Seasonal Dynamics and Stability of Zooplankton Community in Mytilus coruscus Farming Areas

Large-scale mussel farming affects marine plankton, yet the seasonal dynamics and stability of zooplankton community in mussel farms remain poorly understood. To evaluate the seasonal variation of zooplankton community and potential aquaculture impacts, surveys on zooplankton community in mussel farms near Gouqi Island were carried out in October 2022 (autumn), May 2023 (spring), and August 2023 (summer). The results indicated that copepods were the dominant group across three seasons. The zooplankton community structure showed significant seasonal variation (permutational multivariate analysis of variance [PERMANOVA], p = 0.001), with water temperature (WT) and salinity (SAL) identified as the primary environmental drivers by redundancy analysis (RDA). Zooplankton abundance peaked in summer, driven by the dominance of small-sized copepod Paracalanus parvus, while autumn exhibited the highest species diversity. Meanwhile, abundance–biomass comparison (ABC) curves indicated a K-strategist-dominated zooplankton community in spring (W > 0) and a structural shift toward opportunistic r-strategists in summer and autumn (W < 0). Crucially, rather than strictly reflecting anthropogenic disturbance, this zooplankton community miniaturization primarily indicates natural seasonal succession driven by environmental fluctuations. However, we hypothesize that this natural shift may be compounded by top-down grazing pressures from mussels. These findings emphasize the dominance of natural seasonal drivers, with aquaculture activities acting as plausible interacting mechanisms, providing a crucial baseline for future monitoring.