Description: Several floristic studies on macroalgae of Svalbard have been published, but as access to the archipelago is difficult, these studies are scattered and often only cover single sites and habitats. Kongsfjorden, Isfjorden and Hornsund are the three most comprehensively investigated areas, and most of the species information comes from these three fjords. Quantitative and structured sublittoral sampling has been undertaken along depth transects and along the fjord only in Kongsfjorden. Clear dif-ferences are found from the outer to the inner parts of the fjord. Macroalgal biodiversity data from Kongsfjorden are presented in detail and compared to data for the whole archipelago. In total 197 species of macroalgae have been recorded for Svalbard; 84 of these occur in Kongsfjorden. The current taxonomic status of some species is discussed. Changes in the macroalgal flora during the last decades for Svalbard in general and in Kongsfjorden in particular, are summarised and possible causes discussed. Information on biodiversity of microphytobenthos is very scarce, and investigations in Kongsfjorden on benthic diatoms from soft bottom and biotic surfaces provide the first floristic information available. A total of 69 diatoms species have been identified and form a first baseline for a high-latitude fjord system. Biodiversity is relatively low compared to other sandy marine shallow water areas of temperate regions as indicated by the Shannon-Weaver index. Some data on epiphytic diatoms colonising seaweeds are available. Benthic diatoms colonise large parts of Kongsfjorden in high abundances and, in addition to macroalgae, are important as primary producers and therefore also for trophic relationships in the harsh Arctic environment.

Description: Due to its Arctic location at 79°N, Kongsfjorden in Svalbard experiences strong seasonality in light climate, changing from polar night to midnight sun. Sea ice conditions and the optical properties of seawater further modify the amount and the spectral composition of solar radiation penetrating into the water column, thus defining the underwater light climate in Kongsfjorden. Light represents one of the major shaping factors for the entire marine ecosystem. A number of studies focusing on implications of the underwater light for marine organisms have beenconducted in Kongsfjorden, generating diverse datasets on seawater optical properties, scattered over time and space. This review synthesizes the fragmentary information available from the literature as well as presenting some unpublished data, and discusses the underwater light climate and its main controlling factors in Kongsfjorden. Furthermore, we provide a short synopsis about the relevance of light for different components of an Arctic marine ecosystem, exemplified by studies carried out in Kongsfjorden. Due to its year-round accessibility and its high-Arctic location, Kongsfjorden has become a prime fjord for studying how the strong seasonal changes in light availability, ranging from polar night to midnight sun, affect marine life with respect to primary production, behavioural aspects and synchronization of growth and reproduction.

Description: Kelps (brown algae of the order Laminariales) build highly complex and productive underwater forests and possess microscopic and macroscopic life stages. The microscopic stages (spores, gametophytes, juvenile sporophytes) are usually more sensitive to environmental stressors and may form a bottle-neck for the survival of the population. Future Arctic kelp forests will be especially affected by elevated temperatures and increased sedimentation. Knowledge on grazer impact is still rudimentary. In order to investigate how global change in interaction with grazing may shape future Arctic kelp systems we performed laboratory experiments (2 x 3 x 2 factorial design) on early life stages of the kelps Alaria esculenta, Laminaria digitata and Saccharina latissima from Arctic Kongsfjorden (Svalbard). Spores were exposed to ambient and elevated summer temperatures in combination with 3 levels of sediment and 2 levels of grazing by the limpet Margarites helicinus. The germination and formation of juvenile sporophytes was strongly inhibited in all species with increasing sediment cover, clearly showing the strongest negative effect on sporophyte development of all tested variables. Grazers interacted with temperature and sedimentation affecting kelps in a species-specific way. They had a strong impact on the number of developing sporophytes partially counteracting the negative impact of sedimentation. We conclude that the structure of kelp communities can be shaped by abiotic and biotic variables acting on early developmental stages and that global warming has the potential to alter the strengths and direction of these effects, which may lead to future shifts in community structure.

Description: In Arctic macroalgal belt ecosystems, macrozoobenthic production is thought to be an important link between primary production and higher trophic levels. Macrozoobenthic biomass and secondary production were studied along transects (2.5-15 m depth) in the macroalgal belt at Hansneset in Kongsfjorden, Svalbard, from 2012 to 2013. At 2.5m, the standing stock reached its maxima of 174.8 ± 54.4 g ash free dry weight per 1 m2, while density (4341 ind. m-2± 1127 95% CI) and production (7.0 g C m-2 y-1 ± 2.8 95% CI) were highest at 5 m water depth in 2012/13. Compared to a study from 1996/98, this re-sampling indicated a drastic change in the depth-distribution of macrozoobenthic biomass and secondary production at Hansneset. While both biomass and secondary production increased with water depth in 1996/98, this pattern was inversed in 2012/13 owing to a tenfold increase of biomass and secondary production in the upper most sublittoral (2.5-5 m). Variability of macrozoobenthic biomass and secondary production corresponded to differences in the physical environment and macroalgal vegetation along the depth gradient. In the last decade, the number of ice free days per year increased probably due to Arctic warming. As a result, shallow rocky habitats (2.5-5 m) are less affected by ice scouring, thereby opening new space for colonization by benthic fauna. However, faunal secondary production was low compared to macroalgal primary production, indicating a considerable export of most of the algal production from the shallow habitats to the adjacent areas.

Description: Arctic West Spitsbergen in Svalbard is currently experiencing gradual warming due to climate change showing decreased landfast sea-ice and increased sedimentation. In order to document possible changes in 2012–2014, we partially repeated a quantitative diving study from 1996 to 1998 in the kelp forest at Hansneset, Kongsfjorden, along a depth gradient between 0 and 15 m. The seaweed biomass increased between 1996/1998 and 2012/2013 with peak in kelp biomass shifted to shallower depth, from 5 to 2.5 m. The kelp biomass at 2.5 m was 8.2-fold higher in 2012/2013 (14 kg fresh biomass m-2) than in 1996/1998 and mostly due to an increase in the kelp Laminaria digitata. This resulted in a very high density of 2- to 8-year-old kelp (70 ind. m-2) and a high leaf area index of nearly 10 at 2.5 m. The entire zonation seemed to have shifted upwards to shallower depth, since also the lower depth limit of most dominant brown algae was shallower as well as the biomass maximum of several taxa. The cumulated annual photosynthetic active radiation at 15 m depth (42 mol m-2 year-1) determined the current depth limit of kelps. Changes also resulted in an altered seaweed community pattern. The complex pattern of change was probably driven by opposing effects of coacting environmental drivers, namely lack of ice-scouring, elongation of the open-water period and deterioration of the underwater irradiance climate. The results are interpreted as a consequence of Arctic warming probably reflecting a typical scenario for change along other Arctic shores in near future.

Description: The natural environment of Antarctic seaweeds is characterized by changing seasonal light conditions. The ability to adapt to this light regime is one of the most important prerequisites for their ecological success. Thus, the persistence of seaweeds depends on their capacity to maintain a positive carbon balance (CB)for buildup of biomass over the course of the year. A positive CB in Antarctica occurs only during the ice-free period in spring and summer, when photosynthetically active radiation (PAR, 400–700 nm) penetrates deeply into the water column. The accumulated carbon compounds during this period are stored and remobilized to support metabolism for the rest of the year. Over the last decades climate warming has induced a severe glacial retreat in Antarctica and has opened newly ice-free areas. Increased sediment runoff, and reduced light penetration due to melting during the warmer months, may lead to a negative CB with changes in the vertical distribution of seaweeds. Furthermore, warmer winters and springs result in earlier sea-ice melt, causing an abrupt increase in light, compensating the reduction in PAR in summer or increasing the annual light budget. Studies performed in Potter Cove, Isla 25 de Mayo/King George Island, reveal that algae growing in newly ice-free areas did not acclimate to the changing light conditions. Lower or even negative CB values in areas close to the glacier runoff seem to be primarily dependent on the incoming PAR that finally determines the lower distribution limit of seaweeds. The present chapter discusses how carbon balance respond to the changing Antarctic light environment and its potential implications for the fate of benthic algal communities.

Description: Macroalgae is a central part of marine shelf ecosystems in the Arctic, both as primary producers and as habitat builders and may contribute substantially to the carbon export into the deep sea. In Kongsfjorden we quantified the zonation of visually dominant macroalgal taxa and of detached macroalgae from underwater videos taken in summer 2009 at six transects between 2 to 138 m water depth. Four transects were located at the south shore along the length axis of the fjord (Kongsfjordneset, Brandal, Prince Heinrich Island, Tyskahytta). Two further transects investigated the steep bedrock of Hansneset with a west-east orientation 50 m apart from each other: Hansneset 1 (north) and Hansneset 2 (south). The georeferenced data (date, depth, coordinates) of all transects were linked to the timecode of the video and imported into a geographic coordinate system (GIS). Presence/absence and cover data of macroalgae along the transects was collated into the GIS. The resulting shape files provide useful information for further investigations of macroalgae in the fjord and the geographical information may enhance the repeatability of the investigation in the future.

Description: Spores represent the most vulnerable life history stage of kelps. While UV-induced inhibition of spore germination has been readily documented, the impact of in situ underwater radiation below kelp canopies has been largely overlooked. We determined spectral composition and intensity of underwater radiation along a density gradient in an Alaria esculenta kelp forest at 3 m depth in Kongsfjorden, Svalbard. Accordingly, we set up a laboratory experiment simulating five different radiation conditions corresponding to irradiances under very dense to no canopy cover on a cloudless summer day. Spore responses (photosynthetic quantum yield, pigment and phlorotannin contents, swimming activity, and germination success) were determined after 4, 8, 16, and 24 h of exposure. In situ spectral radiation composition differed strongly from conditions applied in previous studies, which underestimated photosynthetically active radiation and overestimated UV-radiation effects. Furthermore, spore solutions differed significantly in quantum yield, pigment, and phlorotannin contents upon release. Nevertheless, spores reacted dynamically to different radiation conditions and exposure times. Highest radiation (PAR 61.8 W m−2, 1.9 W m−2 UVA, 0.01 W m−2 UVB) caused photodamage after exposure for ≥ 8 h, while intermediate radiation led to photoinhibition. Lowest radiation (PAR 0.23 W m−2, 0 W m−2 UVA, 0 W m−2 UVB) caused inconsistent reactions. There was a reduction of absolute pigment content in all treatments, but reduction rates of photosynthetic pigments were significantly different between radiation treatments. Soluble phlorotannin content decreased under all conditions but was not significantly affected by experimental conditions. High radiation reduced swimming activity of spores, but experimental conditions had almost no effect on germination success. Consequently, it seems unlikely that in situ radiation conditions negatively affect spores in present and future radiation scenarios.