MEPS 747:35-48 (2024)  -  DOI: https://doi.org/10.3354/meps14698

ABSTRACT: Phytoplankton sinking rates (Srs) are often considered to be the predominant method through which organic matter is delivered out of the surface euphotic zone. Although the phytoplankton size structures (PSSs) are considered to be responsible for the Srs, which are in turn regulated by nutrient levels, their interactive effects on Srs are not so well studied. To address this knowledge gap, we measured the PSSs and their Srs, along with environmental variables including nitrate, nitrite, ammonium, phosphate, temperature (T), salinity, and dissolved oxygen in summer and winter in a mussel farm. We found spatial variations in PSSs in both seasons. Owing to the size-selective feeding of mussels, the activity of the mussel farm may influence the spatial distribution of the phytoplankton community. The dominant PSSs were >20 and <20 µm phytoplankton in summer and winter, respectively, suggesting seasonal variations in PSS. These dominant size groups were negatively related to nitrate in summer and positively related to T in winter, suggesting that nitrate and T may have played roles in influencing the PSS. In summer, micro- and pico-sized phytoplankton strongly affected their Srs in opposite directions, which were in turn mainly regulated by nitrate, indicating that 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 a mutual interaction between Sr and nitrate. These results imply that in a mariculture system, the drivers of phytoplankton carbon export may involve multiple factors with complicated interactive effects.

KEY WORDS: Environmental variables · Nitrate · Phytoplankton · Sinking rate · Size structure