Effect of elevated CO2 on organic matter pools and fluxes in a summer, post spring-bloom Baltic Sea plankton community.

Ocean acidification is expected to influence plankton community structure and biogeochemical element cycles. To date, experiments with nutrient stimulated blooms have been primarily used to study the response of plankton communities to elevated CO₂. In this CO₂ manipulation study, we used large-volume (~ 55m³) pelagic in situ mesocosms to enclose a natural, post spring-bloom plankton assemblage in the Baltic Sea to investigate the response of organic matter pools to ocean acidification. In the mesocosms, fCO₂ was manipulated yielding a range of average fCO₂ of 365 to ~ 1231 µatm with no adjustment of naturally available nutrient concentrations. Plankton community development and key biogeochemical element pools were subsequently followed in this nitrogen-limited ecosystem over a period of seven weeks. We identified three distinct phases based on temperature fluctuations and plankton biomass: a warm, productive period with elevated chlorophyll a and particulate matter concentrations (Phase I), a decline in autotrophic biomass coinciding with cooler water temperatures associated with lower incoming photosynthetically active radiation (PAR) and higher zooplankton grazing pressure (Phase II), and a steady state phase with low net change in particulate and dissolved matter pools (Phase III). We observed higher sustained chlorophyll a and particulate matter concentrations (~ 25% higher) and lower inorganic phosphate concentrations in the water column in the highest fCO₂ treatment (1231 µatm) in Phase III. Size-fractionated phytoplankton pigment analyses indicated that these differences were driven by picophytoplankton (< 2 µm) and were already established early in the experiment during Phase I. However the influence of picophytoplankton on bulk organic matter pools was masked by high biomass of larger plankton until Phase III when the small size fraction (< 2 µm) contributed up to 90% of chlorophyll a. Furthermore, CO₂-related differences in water column suspended matter concentrations were not reflected in sinking material flux. Our results from this study indicate that ocean acidification could have significant and sustained impacts on pelagic biogeochemical element pools in nitrogen-limited ecosystems. [ABSTRACT FROM AUTHOR]