC, 70% of its 1100 strains (100% of 90 macroalgae) are currently available through the Australian National Algae Supply Service (CSIRO, Hobart, Tasmania).

3.3 Population genetics, contemporary and historical connectivity with mainland Australia

Although Tasmania is currently an island, it has historically been connected to mainland Australia when sea levels dropped during glaciation events including as recently as ∼14,000 years ago (Lambeck and Chappell 2001). Contemporary patterns in Tasmanian seaweed population genetics, and relationships with mainland populations, reflect a combination of connectivity during these periods of lower sea levels, along with the dominant oceanography of the region which is characterised by the poleward flowing Zeehan current in western Victoria and western Tasmania and the poleward flowing East Australian Current in eastern regions (Ayre et al. 2009; Fraser, C.I. et al. 2009; Weber et al. 2017; Figure 1). For example, chloroplast and mitochondrial sequences from six dominant seaweed species in Tasmania (Durvillaea potatorum, Ecklonia radiata, Hormosira banksii, Lessonia corrugata, Macrocystis pyrifera and Phyllospora comosa) showed small phylogeographic variation among populations (Durrant et al. 2015; Fraser, C.I. et al. 2009; Mueller et al. 2018). Species with a greater dispersal capacity, for example M. pyrifera and P. comosa, have a greater population connectivity than negatively buoyant species (e.g. H. banksii and L. corrugata).

There is evidence of divergence of haplotypes between eastern and western regions of Tasmania for Durvillaea potatorum, Ecklonia radiata, Hormosira banksii and Lessonia corrugata. Although there are some unique haplotypes in Tasmanian populations relative to conspecifics from the mainland, eastern Tasmanian haplotypes are found in eastern mainland populations and western Tasmanian haplotypes are found in western mainland populations (Durrant et al. 2015; Fraser, C.I. et al. 2009; Mueller et al. 2018). For L. corrugata, which has a limited capacity for dispersal, the availability of intervening habitat (i.e. rocky reef) appears to be a key determinant of population connectivity rather than geographic proximity (Durrant et al. 2015).

Overall, these findings suggest that eastern Tasmania (and the southeast coast of mainland Australia) was a refuge for temperate macroalgae (and other marine organisms, Ayre et al. 2009; Di Battista et al. 2014) during periods of glaciation and that western Tasmania, which experienced intense cooling from subantarctic waters during that time, was subsequently re-invaded in postglacial periods, most likely from southern or western Victoria (Fraser, C.I. et al. 2009; Mueller et al. 2018). More broadly, the limited phylogeographic variation observed in Australian seaweeds, in conjunction with other intra- and interspecific phylogenetic relationships (e.g. Coleman 2013; Starko et al. 2019), suggests most of these now-dominant species are likely to be relatively recent arrivals to the region (∼3 million years ago, MYA). Members of the order Fucales, including Durvillaea spp., likely have a longer history as they originated in the Australasian region (Cheshire et al. 1995; Weber et al. 2017). In future work, an emphasis on finer-scale genetic variation and diversity using more variable markers such as micro-satellites or single nucleotide polymorphisms (SNPs) would enhance understanding of population-level processes such as gene flow, population connectivity and natural selection (e.g. Coleman et al. 2011; Wood et al. 2021). For example, a recent study using genotype-by-sequencing (GBS) differentiated two genetically distinct Macrocystis pyrifera populations, separating into Northern and Southern regions (Iha et al., unpublished). This GBS analysis revealed a greater population diversity and less connectivity between populations than was evident from previous genetic studies. Further, research has focussed on only large brown seaweeds and we have no knowledge of the population genetics of Tasmanian Rhodophyta, the most specious phylum, or Chlorophyta. A greater knowledge of key species from these phyla is critical to the developing Tasmanian seaweed aquaculture industry (see below).

4.1 Distribution of seaweeds along wave-exposure gradients and depth gradients

Examples of the dominant species of Tasmanian seaweeds, and communities, are pictured in Figure 2. The distribution and abundance of the dominant large brown seaweeds within Tasmania are dependent on the availability of rocky substrata, and other key environmental factors, including wave exposure and depth (Edgar 1984; Hill et al. 2010; Sanderson and Thomas 1987). Wave exposure varies enormously around Tasmania: there is typically extremely high exposure on the west and south coasts, moderate exposure on the east coast and low wave exposure on the north coast (Butler et al. 2020). The effect of wave exposure on the distribution of habitat-forming brown seaweeds differs between genera and is partly related to their morphology. On more wave-exposed coasts, Durvillaea spp. often dominate intertidal and shallow subtidal habitats while Phyllospora comosa and Ecklonia radiata are also abundant but occur deeper than Durvillaea spp. (Edgar 1984). These species have flat blade morphologies that are more conducive to hydrodynamic streamlining in wave-exposed sites (Hurd 2000). In contrast, Sargassum spp. and Cystophora spp., which have many lateral branches that would increase drag forces on their small holdfasts, are found in more sheltered shallow subtidal environments, and their occurrence and abundance declines with increasing exposure (Edgar 1984; Hill et al. 2010).

Figure 2:

Examples of the diversity of seaweed species in Tasmania. (A) Hormosira banksii, (B) Asparagopsis armata, (C) Lessonia corrugata, (D) Durvillaea potatorum, (E) front: Caulerpa trifaria; back: Phyllotrichia verruculosa, (F) Macrocystis pyrifera, (G) Phyllospora comosa, (H) Delisea sp., (I) assemblage of red seaweeds including Ballia callitricha, Corallina officinalis, Delisea plumosa, Phymatolithon maisonianum, Pterocladiella capillacea, Spnderphycus coriaceus, (J) Durvillaea spp. and Xiphophora gladiata, (K) reef assemblage including: Ecklonia radiata, Caulerpa trifaria, Plocamium cirrhosum, (L) encrusting red algal assemblage, (M) Ecklonia radiata, (N) assemblage of crustose coralline algae, (O) Sargassum sp. Photo credits: (A, D, E, F) Joanna Smart, (B) Elysha Kennedy, (C) Michael Guiry (AlgaeBase), (G, K) Matthew Doggett, (N) Elisabeth (Beth) Strain, (I) Christopher Cornwall, (J) Scott Ling, (H and L) Cayne Layton, (M) Jo Lane, (O) Emilliano Cimoli.

While Durvillaea spp. dominate the low intertidal on high energy shores of the east, south and west coasts, Lessonia corrugata can be locally abundant in low intertidal habitats on medium energy shores principally in southwestern Tasmania (Barrett et al. 2001; Edgar 1984) and Hormosira banksii forms extensive beds on the low wave-energy reefs of the north coast and sheltered embayments on the east coast (Lewis et al. 2021; Mueller et al. 2015). On the north coast, thalli of Hormosira banksii are smaller (both shorter fronds and smaller vesicles) which may be a response to the different tidal regime that results in longer exposure to the air (Gemelli et al. 2019; Mueller et al. 2015).

Depth and light availability are also important variables explaining the distribution and abundance of large brown seaweeds that generally require moderate levels of light. On most of the open coasts of Tasmania, Ecklonia radiata, Cystophora spp., Phyllospora comosa and Sargassum spp. are the dominant taxa at depths >5 m (Edgar 1984; Sanderson 1990a), although Durvillaea spp. can be found down to 20 m depth on the extremely wave-exposed south coast (Edgar 1984). In particular, E. radiata dominates reefs with moderate to high wave energy from ∼10 to 25 m depth where it forms forests of close to 100% cover (Wernberg et al. 2019). In deeper waters, a decrease in E. radiata abundance is strongly related to the reduced availability of light and rock substratum, although forests of ∼40–50% cover can still be found at 40 m on the east coast of Tasmania (Marzinelli et al. 2015). Prior to its decline, M. pyrifera was also abundant at depths between 10 and 20 m on open coasts (Butler et al. 2020; Johnson et al. 2011) and in many places it has been replaced by E. radiata and/or P. comosa (Ling and Keane 2018). These patterns are, however, locally variable and further study is underway to examine these trends and associated environmental drivers, including the use of autonomous underwater vehicles (AUV, e.g. Perkins et al. 2020), remotely operated vehicles (ROV, e.g. Sward et al. 2021) and towed video technology which is enhancing understanding of seaweed ecology in Tasmania at greater depths.

4.2 Seasonal patterns of growth, standing stock and demography

Seasonal patterns of growth have been documented for eight species of brown seaweed, but no red or green seaweeds. For Ecklonia radiata, maximum blade elongation rates occur in summer (Nov–Dec) and minimum in winter (June–July, Table 2). These rates are higher than those recorded in other regions of Australia (0.03–0.22 cm d⁻¹) but similar to those in southwest New Zealand (0.06–0.45 cm d⁻¹; Wernberg et al. 2019). For juvenile E. radiata (initial size 50–150 mm long) growth rates also vary with season, being highest in autumn and lowest in spring and summer (Layton et al. 2019b). In contrast, for Macrocystis pyrifera, growth was maximal in spring and minimal in late summer (Sanderson 1990a). For the fucoids, Phyllospora comosa has maximal growth rates in summer compared to winter (Sanderson 1990a) and, similarly, percentage cover in the D’Entrecasteaux Channel declines in winter, most likely due to increased wave action removing seaweeds and reduced growth rates, before increasing in summer (White et al. 2021). A seasonal pattern of maximum seaweed length was recorded for Sargassum fallax (recorded as S. bracteolosum) at Fancy Point, with the longest seaweeds recorded in December-January, and shortest between February and April (Edgar 1983b). No clear seasonal patterns in longest length were recorded for Phyllotricha verruculosa (recorded as Sargassum verruculosum), Cystophora retroflexa and Caulocystis cephalornithos, and the small (<14 cm) dictyotalean seaweed Zonaria turneriana (Edgar 1983b). The studies of Edgar (1983b) and Sanderson (1990) provide a vital baseline of information that, if repeated at the same sites, may allow the detection of changes in patterns of seasonal growth rates and cycles over the past four decades, during which time surface sea temperatures have increased (Johnson et al. 2011).

A few studies have measured standing stocks of Tasmanian seaweeds, again focussing on large brown seaweeds. The highest known standing stock is 68 kg ww m⁻² in the low intertidal (0 m) for Durvillaea potatorum (probably mixed with D. amatheiae) at Eaglehawk Neck, which compares to 20 kg m⁻² at 2 m depth; the lower biomass at 2 m was due to a high density of small, young seaweeds compared to a lower density of large, mature seaweeds at 0 m (Cheshire and Hallam 1988). Standing stocks at 14 m depth at George III Reef were 36 kg m⁻² (Sanderson 1990a). These standing stocks are greater than those of D. antarctica in New Zealand (10–24 kg ww m⁻², South and Hay 1979). The standing stocks of laminarian and other fucalean communities are lower than those of Tasmanian Durvillaea spp. at 0.2–8 kg m⁻² for a mixed community at Fancy Point (Edgar 1983a), 1–8 kg m⁻² for E. radiata beds at Maria Island, George III Reef and Bicheno (Sanderson 1990a) and 2.3 kg m⁻² for Phyllospora comosa at Bicheno (Sanderson 1990a).

The instantaneous or annual production rates of kelps and other canopy forming species in Tasmania is unknown with the exception of the study conducted by Randall et al. (2019), who continuously measured productivity of an Ecklonia radiata bed over 11 days in a semi-sheltered bay in eastern Tasmania and found that kelp contributed about half of the total primary production of the macroalgal community on the reef. This finding is in line with work in New Zealand showing that the fucoid canopy of an intertidal community accounted for about half of the total primary production (Tait and Schiel 2018).

The demography of native Tasmanian seaweeds is not well studied except for Ecklonia radiata and Hormosira banksii. Detailed demographic studies of E. radiata in Tasmania revealed some interesting differences from mainland populations, for example, it is larger and more fecund than mainland conspecifics which may be related to more optimal temperatures and nutrients in the cooler Tasmanian waters (Mabin et al. 2013). Reproduction (i.e. sori production and zoospore release) occurs all year round for E. radiata in Tasmania but generally peaks in autumn and winter, and is lowest in summer (Mabin et al. 2013; Sanderson 1990a; Tatsumi et al. 2022). This is different to at least some mainland populations of E. radiata (e.g. Western Australia), where the peak in zoospore production/release is earlier (late summer and early autumn) with zoospore production/release outside of that season being negligible (Giraldo-Ospina et al. 2021; Mohring et al. 2014; Wernberg et al. 2019).

The settlement, recruitment, and post-recruitment growth of Ecklonia radiata is affected by abiotic changes linked to changes in adult kelp density and patch size (Layton et al. 2019a,b, 2020a; Tatsumi et al. 2021). High densities of adult E. radiata provide suitable environments (reduced light, water flow, sediment accumulation and scour) for the settlement and recruitment of microscopic juveniles, whereas the growth of already established older juveniles can be higher in areas where the adult canopy is sparse (Flukes et al. 2014; Layton et al. 2019b; Tatsumi et al. 2021). Thus, altered densities of adult E. radiata may facilitate different demographic processes, similar to how variations in E. radiata density facilitate associated communities (Flukes et al. 2014; Shelamoff et al. 2019, 2020a,b; Wernberg et al. 2005). The percentage cover of E. radiata within Tasmanian macroalgal beds has increased in eastern Tasmanian, particularly in the south and southeast, from 2001–2002 to 2016–2017, which is likely driven by increasing seawater temperatures and concomitant reduction in heavy shading once provided by dense and tall Macrocystis pyrifera forest canopy (Ling and Keane 2018). A similar trend was seen within Marine Protected Areas (MPAs) in this region over the period 1992–2002 as part of MPA long-term monitoring (Barrett et al. 2009), again inferred to be in part due to the concurrent decline in M. pyrifera. However, Barrett et al. (2009) also demonstrated the remarkable stability of many canopy-forming and understorey species at decadal scales, with the percentage cover of many common species varying by no more than ± 50% of their recorded initial cover over this time period.

A detailed demographic study of a ‘dwarf’ population of Hormosira banksii at Beechford on the north coast of Tasmania showed that although recruitment occurred throughout the year it peaked during summer (Lewis et al. 2021). This higher recruitment in summer is consistent with H. banksii in New Zealand (Schiel and Taylor 1999) but different to Victoria where recruitment peaked in April (Bellgrove et al. 2004). Moreover, post-recruitment survivorship was consistently high: up to 80% over 18 months (Lewis et al. 2021). This dwarf morphology of H. banksii is relatively slow growing with individuals taking approximately 5 years to grow from recruits to the largest size class of 10 cm (Lewis et al. 2021).

Otherwise, there is limited information on the demography of other large brown seaweeds, including Macrocystis pyrifera, Lessonia corrugata and Durvillaea spp. and of all green and red seaweeds from Tasmania including two species that are the focus of new aquaculture efforts: Asparagopsis armata, which is being grown as a feed supplement to mitigate methane production by ruminants, and species in the genus Caulerpa, which are being considered for human food. Understanding the fundamentals of annual growth cycles, when and how seaweeds reproduce, and their life cycles, is becoming increasingly important as we tackle responses of seaweeds to climate change (e.g. Leal et al. 2021) and develop a seaweed aquaculture industry (Charrier et al. 2017).

4.3 Subtidal seaweed as habitat-forming ecosystem engineers

Seaweeds of the orders Laminariales and Fucales are ecosystem engineers that provide a complex three-dimensional structure, and directly or indirectly modify resources for themselves and other organisms (Jones et al. 1994). In Tasmania, the structure of Ecklonia radiata forests modifies subcanopy irradiance, seawater chemistry, water flow and sedimentation and all of these effects are influenced by the density and/or size of E. radiata patches (Britton et al. 2016; Cornwall et al. 2015a; Flukes et al. 2014; Layton et al. 2019a,b; Ling et al. 2020). These modified abiotic factors influence the recruitment, growth and survivorship of microscopic and macroscopic juvenile E. radiata (Layton et al. 2019a,b; Tatsumi et al. 2021), demonstrating a positive intraspecific feedback on demographic functions. Abiotic alterations by E. radiata also strongly influence the composition and productivity of associated benthic macroalgal and sessile invertebrate assemblages (Edgar et al. 2004; Shelamoff et al. 2019a,b, 2022b), and the diversity and abundances of other species including mobile macroinvertebrates and fishes (Shelamoff et al. 2020b, 2022a). The living and decaying tissue of E. radiata and the attached epiphytes are directly consumed by herbivorous and detritivorous invertebrates such as amphipods, gastropods and sea urchins that are pivotal for secondary productivity and provide food resources for higher trophic levels including species of commercial interest such as southern rock lobster, Jasus edwardsii (Palinuridae) (Ling et al. 2009a,b; Shelamoff et al. 2020a). Surprisingly, Shelamoff et al. (2020a) found that epifaunal secondary production in the understorey decreased with patch size but was highest in high-density patches of E. radiata and in patches of turf algae in the absence of kelp that had lower epifaunal diversity.

Seaweeds facilitate the settlement and recruitment of invertebrates and can affect species richness. For example, both Macrocystis pyrifera and Ecklonia radiata appear to provide physical cues and/or release (unidentified) water-borne chemical cues that attract settling southern rock lobster puerulus larvae and provide refuge that improves post-settlement survivorship leading to elevated recruitment (Hinojosa et al. 2015; Shelamoff et al. 2022a). Ecklonia radiata also facilitates the recruitment of native oysters (Ostrea angasi) and the establishment of other sessile invertebrates through their capacity to suppress competition from algal turfs and modify water flow and particle deposition rates (Layton et al. 2019a,b; Shelamoff et al. 2019a,b). Moreover, increases in E. radiata density increase the overall abundance and richness of the fish assemblage, although it does not enhance the recruitment of certain cryptobenthic fish species (Shelamoff et al. 2020b).

More broadly, the structure provided by seaweed is a strong driver of epifaunal invertebrate communities (Ling 2008), with increasing densities of these invertebrates associated with highly branched filamentous seaweeds (i.e. turfs) compared to less branched foliose species (Shelamoff et al. 2020a), although individuals are smaller in size within turf algae (Edgar 1983a; Fraser, C.I. et al. 2021). Large mobile invertebrates (primarily decapods) are more associated with turfing algae on reefs, which supports high epifaunal secondary productivity, than with the macroalgal canopy (Edgar et al. 2004; Shelamoff et al. 2020a, 2022a). Turf- and foliose algal-dominated assemblages in Tasmania support distinctly structured assemblages of epifauna, with the structure of each closely resembling those found elsewhere in Australia (Edgar 1983a,c; Fraser, C.I. et al. 2020). Increasing richness of epifaunal species is correlated with increasing seaweed species richness (Shelamoff et al. 2020a). Overall, this body of work highlights the critical role of seaweed in facilitating the diversity and productivity of coastal marine ecosystems of Tasmania.

4.4 Invertebrate grazer – seaweed interactions, and trophic cascades

Herbivores have distinct effects on the structure and function of Tasmanian reef ecosystems. At local scales, herbivory can structure reef-scapes and in extreme cases lead to a complete and persistent absence of fleshy/erect macroalgae. This is well-documented on subtidal reefs where the impacts of herbivory are strikingly obvious owing to the formation of barren grounds caused by overgrazing of kelp by sea urchins (Ling 2008; Ling et al. 2010, 2015; Perkins et al. 2015, 2020).

Sea urchin abundance and grazing impacts have increased dramatically in Tasmania in recent decades due to a population explosion of the range-extending diadematid sea urchin Centrostephanus rodgersii (Johnson et al. 2005, 2011, 2013; Ling 2008; Ling and Keane 2018; Ling et al. 2009a,b, 2015; Perkins et al. 2020). Since the first detection of an individual urchin on the mainland Tasmanian coast at St. Helens in 1978, the population of C. rodgersii in eastern Tasmania has now reached ∼20 million (Ling and Keane 2018). From 2001 to 2017, the cover of urchin barrens on eastern Tasmanian reefs caused by C. rodgersii increased from ∼3% to ∼15% and is on target to reach ∼50% of eastern Tasmanian reefs in coming decades (Ling and Keane 2018). Observed and expected losses of kelp forests will continue to drastically reduce available habitat and food resources for ∼150 kelp-associated taxa (Sanderson 2003), including commercially fished abalone and southern rock lobsters, that are largely absent from urchin barrens (Johnson et al. 2005, 2011, 2013; Ling 2008; Ling and Keane 2018; Strain and Johnson 2009, 2013).

In addition to the increasing grazing effect of Centrostephanus rodgersii, the native short-spined sea urchin Heliocidaris erythrogramma is also a key herbivore on subtidal Tasmanian reefs (Ling et al. 2010). While C. rodgersii overgrazes kelp on wave-exposed eastern Tasmanian reefs (Johnson et al. 2005; Ling et al. 2009a,b), H. erythrogramma predominantly overgrazes kelp on moderately sheltered reefs along the north, east and south-east coasts of Tasmania (Ling et al. 2010). Moreover, in contrast to the obligate scraping mode of C. rodgersii, the foraging behaviour of H. erythrogramma can change from feeding only on drift algae to the more destructive grazing of attached kelp when drift-kelp is absent (Kriegisch et al. 2019).

Relative to historical baselines, and highly visible by comparing fished zones with marine protected areas (MPAs), sea urchin predators have been functionally extirpated across much of the Tasmanian coastline (Barrett et al. 2009; Ling and Johnson 2012; Ling et al. 2009a,b; Pederson and Johnson 2006). Large southern rock lobsters, Jasus edwardsii are known predators of both long and short-spined sea urchins, and field experiments show increased predation mortality of urchins in the presence of rebuilt lobster populations relative to fished reefs (Johnson et al. 2013; Ling and Johnson 2012; Ling and Keane 2021; Ling et al. 2009a,b; Pederson and Johnson 2006). Likewise, monitoring of ecological processes within Tasmania’s east and southeast coast MPAs has shown that where predators, including lobsters, are protected from fishing, urchin populations are controlled, and reefs remain resilient to barren formation (Barrett et al. 2009; Perkins et al. 2020). While predator abundance, and thus resilience of kelp forests is historically low outside of MPAs, particularly for eastern Tasmania, the “East Coast Rock Lobster Rebuilding Strategy” explicitly aims to recover large lobster abundance towards increasing predation on urchins to reduce the risk of barrens formation (https://dpipwe.tas.gov.au/).

Other grazing or browsing species that exert a weaker or smaller-scale influence on algal assemblages include the herring cale (Olisthops cyanomelas) which has increased in abundance in Tasmania with warming of Tasmanian coastal waters (Barrett et al. 2014), blacklip abalone (Haliotis rubra) (Strain and Johnson 2012) and other species of gastropods (Wernberg et al. 2019). All of these herbivores have specialised feeding mechanisms e.g. herring cale attacking the meristem of kelp rather than the laminae which can be destructive to the kelp. The removal of abalone results in encrusting coralline algae becoming overgrown with sessile invertebrates, filamentous and foliose algae, and the filamentous algae/sediment matrix (Strain and Johnson 2010, 2012). Abalone avoid this overgrown habitat such that heavy fishing of abalone can lead to a transition to a low preference habitat for them (Strain and Johnson 2012). The transition thus appears to represent a form of discontinuous phase shift in benthic community composition (Steneck and Johnson 2014) maintained by positive feedback, and which could have significant negative consequences for the abalone fishery.

4.5 Competition and facilitation among seaweed

Positive and negative interactions among seaweed species are well known (Bennett and Wernberg 2014; Bennett et al. 2015; Edwards and Connell 2012) but are not well-studied in Tasmanian seaweed communities. Nonetheless, the removal of the Ecklonia radiata canopy frees up resources (such as irradiance, which declines by up to ∼90% beneath a full E. radiata canopy, (Layton et al. 2019b) for competitors that can shift the benthic community toward a foliose algae-dominated state (Flukes et al. 2014). Under a full canopy, the community has a much higher cover of sponges, bryozoans, and encrusting coralline algae. The increase in foliose algae following E. radiata canopy loss can in turn negatively impact kelp recruitment, which is lower under a high cover of understorey algae (Tatsumi and Wright 2016). Additionally, the removal of mixed brown algal canopies (primarily fucoids and E. radiata), allowed high recruitment of invasive Undaria pinnatifida, although the response of U. pinnatifida varies with time of year and other levels of disturbance (Edgar et al. 2004; Valentine and Johnson 2003, 2004).

Undaria pinnatifida was first recorded in Tasmania in 1988 (Sanderson and Barrett 1989), and it has established as a winter annual with a seasonal phenology similar to that in other geographic regions that it has invaded (Schaffelke et al. 2005). Experiments to eradicate or control U. pinnatifida populations by harvesting sporophytes in the Tinderbox marine reserve showed that the gametophyte phase acts as a ‘seed bank’ that facilitated the annual re-establishment of the sporophyte generation; after 2.5 years there was no effect of sporophyte removal on the U. pinnatifida population (Hewitt et al. 2005). Initial concern that U. pinnatifida would outcompete and displace native seaweeds (stemming from observations of extensive macroalgal beds previously dominated by a canopy of Ecklonia radiata and fucoids being replaced by mono-specific stands of U. pinnatifida in Mercury Passage, e.g. Sanderson 1990b) proved unfounded when experiments demonstrated that U. pinnatifida was merely ‘tracking’ disturbances that removed native species, and that in the absence of disturbance natives would eventually reclaim dominance (Johnson et al. 2004; Reeves et al. 2018; Valentine and Johnson 2003, 2004). Thus, despite high potential for dispersal in U. pinnatifida (Sliwa et al. 2006), after more than three decades since its arrival in eastern Tasmania it remains largely a minor component of the canopy of macroalgal beds.

It is more likely that a greater risk to Ecklonia radiata and native fucoids maintaining closed-canopy macroalgal beds in Tasmania is the proliferation of filamentous algal turfs and the sediments they accumulate to form a semi-consolidated sediment matrix that severely limits recruitment of large fucoid and laminarian species (Valentine and Johnson 2005a,b). Proliferation of turf algae at the expense of E. radiata in Port Phillip Bay in Victoria (Reeves et al. 2018, 2022), and of other canopy-forming species elsewhere in the world (e.g. Filbee-Dexter and Wernberg 2018; Strain et al. 2014) typically occurs on low wave-exposure reefs subject to poor water quality and other human-instigated stressors, highlighting that attention to water quality in Tasmania should remain a management priority.

4.6 Restoration

One emerging tool in the conservation and management of seaweed-dominated ecosystems is habitat restoration, the science and practice of which is currently undergoing substantial expansion (Eger et al. 2022; Layton et al. 2020b; Morris et al. 2020). Within Australia there have been few attempts to restore kelp forests, however the earliest reported work comes from Tasmania, and outlines attempts to restore areas of disappearing Macrocystis pyrifera (Layton et al. 2020b; Sanderson 2003). The projects realised some success, with giant kelp established at one of the >10 sites; however, this too disappeared after some time in keeping with the persistent decline of giant kelp in Tasmania due to climate change and ocean warming (Butler et al. 2020; Johnson et al. 2011; Steneck and Johnson 2014). This illustrates the necessity not only to understand the practical and methodological aspects of kelp forest restoration, but also to address and understand the drivers of habitat decline (Abelson et al. 2020). Work is currently underway in Tasmania that has identified and planted giant kelp genotypes that display increased tolerance of warm water as the foundation of ongoing restoration efforts (Layton and Johnson 2021).

Additional research from Tasmania has increased understanding of the factors likely affecting the successful restoration of kelp forests. Valentine and Johnson (2005) illustrated that, even after the removal of grazing urchins, heavy inoculation with Ecklonia radiata kelp spores was unable to promote kelp reestablishment – presumably due to recruitment inhibition by the turf algae and sediments that had proliferated in the absence of the kelp (also see Layton et al. 2019b). Conversely, if healthy kelp forests are close to denuded areas (e.g. urchin barrens), the removal of urchins can facilitate rapid natural recovery of kelp and other macroalgae (Ling 2008) – however >90% of urchin biomass has to be removed from a given area in order to promote natural macroalgal recovery (reviewed by Ling et al. 2015; Marzloff et al. 2016). Layton et al. (2019b, 2021 demonstrated a method for the successful transplantation of >1000 adult E. radiata on artificial reefs in Tasmania. Following abundant natural recruitment from the transplanted kelp, some of those reefs eventually supported self-sustaining areas of E. radiata, but crucially, this only occurred where there were adequate adult E. radiata to facilitate juvenile conspecifics via modification of the physical environment.

5.1 Dissolved inorganic carbon uptake mechanisms

The inorganic carbon physiology of some Tasmanian seaweed communities is globally unique. Cornwall et al. (2015b) revealed that, based on carbon stable isotope signatures and pH-drift experiments, up to 90% of populations at Tinderbox and Ninepin Point do not operate a carbon-dioxide concentration mechanism (CCM), i.e. they rely on dissolved CO₂ and cannot take up bicarbonate (termed non-CCM seaweeds). This finding contrasts sharply with other regions studied – Southern New Zealand, the Great Barrier Reef and a volcanic vent in Italy – where only ∼5% are non-CCM (Cornwall et al. 2017; Diaz-Pulido et al. 2016; Hepburn et al. 2011). In Tasmania, almost all non-CCM species are red seaweeds, and these communities provide a unique opportunity to understand the underpinning physiological mechanisms of inorganic carbon uptake as, internationally, previous work has focussed on green and brown seaweeds. Further, the photosynthetic rates of non-CCM seaweeds are unlikely to be saturated at present-day pCO₂ levels and this has implications for their response to ocean acidification (see below). As carbon and nitrogen metabolisms are tightly linked, non-CCM seaweeds are also predicted to have unique features of their nitrogen physiology, which is yet to be studied in detail but warrants attention (Paine et al. 2021b).

5.2 Photosynthesis and respiration

Net photosynthetic and respiration rates have been measured for a range of species, in the laboratory, focussing on red seaweeds due to their interesting carbon physiologies (Cornwall et al. 2015b). Rates are within the ranges reported for other species worldwide, and there are no clear differences between seaweeds with and without CCMs (Table 3). However, there are no published studies on the effects of light (photosynthesis-irradiance curves) or dissolved inorganic carbon (photosynthesis-DIC curves) on photosynthetic rates and this is a substantial omission in our understanding of the photosynthetic physiology of Tasmanian seaweeds.

5.3 Nitrogen physiology and C:N ratios

Typical of temperate regions worldwide, the two forms of inorganic nitrogen available for uptake by Tasmanian seaweeds are nitrate (NO₃ ⁻) and ammonium (NH₄ ⁺), and concentrations vary spatially and temporally with season and anthropogenic influences. There has not been a systematic study of seasonal patterns of NO₃ ⁻ and NH₄ ⁺ in Tasmanian waters, however concentrations of NO₃ ⁻ on the east coast are <3 μM year-round (Smart et al. 2022; Thompson et al. 2009), although a point measurement at Bruny Island in winter 2021 was ∼6 µM (E.R. Paine pers comm). These concentrations of total inorganic nitrogen are much lower than in other temperate regions, for example winter NO₃ ⁻ concentrations of ∼10 µM in the north Atlantic and ∼20 µM in the north Pacific (Hurd et al. 2014), indicating that Tasmanian waters are comparatively oligotrophic. The inorganic nitrogen uptake rates of only a few species have been studied. For ammonium, Macrocystis pyrifera, Lessonia corrugata, and Ecklonia radiata uptake rates are saturable with a maximum uptake rate (V _max) of 200, 45.8 and 45 µmol NH₄ ⁺ gDW⁻¹ h⁻¹ and half-saturation constants (K _s) of 361.3, 104.2 and 121 µM, respectively (Smart et al. 2022). Phyllospora comosa (order Fucales) showed evidence of both passive and active uptake mechanisms (biphasic uptake) for spring and autumn but not summer (i.e. two of three study seasons); this is only the third time globally that bi-phasic uptake has been reported for a brown seaweed – all of which were members of the Order Fucales (Smart et al. 2022).

The effect of light on uptake and assimilation of nitrate and ammonium for the non-CCM red seaweed Hemineura frondosa found that saturating irradiance (150 μmol photons m⁻² s⁻¹) resulted in increased uptake rates of ammonium and nitrate compared to limiting (30 μmol photons m⁻² s⁻¹) irradiance. This finding was attributed to an increased utilization of stored tissue nitrogen under low light, rather than the allocation of energy to the assimilation of new nitrate via nitrate reductase – the activity of which did not change with irradiance – which is an energetically costly process (Paine et al. 2021b).

The ratio of tissue carbon: nitrogen can be used as an indicator of whether seaweed growth is nitrogen limited and also of the nutritional content of a seaweed, because higher nitrogen content is related to a higher protein content (Hurd et al. 2014). A ‘rule of thumb’ is that seaweeds with a C:N <15–20 are nitrogen sufficient, and those >20–25 are nitrogen-limited for growth (Hurd et al. 2014). The C:N ratios of red and green Tasmanian seaweeds are within the ranges of species studied elsewhere (Sheppard et al. unpublished data). However, the Tasmanian seaweeds Durvillaea spp., Phyllospora comosa, Lessonia corrugata and Ecklonia radiata have some of the highest C:N ratios globally, with values for E. radiata of up to 100, compared to the global mean of 27.2 (Sheppard et al. unpublished data). C:N ratios in juvenile P. comosa (Flukes et al. 2015) and Macrocystis pyrifera (Mabin et al. 2019a) from Fortescue Bay in SE Tasmania were ∼15 which is likely due to the much lower structural carbon content of juveniles compared to adults. C:N ratios varied with depth, season and year for adult E. radiata (∼13–21), and with season and year for P. comosa (range ∼21–32) with complex interactions among these factors. Macrocystis pyrifera showed large seasonal fluctuations in C:N ratios from means of ∼10 in winter to ∼45 in summer (Flukes 2015). Overall, the comparatively high C:N ratios of Tasmanian brown seaweeds indicate that they either have nitrogen-limited growth or a low nitrogen requirement for growth: the ability to grow productively in relatively low ambient inorganic nitrogen concentrations is likely an adaptation to relatively oligotrophic waters. Future work to elucidate the physiological and biochemical mechanisms of such adaptations by comparing M. pyrifera, which has a wide biogeographic distribution, from Tasmania with other locations (Chile, Canada, New Zealand, USA) will be important in assessing population responses to the ongoing global decline in nitrate concentrations due to climate change (Pörtner et al. 2014).

5.4 Water motion

Water motion is a key driver of seaweed primary production through its influence on seaweed morphology and the thickness of velocity and diffusion boundary layers that affect rates of photosynthesis, nutrient uptake, and growth (Hurd 2000, 2015). Field studies comparing seawater flows above and within Tasmanian seaweed beds (Ninepin Point and Tinderbox) revealed that velocity and turbulence are reduced by 90% irrespective of background flow regimes, due to dampening by the seaweeds (Kregting et al. 2021). Velocities within the beds were 3 cm s⁻¹ and this work adds to the small number of studies that report substantial flow attenuation by natural seaweed beds (see Table 1 in Kregting et al. 2011, 2021). Reductions of mainstream flows of ∼55% beneath Ecklonia radiata canopies have also been observed at various depths in Tasmania (Layton et al. 2019b). In flume experiments at water velocities of 6 cm s⁻¹, communities of understorey algae and small kelp were each found to be capable of reducing flows by 30 and 60%, respectively (Layton et al. 2019a). Diffusion boundary layer (DBL) thickness at the surface of E. radiata blades in flows of 0.5 and 8 cm s⁻¹ was 0.5 and 0.1 cm, respectively, but when the blade surface was covered by the bryozoan Membranipora membranacea, the DBL thickness was between 80 and 200% thicker than for bare blades (Noisette and Hurd 2018). Thick DBLs allow the seawater chemistry at the surface of seaweeds to be biologically modified and may act as a refuge for calcifying organisms such as M. membranacea from the ongoing reduction in seawater pH due to ocean acidification (Hurd 2015; Layton et al. 2019b; Noisette and Hurd 2018; Noisette et al. 2022).

Recent studies are revealing that Tasmanian brown seaweeds may have unique nitrogen metabolisms that allow them to grow in seawater that has persistently low nitrogen concentrations compared to well-studied temperate regions of the northern hemisphere (Hurd et al. 2014). Communities of red seaweeds are uniquely dominated by non-CCM species to the extent that CCM-red seaweeds can be difficult to locate for experimental work along the south-east coast (D. Britton pers comm). Tasmania’s rich and diverse flora provides the ideal system to study physiological mechanisms by which seaweeds metabolise carbon and nitrogen and allow better understanding of how these unique systems will acclimate and adapt to climate change. Further to this, an underpinning physiological knowledge is needed to domesticate seaweeds for aquaculture, and to explore potential bioactive compounds.

6.1 Ecological changes in diversity and range shifts that can be directly linked to ocean global change

Ocean warming along Tasmania’s east coast is 3–4 times the global average due to the strengthening of the East Australian Current, making Tasmania a ‘natural laboratory’ for climate change research (Johnson et al. 2011). As discussed above (Invertebrate grazer – seaweed interactions and trophic cascades), this has led to the range-expansion of the sea urchin Centrostephanus rodgersii into eastern Tasmania and, subsequently, significant declines in kelp and seaweed assemblages (Johnson et al. 2005, 2011; Ling 2008; Ling and Keane 2018; Ling et al. 2009a,b; Perkins et al. 2015). Since the 1950s, eastern Tasmania has also suffered ∼ 95% declines in giant kelp (Macrocystis pyrifera) forest surface canopy cover, which is attributed to the increasing influence of the warm, nutrient-poor waters of the EAC in Tasmania (Butler et al. 2020; Johnson et al. 2011; Steneck and Johnson 2014), with overgrazing by sea urchins likely to have exacerbated the problem (also see Ling and Keane 2018). Consequently, in 2012 M. pyrifera forests of southeast Australia became the first marine community listed as Endangered under the Australian Federal Government Environment Protection and Biodiversity Conservation Act (Evans et al. 2017). Where those losses have occurred, M. pyrifera has been largely replaced by Ecklonia radiata, which is more thermally tolerant, or urchin barrens (Johnson et al. 2011; Ling and Keane 2018).

In addition to Centrostephanus rodgersii, numerous other marine species are moving south with the EAC into Tasmania’s waters. In fact, Tasmania has more records of range shifting species than any other marine region in Australia, and >80 new marine species have been recorded in Tasmania in recent decades (Gervais et al. 2021). While the vast majority of investigations of range expansions have focussed on fishes and other animals, range shifts are also likely to have occurred for seaweed (Wernberg et al. 2011). Ultimately, with the ongoing threat of climate change, declines of key habitat-forming species such as the kelps and an increasing number of novel species, Tasmania’s marine ecosystems are currently experiencing unprecedented change.

6.2 Future projections of changes in seaweed assemblages based on laboratory manipulative studies

To help understand ecological shifts that are ongoing in Tasmania, a range of manipulative experiments have been conducted assessing how seaweeds will respond to ocean global change. Studies have primarily examined the effect of ocean acidification in isolation (Britton et al. 2016, 2019; Cornwall and Hurd 2019; van der Loos et al. 2019) and warming in conjunction with nutrient limitation (Fernándezet al. 2020; Flukes. et al. 2015; Mabin et al. 2013; Schmid et al. 2020). However, the combined drivers of warming and acidification (Britton et al. 2020a; 2021), warming and light (Paine et al. 2021a), and warming, light and nutrient limitation (Mabin et al. 2019a) have also been investigated.

The effects of ocean acidification on the responses of fleshy (i.e. non-calcifying) seaweeds is thought to depend on their inorganic carbon uptake strategies (Hepburn et al. 2011). Given the large proportion of non-CCM seaweeds in Tasmania (Cornwall et al. 2015b), this island presents a unique opportunity to study how the response of seaweeds to ocean acidification is mediated by their inorganic carbon uptake mechanisms. The response of both non-CCM and CCM species to elevated CO₂ have been tested with varying responses. Non-CCM species tested (Plocamium dilatatum, Callophyllis lambertii and Craspedocarpus ramentaceus) in medium-term (1–2 weeks) laboratory growth experiments did not display elevated growth rates, however net photosynthesis increased in C. lambertii (Britton et al. 2019; van der Loos et al. 2019). Short-term (hours) exposure to elevated CO₂ increased dissolved inorganic carbon uptake in three species (Plocamium angustum, Heminuera frondosa and Rhodymenia sp.), however this translated into an increase in photosynthesis only for P. angustum (Cornwall and Hurd 2019). Species with a CCM appear likely to down-regulate the energetically costly CCM to rely more on diffusive CO₂ as an inorganic carbon uptake strategy (Britton et al. 2016, 2020a; van der Loos et al. 2019). However, it is unclear whether this will lead to a widespread benefit for CCM species because only the red seaweed Lomentaria australis showed an increase in growth rates following CCM down-regulation (van der Loos et al. 2019). No increases in growth were detected for the kelp Ecklonia radiata (Britton et al. 2016) or the fucoid Phyllospora comosa (Britton et al. 2020a), despite evidence of CCM down-regulation. In the only study testing responses of calcified seaweeds, mixed CCA assemblages were highly sensitive to the combined effects of warming and ocean acidification, with negative effects detected in conditions projected to occur by 2030 (Britton et al. 2021).

The effects of warming have been predominately negative for the species tested so far, with temperatures at and above 22 °C negatively affecting growth in Ecklonia radiata gametophytes and microscopic sporophytes (Mabin et al. 2013, 2019b), Phyllospora comosa juveniles (Flukes et al. 2015) and Macrocystis pyrifera juveniles (Mabin et al. 2019a). Temperature also affects germination of M. pyrifera gametophytes and differences between the northern and southern Tasmanian populations are evident (Iha et al., unpublished), with a germination temperature of 14–15 °C for northern strains and 13–14 °C for southern strains, while temperatures above 19 °C were fatal. The number of days until germination also varied between northern and southern populations, with southern population gametophytes germinating after 11 days and northern population after 15–19 days. However, thermal performance is enhanced with nitrogen sufficiency in M. pyrifera (Fernández et al. 2020; Schmid et al. 2020) and elevated CO₂ concentrations in P. comosa (Britton et al. 2020a). The remodelling of cell membrane fatty acids is an important mechanism of acclimation to high temperature, nutrient limitation and ocean acidification (Britton et al. 2020a; Schmid et al. 2020). Gametophytes of the endemic kelp Lessonia corrugata appear to be highly sensitive to elevated temperatures with a thermal optimum of 17 °C detected regardless of light levels, and strongly negative responses above this temperature (Paine et al. 2021a).

6.3 Local anthropogenic stressors

As urban density is highest in coastal regions, local anthropogenic stressors can impact seaweeds, manifesting in a variety of ecosystem responses (e.g. Strain et al. 2014, 2020). These stressors include the addition of excess nutrients, heavy metals and sedimentation from increased urbanisation, land uses in the catchment such as agriculture, and coastal industries including finfish aquaculture. In Tasmania, the majority of research in this area has focused on assessing the effects of excess nutrients from salmon aquaculture (Fowles et al. 2018a; Oh et al. 2015; White et al. 2022), heavy metals and sedimentation via storm water drains and shipping activities (Fowles et al. 2018a; Ling et al. 2018).

The addition of nutrients and heavy metals into coastal systems can impact seaweed directly through uptake but also through the interaction with excess sediment (Strain et al. 2015), decreased light availability (Kavanaugh et al. 2009), and other stressors all of which can indirectly impact seaweed communities and their trophic interactions (White et al. 2018). The response of Tasmanian seaweed communities to excess nutrients is comparable to that described by Pearson and Rosenberg (1978) such that, when systems are enriched, longer-lived and more stable species are progressively replaced with faster growing ephemeral species (Fowles et al. 2018a; Oh et al. 2015; White et al. 2022). In Tasmania, this manifests in the decline of brown seaweeds such as Ecklonia radiata, Dictyopteris muelleri, Zonaria spp. and foliose red seaweeds (Fowles et al. 2018a,b) and the proliferation of green seaweeds including Ulva, Cladophora, and Chaetomorpha billardierii, the red seaweed Asparagopsis armata, and a variety of turfing, epiphytic and filamentous seaweeds (Fowles et al. 2018a; Oh et al. 2015). These fast-growing species can reduce the availability of light for other species for space, and the combined effects of nutrient enrichment and sedimentation can lead to greater shifts in the overall community dynamics with larger kelps being replaced by smaller turf-forming seaweeds (Ling et al. 2018; Oh et al. 2015; Schiel and Gunn 2019; Strain et al. 2014). Increased cover of the matrix created by filamentous turfs and accumulated sediment represents a discontinuous phase shift in community dynamics since the sediment matrix greatly inhibits or prevents recruitment of large canopy-forming species (Filbee-Dexter and Wernberg 2018; Reeves et al. 2018, 2022; Strain et al. 2014; Valentine and Johnson 2005a,b).

In the Tasman Peninsula region of Tasmania, local environmental impacts on seaweed appear to be strongly influenced by wave exposure (White et al. 2022). In more exposed locations, nutrients and heavy metals are more dispersed compared to sheltered locations (Fowles et al. 2018a; Oh et al. 2015). As wave exposure is also a fundamental driver of seaweed community composition (Edgar 1984; Hill et al. 2010), understanding the relationship between exposure and nutrient enrichment is key to evaluating the vulnerability or resilience of seaweed communities in any given location.

6.4 Seaweeds as bioindicators and biomonitors of nutrient and metal pollution

Seaweeds have also successfully been used as bioindicators for nutrient and heavy metal pollution in Tasmania. For example, changes in the responses of different seaweed functional groups (e.g. canopy-forming algae, enrichment indicator species and encrusting seaweeds) have been used to provide insights into the potential environmental effects of nutrient addition from salmon farming in south-east Tasmania, at both local (Fowles et al. 2018a; Oh et al. 2015; White et al. 2022) and regional (White et al. 2021) spatial scales.

Stable isotopes are tools used to track sources of nutrient enrichment. Where there are excess nutrients in the water column, green and red seaweeds will preferentially assimilate the lighter nitrogen isotope, with prolonged exposure changing the ratio of nitrogen in seaweed tissues (Howarth et al. 2019; Viana and Bode 2013). For example, in the Derwent estuary, changes in the isotopic signature of nitrogen in Ulva spp. revealed that the two major sources of nitrogen in the estuary are sewage and natural marine activities (van Os 2020). Further, Ulva australis is a useful bioindicator of heavy metal pollution including arsenic, cadmium, copper, lead, selenium, and zinc (Farias et al. 2017a,b, 2018, 2019). Ulva australis accumulated zinc in relatively high concentrations, with spatial variation in tissue concentrations of zinc within the estuary reflecting relative concentrations in seawater (Farias et al. 2018, 2019). Bioindicator and biomonitor research in Tasmania has highlighted a key role of seaweeds in the monitoring and assessment of impacts of various anthropogenic stressors.

7.1 Bioactive compounds and natural products

Seaweeds are a rich source of natural products and 1000s of compounds with biological activity have been identified (Holdt and Kraan 2011; Ntie-Kang and Svozil 2020). This is particularly the case for red seaweeds which contain compounds currently used in a range of applications including cosmetics, pharmaceuticals, human food and health, and as supplements in agriculture (Aziz et al. 2020). For example, the red algal genus Laurencia has over 1000 secondary metabolites described (Davis and Vasanthi 2011). However, with very few exceptions, we know little about the natural products of Tasmanian seaweeds.

7.2 Fucoidan

Since the early 2000s, Tasmania has established a successful commercial fucoidan extraction industry, Marinova Pty Ltd, initially developed from harvesting the introduced kelp Undaria pinnatifida (Sanderson 1990b; Valentine and Johnson 2004) but more recently based on imported U. pinnatifida and other species including Ascophyllum nodosum from Canada. Fucoidans are sulfated, complex, fucose-rich polymers found in brown seaweeds and echinoderms (Fitton 2011; Fitton et al. 2015). Their function in brown seaweeds is to protect against pathogens, with the highest concentrations associated with the reproductive parts of the algae. Commercially, fucoidans have been used as an ingredient in food supplements for at least two decades, where they support human health in a complementary setting and are also used as topical cosmetic ingredients, but they are not currently used in any therapeutic applications classified as “pharmaceutical” (Citkowska et al. 2019; Fitton et al. 2015). Fucoidans have also been studied for their potential use as anticoagulants, anti-inflammatory, and anticancer agents in vivo. They are known as viral entry blocking agents and are effective against a wide range of coated viruses and some bacteria and have well-documented effects on cancer cell cycle arrest, enzyme inhibition, thrombolytic activity, and immune modulation (Gueven et al. 2020; Park et al. 2022; Zayed et al. 2022).

Fucoidans have structures that are species-specific, with properties that vary in composition, depending on the source biomass used. The polysaccharide backbone is typically composed of fucose monomers, but can also have significant contributions from galactose, xylose, arabinose, and rhamnose. Worldwide, fucoidans are extracted from Fucus vesiculosus, Ascophyllum nodosum, Ecklonia spp., U. pinnatifida, Cladosiphon spp., Laminaria japonica, Macrocystis pyrifera, and Kjellmaniella crassifolia with manufacturers of bulk fucoidan operating in Japan, Russia, China, South Korea, Vietnam, Taiwan, and Australia. Although commercial fucoidan extraction from Tasmanian macroalgae has focussed on the introduced U. pinnatifida, other species have also been examined and here we report new information on the presence of fucoidan in native Tasmanian seaweeds within the family Sargassaceae (Cystophora polycystidea, Cystrophora retorta, Cystophora moniliformis and a Sargassum sp.; Table 4). Each of these Tasmanian species yielded a water-soluble co-extract rich in both fucoidan and polyphenol – the latter expressed as polyphloroglucinol equivalents. Interestingly, the fucoidan fractions included relatively high galactose contents, contrasting with the high abundance of fucose in many fucoidans extracted from other species within the order Fucales, such as Fucus vesiculosus. This high galactose content is not without precedent, however, with similar fucose:galactose ratios observed in extracts from the order Laminariales such as Ecklonia radiata and Lessonia sp. (Fitton 2011; Zayed et al. 2022).

7.3 Fatty acids

Seaweeds have gained increasing attention as a source of fatty acids, especially long chain polyunsaturated fatty acids for applications in functional foods and dietary supplements (van Ginneken et al. 2011). A 2018 screening of 61 Tasmanian seaweeds species indicated a promising resource for omega-3 fatty acids (Schmid et al. 2018). The survey comprised 11 Chlorophyta, 17 Phaeophyceae (Ochrophyta), and 33 Rhodophyta and showed that total fatty acid (TFA) concentrations varied considerably (between 0.6 and 7.8% of dry weight) between species. On average the Phaeophyceae exhibited the highest concentrations followed by Chlorophyta and Rhodophyta (Schmid et al. 2018). Results of the phylum-specific fatty acid profiles were in line with other studies around the world (Galloway et al. 2012; Kumari et al. 2013; Schmid et al. 2014). The fatty acid profile of most species was characterised by high proportions of polyunsaturated fatty acids (PUFA), and a low ratio of n-6/n-3 PUFA, which is beneficial when considering seaweeds for foods (Simopoulos 2002). An investigation of seasonal patterns in fatty acid composition at various Tasmanian locations (Bicheno, Coal Point, Mouldy Hole) indicate that they vary substantially with season and site, indicating that multiple environmental drivers influence fatty acid composition (Britton et al. 2020b).

7.4 Liquid fertilizers

Tasmania has two established companies that produce liquid fertilizers, Seasol™ established in 1974 (Arioli et al. 2015), and Natrasol™ and, for both, the raw material is from local beach-cast Durvillaea spp. and imported Ascophyllum nodosum. Seaweeds contain a range of compounds including macro- and microelements (N, P, K, etc.), amino acids, vitamins, polysaccharides, and plant hormones (including cytokinins, auxins, abscisic acid) that have enormous benefits for agricultural and horticultural production, including enhanced crop yield, improved plant development like flowering and leaf development and fruit set, as well as enriching the soil microbiology and water holding capacity. Applications of seaweed extracts can enhance the tolerance of crops to a wide range of abiotic and biotic stresses such as enhanced ability to tolerate climatic stresses, plant disease, and improved resistance to insect and fungal attack such as mould and mildew (Arioli et al. 2015; Righini et al. 2018; Roberts et al. 2015).

7.5 Seaweed aquaculture including IMTA

The Australian seaweed industry, including Tasmania, is small but expanding rapidly. It has a current estimated value of AU$3 million, with the main source of seaweeds being wild harvest but, with the development of new aquaculture species, it is predicted to grow substantially in the upcoming decades (Kelly 2020). Much of this growth is predicted to be centred around the southern temperate regions, as demonstrated by the 5248 ha of marine farming leases allocated for seaweed aquaculture and other species in Tasmania (Kelly 2020). In Tasmania, there are three seaweed-related industries: (i) alginates, (ii) fertilisers and feeds for agriculture, and (iii) fucoidan bioactive compounds (see above, Lee et al. 2012). The alginate and fertiliser industries are primarily driven by the collection of beach-cast bull kelp (Durvillaea spp.) since the 1970s by Kelp Industries Pty Ltd. on King Island, Tasmania (Supplementary Table S1). Other Tasmanian based enterprises that collect bull kelp, albeit in smaller amounts, are TasKelp Ltd (King Island) and Kelpomix Ltd. (Granville Harbour). Overall, bull kelp from King Island supplies ∼5% of the world’s production of alginates, providing the island with an income of about AU $2.5 million. In 2017, the Tasmanian state government introduced a formal management plan under the Living Marine Resources Management Acts (1995) for the collection of beach-cast native marine plants and the harvest of some introduced pest species, including wakame (Undaria pinnatifida) for commercial purposes. Kai Ho Ocean Treasure harvests U. pinnatifida which is dried and sold as wakame. Market testing by Kai Ho of local seaweed species harvested under permit in 2016–2018 indicated strong interest for products from Lessonia corrugata, Macrocystis pyrifera, Ulva spp., Chaetomorpha coliformis, Codium fragile and the introduced Grateloupia turuturu as foods (J.C. Sanderson pers. comm.).

There is considerable interest in farming seaweeds in Tasmania, building on early work that identified various native species (47 reds, 31 greens, 18 browns) as suitable for the edible market, and trial cultivations of Macrocystis pyrifera and red seaweeds (e.g. Gracilaria spp. and Gelidium spp.) in the late 1980s and early 1990s – which focussed on the supply of feed for abalone (Scott and Sanderson 1994). There is currently no formal legislation on the source location of seedstock with respect to distance to the farm site nor regulations regarding at-sea farming of selectively bred high performing strains in Tasmania. At the time of writing, however, commercial and research permits are provided on a case-by-case basis. A permit application typically includes a description of the target species, an environmental impact assessment and monitoring plan, and some consideration of biosecurity and provenance of seedstock as well as the scale of the farming operations.

Ongoing research and development projects aim to initiate and expand seaweed cultivation in Tasmanian waters (Table S1). These include the development of kelp (Ecklonia radiata, Lessonia corrugata and Macrocystis pyrifera) aquaculture as part of an Integrated Multi-Trophic Aquaculture (IMTA) with salmon. Of these species, M. pyrifera has the highest ammonium uptake rates at both low and high concentrations, making it the preferred species (Smart et al. 2022). Optimal nursery conditions for reproduction and growth of juvenile sporophytes are: E. radiata = 15 °C and 30 µmol photons m⁻² s⁻¹, L. corrugata = 12 °C and 60 µmol photons m⁻² s⁻¹, and M. pyrifera = 12 °C and 30 or 60 µmol photons m⁻² s⁻¹ (Visch et al., unpublished). Lessonia corrugata gametophytes had a very narrow range of temperature over which gametophytes grew optimally at 15.7–17.9 °C (Paine et al. 2021a). The first trials of near-shore cultivation of M. pyrifera in south-eastern Tasmania indicate that, to optimise yield and quality of the cultured biomass, the kelp deployed in April (autumn) should be harvested in July-August (mid to late winter) (Biancacci et al. 2022a). The nutritional composition and heavy metal profile M. pyrifera and L. corrugata cultured in proximity to salmon and mussel farms indicate that they are suitable for human and animal consumption, except for the iodine content which was above the maximum tolerable level in dried seaweed established by Food Standards Australia New Zealand (Biancacci et al. 2022b). A similar result was found for wild-harvested M. pyrifera proximal and distal to salmon farms (Biancacci et al. 2022c). Modelling studies suggest that commercial scale aquaculture of M. pyrifera alongside salmon farms can significantly reduce nitrogen concentrations and chlorophyll levels (from phytoplankton) in adjacent waters, indicating the potential for nitrogen mitigation by the kelp (Hadley et al. 2018).

Another focus is the cultivation of Asparagopsis armata, which is native to Tasmania, as a livestock feed supplement to reduce methane emissions via enteric fermentation in ruminants. Methane emissions from ruminants contribute 9.2% of Australia’s total Greenhouse Gas emissions (∼14% globally) and small amounts of Asparagopsis containing the bioactive compound bromoform, fed to ruminants, reduce methane production by between 80 and 98% (Kinley et al. 2020; Roque et al. 2021; Stefenoni et al. 2021). Although there have been concerns raised about the potential toxicity of bromoforms via absorption by livestock and humans and potential ozone depletion due to mass cultivation of Asparagopsis, these risks appear very low (Glasson et al. 2022; Jia et al. 2022). Large-scale cultivation of Asparagopsis is suggested as a priority for the emerging Australian seaweed aquaculture industry (Kelly 2020) and in 2019 Sea Forest Ltd established a marine farm at Triabunna on the east coast to grow Asparagopsis as a feed supplement for livestock. Initial work in Tasmania has focussed on both on-land and ocean-based farming where the latter occurs via regrowth of gametophytes utilising the naturally occurring ‘barbs’ on thalli that hook onto ropes (Wright et al. 2022).