MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps14698

ABSTRACT: Phytoplankton sinking rates (Srs) are often considered to be a predominant method for delivering organic matter out of the surface euphotic zone. Although the phytoplankton size structures (PSSs) are considered to be responsible for the Sr, which are in turn regulated by nutrient levels, their interactive effects on Sr have been less examined. To address this gap, we measured the PSS and their Sr, and environmental variables including nitrate, nitrite, ammonium, phosphate, temperature, salinity and dissolved oxygen in summer and winter in a mussel farm. We found spatial variations of PSSs in both seasons. Since the size selective feeding of mussels, the mussel farm activity may influence the spatial distribution of phytoplankton community. The dominant PSSs were >20 and <20 μm phytoplankton in summer and winter, respectively, suggesting the seasonal variations of PSS. These dominant size groups were negatively related to nitrate in summer, while positively related to temperature (T) in winter, suggesting that nitrate and T may have played roles in affecting the PSS. Interestingly, in summer, the micro- and pico-sized phytoplankton strongly affected their Sr in opposite directions, which were in turn regulated majorly by the nitrate, indicating the nitrate may affect the Sr indirectly via PSS. Meanwhile a direct effect of nitrate on Sr was found in both seasons but in opposite directions, suggesting the mutual interaction between Sr and nitrate. These results imply that in a mariculture system, the drivers of phytoplankton carbon export may involve in multiple factors with complicated interactive effects.