Description: Understanding how changes in limiting nutrient availability affect life in the oceans requires interdisciplinary efforts. Here we illustrate this with an example of silicon, one of the most common elements on land which bioavailable form, silicic acid (Si(OH)4), is a limiting nutrient for silicifying primary producers, such as diatoms. Silicic acid concentrations in the pelagic polar and subpolar North Atlantic have declined by 1-2 μM during spring pre-bloom conditions over the past 25 years. Many coastal areas of the North Atlantic region also face decreased relative availability of silicon due to increased riverine supply of nitrogen and phosphorus and stable or declining loads of silicon. Both declining silicic acid concentrations and declining silicon to nitrogen (Si:N) ratios limit the growth of diatoms, which are major primary producers contributing up to a quarter of global primary production. To assess the effects of declining silicon availability on phytoplankton communities we conducted a mesocosm experiment manipulating Si:N ratios and copepod grazing pressure on phytoplankton communities from the Baltic Sea. Declining Si:N ratio affected not only diatom abundance and relative biomass but also their species composition and overall plankton diversity. Our results illustrate the importance of silicon in structuring community composition at the base of temperate marine food webs. Changes in silicic acid concentrations and Si:N ratios, therefore, may have far-reaching consequences on oceanic primary production and planktonic food webs. The decline in silicon concentrations in polar and subpolar North Atlantic waters is attributed to natural multi-decadal variability but is likely amplified by reduced ocean mixing due to increased water temperatures, illustrating the need of international efforts to curb global climate change. The decline in Si:N ratios in coastal oceans also highlights the need for further reduction of nutrient pollution and improved river basin management. This may require interdisciplinary and international approaches to manage anthropogenic perturbations of the silicon cycle.

Description: Diatoms often dominate phytoplankton in temperate, polar and upwelling regions. Decreases in silicate availability or silicon to nitrogen (Si:N) ratios may induce silicon limitation in diatoms and lower their proportion within phytoplankton communities. The effects of such changes on the nutritional quality of phytoplankton are not well understood. To examine how changing Si:N ratios affect plankton nutritional value, we applied a range of Si:N ratios on a natural plankton community and manipulated grazing pressure to assess top-down effects of copepod selective grazing. Diatom proportion in phytoplankton increased with increasing Si:N ratios and so did phytoplankton nutritional quality in terms of major fatty acid concentrations, such as polyunsaturated fatty acids, docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids. However, stoichiometric quality (carbon to nitrogen and carbon to phosphorus ratios), DHA:EPA and omega 3:6 (omega 3:omega 6) ratios declined with increasing Si:N ratios, suggesting that proportions between essential compounds in copepod diet may be more favorable in lowered Si:N ratios. Copepods had a negative effect on DHA contents, DHA:EPA and omega 3:omega 6 ratios, indicating possible selective grazing on more nutritious plankton. Our findings show that declining silicate concentrations can affect stoichiometric and biochemical quality of phytoplankton, which copepods can also moderate by selective grazing.

Description: Increasing human activities on land, such as intensive farming, fossil fuel burning and river flow modifications alter nutrient cycles with implications for both terrestrial and marine ecosystems. The nitrogen cycle has been particularly affected: the amount of nitrogen available for primary producers has doubled due to artificial atmospheric nitrogen fixation. Consequently, more reactive nitrogen is reaching oceans via river run-off and atmospheric deposition. The silicon cycle, on the other hand, has been affected in an opposite direction and to a lesser extent: with increasing river damming, more silicate is biologically fixed in dam reservoirs and less of it is reaching the coastal oceans in a reactive form. These changes result in a decline in silicon to nitrogen (Si:N) ratios and can alter the composition of phytoplankton - small, but numerous organisms, providing the base of pelagic marine food webs. Si:N ratios affect phytoplankton composition because nitrogen is required by all phytoplankton and silicon is essential only for certain groups, such as diatoms. Diatoms use silicate to build their porous cell walls and thus silicon availability can limit their growth. These organisms are abundant, especially in nutrient rich waters, and account for as much carbon fixed as the rainforests on land. Multiple experimental studies have shown that diatom proportion declines with decreasing Si:N ratios. Yet further knowledge of how this change in phytoplankton composition may affect the functioning of entire plankton communities is needed to ultimately understand and estimate the impacts of nutrient alterations on higher trophic levels and the marine carbon pump. In this thesis, I experimentally assessed the impacts of changes in Si:N ratios on the complex interactions in the lower pelagic food web. Two mesocosm experiments were conducted where natural Baltic Sea plankton communities were exposed to a range of Si:N ratios and varying copepod grazing pressure. The results showed that a lowered Si:N ratio not only lowers the proportion of diatoms within the phytoplankton community, but also increases the abundance and biomass of non-silicifying groups of plankton, with implications for the quality and quantity of food available for mesozooplankton. Two conceptual models were developed in Chapter I to illustrate food web structures in low and high Si:N environments, concluding that lowered Si:N ratios result in more complex plankton food webs, which are known to lower energy transfer efficiency. An unexpected finding was that some diatom species were not affected by grazing, indicating that the efficiency of the “diatom-copepod” food chain may be moderated by diatom edibility. In Chapter II, this aspect was investigated further, by assessing the effects of altered Si:N ratios on the nutritional value of plankton in terms of fatty acid and particulate nutrient indicators. The results showed that while high Si:N environments can be characterized by higher availability of essential fatty acids, ratios between particulate nutrients and selected fatty acids are more suitable for mesozooplankton when Si:N ratio is lowered. Changes in phytoplankton composition with declining Si:N ratios observed in this thesis are in line with Tilman’s Resource Ratio Theory, which states that ratios of limiting resources can determine the outcome of species competition. The applicability of this theory, however, has been questioned as it does not account for varying concentrations of resources. In Chapter III, this thesis presents evidence from natural communities that plankton composition responds to lowered Si:N ratios in a similar way both when nitrogen and when silicon concentrations are manipulated. However, while nutrient ratios are critical in determining community composition, absolute concentrations largely control the total biomass of phytoplankton. These findings contribute to the discussion on the ecological importance of nutrient ratios and concentrations and stress that resource ratios, in particular the ratio between silicon and nitrogen, can be used in predicting and modelling the outcome of species competition in natural phytoplankton communities. In conclusion, anthropogenic manipulations of nitrogen and silicon cycles can have strong effects on plankton composition, biomass and trophic interactions. This thesis underlines that implications of these changes on higher trophic levels and ecosystem functioning are complex and future studies are needed to understand the role of selective grazing, phytoplankton quality and defense mechanisms in marine food webs.

Description: The Bay of Bengal (BoB) is a 2,600,000 km2 expanse in the Indian Ocean upon which many humans rely. However, the primary producers underpinning food chains here remain poorly characterized. We examined phytoplankton abundance and diversity along strong BoB latitudinal and vertical salinity gradients—which have low temperature variation (27–29°C) between the surface and subsurface chlorophyll maximum (SCM). In surface waters, Prochlorococcus averaged 11.7 ± 4.4 × 104 cells ml−1, predominantly HLII, whereas LLII and ‘rare’ ecotypes, HLVI and LLVII, dominated in the SCM. Synechococcus averaged 8.4 ± 2.3 × 104 cells ml−1 in the surface, declined rapidly with depth, and population structure of dominant Clade II differed between surface and SCM; Clade X was notable at both depths. Across all sites, Ostreococcus Clade OII dominated SCM eukaryotes whereas communities differentiated strongly moving from Arabian Sea-influenced high salinity (southerly; prasinophytes) to freshwater-influenced low salinity (northerly; stramenopiles, specifically, diatoms, pelagophytes, and dictyochophytes, plus the prasinophyte Micromonas) surface waters. Eukaryotic phytoplankton peaked in the south (1.9 × 104 cells ml−1, surface) where a novel Ostreococcus was revealed, named here Ostreococcus bengalensis. We expose dominance of a single picoeukaryote and hitherto ‘rare’ picocyanobacteria at depth in this complex ecosystem where studies suggest picoplankton are replacing larger phytoplankton due to climate change.

Description: Tropical environments with unique abiotic and biotic factors—such as salt ponds, mangroves, and coral reefs—are often in close proximity. The heterogeneity of these environments is reflected in community shifts over short distances, resulting in high biodiversity. While phytoplankton assemblages physically associated with corals, particularly their symbionts, are well studied, less is known about phytoplankton diversity across tropical aquatic environments. We assess shifts in phytoplankton community composition along inshore to offshore gradients by sequencing and analyzing 16S rRNA gene amplicons using primers targeting the V1-V2 region that capture plastids from eukaryotic phytoplankton and cyanobacteria, as well as heterotrophic bacteria. Microbial alpha diversity computed from 16S V1-V2 amplicon sequence variant (ASV) data from 282 samples collected in and around Curaçao, in the Southern Caribbean Sea, varied more within the dynamic salt ponds, salterns, and mangroves, compared to the seemingly stable above-reef, off-reef, and open sea environments. Among eukaryotic phytoplankton, stramenopiles often exhibited the highest relative abundances in mangrove, above-reef, off-reef, and open sea environments, where cyanobacteria also showed high relative abundances. Within stramenopiles, diatom amplicons dominated in salt ponds and mangroves, while dictyochophytes and pelagophytes prevailed above reefs and offshore. Green algae and cryptophytes were also present, and the former exhibited transitions following the gradient from inland to offshore. Chlorophytes and prasinophyte Class IV dominated in salt ponds, while prasinophyte Class II, including Micromonas commoda and Ostreococcus Clade OII, had the highest relative abundances of green algae in mangroves, above-reef, off-reef, and the open sea. To improve Class II prasinophyte classification, we sequenced 18S rRNA gene amplicons from the V4 region in 41 samples which were used to interrelate plastid-based results with information on uncultured prasinophyte species from prior 18S rRNA gene-based studies. This highlighted the presence of newly described Ostreococcus bengalensis and two Micromonas candidate species. Network analyses identified co-occurrence patterns between individual phytoplankton groups, including cyanobacteria, and heterotrophic bacteria. Our study reveals multiple uncultured and novel lineages within green algae and dictyochophytes in tropical marine habitats. Collectively, the algal diversity patterns and potential co-occurrence relationships observed in connection to physicochemical and spatial influences help provide a baseline against which future change can be assessed.

Description: Despite management efforts, anthropogenic nutrient enrichments continue to enhance phytoplankton blooms worldwide. Release of nitrogen and phosphorus compounds not only provides surplus of nutrients but also disbalances their stoichiometry. Declines in the relative availability of dissolved silicon might induce limitation in diatoms, major primary producers with silicified shells. We studied experimentally how nutrient enrichment and resulting decline in dissolved silicon to nitrogen ratios (Si:N) affect the structure and functioning of natural plankton communities. Nitrate was added to create a range of Si:N ratios and phosphate was supplied in Redfield ratio to nitrogen. We also manipulated copepod abundance to understand the top-down effects on communities experiencing nutrient enrichment. Nitrogen and phosphorus additions resulted in a steep phytoplankton biomass increase, followed by a post-bloom decline. Phytoplankton bloom biomass was higher in high nitrogen treatments but during the post-bloom period this trend switched. Biomass was sustained longer in high Si:N treatments, indicating that silicon limitation terminates the bloom. Many diatom species did not benefit from nitrogen and phosphorus enrichment and diatom dominance ceased below Si:N of 0.4:1. Under high grazing pressure, silicate was taken up faster suggesting that silicification is important in diatom defense. Copepods shaped plankton communities via feeding on dinoflagellates, chlorophytes and the diatom Skeletonema costatum but there was no significant effect of nitrogen and phosphorus enrichment on copepod abundance. Our results, combined with previous studies, show that while nutrient concentrations define the total phytoplankton bloom biomass, resource ratios are important in sustaining biomass and determining community structure and composition.

Description: Changes in silicon to nitrogen (Si:N) ratios are known to affect phytoplankton community composition, as silicon is an essential nutrient for diatoms but not for most other phytoplankton. Less is known if and how this ratio affects biochemical composition and stoichiometry of seston. This is of importance, as changes in seston chemistry can have implications on the quality of food available for zooplankton. We applied a range of Si:N ratios and two levels of copepod grazing on a natural Baltic sea plankton community pre-filtered with 125um mesh size filter. Si:N ratios were achieved by adding silicate (at target concentrations of 10, 16, 22, 28 and 34 μmol L−1) and nitrate solutions (at target nitrogen concentration of 40 µmol L-1) to the experimental units at the start of the experiment. Copepod grazing was manipulated by adding 30 individuals of adult Eurytemora affinis copepods per liter to high copepod treatments once phytoplankton bloom has established (day 6 of the experiment). The mesocosm experiment was carried out in summer 2016 and lasted 20 days. The response of particulate carbon, nitrogen, phosphorus was followed by sampling three times per week and fatty acid samples were taken at the end of the experiment. Our data reveals that increasing Si:N ratios result in an increase of particulate carbon, phosphorus, nitrogen and total fatty acid concentrations. Carbon to nitrogen (C:N) and carbon to phosphorus (C:P) ratios increased with increasing Si:N ratios as well as the concentrations of individual essential fatty acids such as DHA and EPA per seston carbon. Enhanced copepod grazing affected C:N, C:P and DHA and ALA concentrations negatively. Consequently, this data illustrates the importance of bottom up effects such as changes in Si:N ratio and top-down controls like copepod grazing in shaping particulate nutrient and fatty acid composition of marine seston.

Description: Changes in silicon to nitrogen (Si:N) ratios are known to affect phytoplankton community composition, as silicon is an essential nutrient for diatoms but not for most other phytoplankton. Less is known if and how this ratio affects biochemical composition and stoichiometry of seston. This is of importance, as changes in seston chemistry can have implications on the quality of food available for zooplankton. We applied a range of Si:N ratios and two levels of copepod grazing on a natural Baltic sea plankton community pre-filtered with 125um mesh size filter. Si:N ratios were achieved by adding silicate (at target concentrations of 10, 16, 22, 28 and 34 μmol L−1) and nitrate solutions (at target nitrogen concentration of 40 µmol L-1) to the experimental units at the start of the experiment. Copepod grazing was manipulated by adding 30 individuals of adult Eurytemora affinis copepods per liter to high copepod treatments once phytoplankton bloom has established (day 6 of the experiment). The mesocosm experiment was carried out in summer 2016 and lasted 20 days. The response of particulate carbon, nitrogen, phosphorus was followed by sampling three times per week and fatty acid samples were taken at the end of the experiment. Our data reveals that increasing Si:N ratios result in an increase of particulate carbon, phosphorus, nitrogen and total fatty acid concentrations. Carbon to nitrogen (C:N) and carbon to phosphorus (C:P) ratios increased with increasing Si:N ratios as well as the concentrations of individual essential fatty acids such as DHA and EPA per seston carbon. Enhanced copepod grazing affected C:N, C:P and DHA and ALA concentrations negatively. Consequently, this data illustrates the importance of bottom up effects such as changes in Si:N ratio and top-down controls like copepod grazing in shaping particulate nutrient and fatty acid composition of marine seston.

Description: Phytoplankton, microzooplankton, copepod and dissolved nutrient data from a mesocosm experiment, which took place in summer 2016. A range of Si:N ratios and two levels of copepod grazing pressure were manipulated on a natural plankton community in Kiel Bay, Southern Baltic Sea, Germany.