Effects of sea surface warming, irradiance, and grazer abundance on organic matter cycling in a plankton community

Ocean warming affects all components of the food-web. Autotrophic processes, however, are less temperature sensitive than those conducted by heterotrophs, and changes in their balance can thus be expected, likely changing biogeochemical cycling in the oceans. The combined response of all food-web processes to temperature and other factors will thus determine the future direction of the biological carbon pump, which transports photosynthetically fixed CO2 to deeper parts of the ocean via sinking biogenic particles or dissolved organic matter. The aim of this study was to investigate the temperature-dependent partitioning of organic matter between dissolved and particulate matter pools and the changes in their stoichiometry, with emphasis on the impact of changes in irradiance or zooplankton abundance. To elucidate these questions, indoor-mesocosm studies were conducted with natural winter plankton communities from Kiel Bight (Baltic Sea) containing all trophic levels from bacteria to mesozooplankton. Chapter 1 addresses the effects of temperature in combination with different irradiance levels. Rising temperature accelerated the production rate of particulate and dissolved organic matter (POM and DOM), while light intensity played a comparatively minor role for the bloom development. Maximum accumulation of POM was significantly reduced at elevated compared to ambient temperature. Although, heterotrophic loss processes were enhanced by temperature, also an increased formation of transparent exopolymer particles (TEP) was observed. This suggests that in a warmer ocean the accumulation of POM is controlled by two counteracting mechanisms: 1) enhanced heterotrophic remineralization processes, leading to a weakening of the biological pump; and 2) enhanced particle aggregation due to TEP and strengthened transport of POM to deeper parts of the ocean. Chapter 2 describes the effects of temperature in combination with three different overwintering zooplankton grazer (copepod) densities. Increasing copepod abundance decreased the ratio of particulate organic carbon to nitrogen and phosphorus. DOC was positively affected by temperature, but when normalized to Chlorophyll a concentration, showed a tendency to decrease with increasing copepod abundance. This study indicates that the enhanced inorganic carbon consumption relative to N and P by phytoplankton under nutrient stress, which was suggested for a warmer sea surface ocean, could be counteracted by the effect of consumer-driven nutrient recycling. Chapter 3 is a comparative data analysis that aims to identify recurrent patterns of biogeochemical parameters in response to an increase in water temperature. Warming stimulated heterotrophic bacterial processes in particular and had an accelerating effect on the temporal development of phytoplankton blooms. Warming increased the rate of POM and DOC accumulation, whereas the magnitude of POM decreased with rising temperature. Chapter 4 presents results on the effects of high and low light conditions during the months January and February. It was shown that it was possible to induce a diatom phytoplankton bloom in January, comparable to one in February. POM accumulation increased with increasing light levels, whereas DOM, TEP and “Coomassie stainable particles (CSP) remained unaffected by the different irradiance levels. However, CSP were found to occur at the same magnitude and size as TEP and might hence be as important for the partitioning of N as TEP are for the partitioning of C, and should receive more attention in future studies on organic matter cycling. The results presented here show that temperature induced changes in biogeochemical cycling can have strong implications for the biological pump, which plays a vital role in Earth’s climate. Additionally, they demonstrate the strong potential for interactive effects by the interplay of temperature with changes in irradiance and grazer abundance.