Declining silicon to nitrogen ratios : Effects on phytoplankton and plankton food webs

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.