Seasonal succession outweighs cultivation effects on phytoplankton metacommunity dynamics in a subtropical IMTA system, southeastern China

Phytoplankton dynamics plays an essential role in the functioning and sustainability of integrated multi-trophic aquaculture (IMTA) systems. We investigated seasonal and spatial variations of phytoplankton communities in oyster Crassostrea angulata , kelp Saccharina japonica and non-cultivation zones of a subtropical marine ranch in southeastern China. High-throughput sequencing of 18S rDNA across 4 seasons and 2 water depths identified 402 operational taxonomic units (OTUs) (239 phytoplankton species), with Dinophyta as the most abundant group (average relative sequence abundance >36%). Contrary to previous studies of aquacultural impacts on phytoplankton communities, alpha diversity and community composition did not differ significantly among cultivation types (Shannon-Wiener indices: 2.620 ± 0.483, 2.606 ± 0.298 and 2.747 ± 0.200 for oyster, kelp, and non-cultivation areas, respectively). Instead, seasonal variation explained the majority of community turnover (R 2 = 0.62, p < 0.001), with diatoms peaking in summer (average relative sequence abundance 23.2%) and dinoflagellates in winter (average 41.5%). Water temperature, dissolved inorganic nitrogen and chlorophyll a were the strongest environmental drivers. Null model analyses indicated that stochastic processes dominated community assembly (76% of β-nearest taxonomic unit index values between -2 and +2). These results demonstrate that natural seasonal succession, rather than aquaculture, governs phytoplankton dynamics in this system. The findings highlight the resilience of phytoplankton assemblages to moderate cultivation pressures and underscore the need for seasonally adaptive management in subtropical mariculture.