Species Composition and Biomass Dynamics of Filamentous Algae and Their Environmental Drivers in Eriocheir sinensis Aquaculture Ponds

Filamentous opportunistic algae, which behave as opportunistic species, are frequently observed in <i>Eriocheir sinensis</i> aquaculture ponds. These algae can entangle <i>Eriocheir sinensis</i> and release harmful substances during decomposition, thereby negatively impacting farming performance. At present, their management largely depends on non-selective herbicides, while fundamental research on species composition and biomass dynamics remains limited. In this study, 19 aquaculture ponds across five <i>E. sinensis</i> farms in Shanghai were monitored over a two-year period. Filamentous algae species were identified using both morphological and molecular techniques, and their biomass and coverage were quantified. Concurrently, physicochemical parameters of the water were measured to analyze algal occurrence patterns and key environmental drivers. A total of 19 species belonging to four genera of the phyla <i>Chlorophyta</i> and <i>Charophyta</i> were identified. <i>Rhizoclonium</i> was the most common genus, followed by <i>Cladophora</i> and <i>Spirogyra</i>. These genera exhibited distinct seasonal succession, with <i>Cladophora</i> and <i>Spirogyra</i> dominating in spring, while <i>Rhizoclonium</i> predominanted in summer and autumn. Filamentous algal biomass reached its peak in May 2024, with a dry weight of 42.92 g/m². The two-way ANOVA results indicated significant main effects of month and region, as well as a significant month × region interaction. The Spearman correlation analysis revealed a strong positive association between algal biomass and pH. This pattern is consistent with the effect where the intense algal photosynthesis raises water pH through the uptake of dissolved carbon dioxide. The total biomass was significantly correlated with the nitrogen-to-phosphorus ratio, suggesting that nitrogen and phosphorus availability influenced algal growth. Moreover, filamentous algal coverage was positively associated with maximum algal biomass. The linear regression analysis further revealed that multiple environmental factors jointly contributed to algal proliferation, with total nitrogen, nitrate nitrogen, and fluorescent dissolved organic matter (fDOM) showing relatively strong associations with maximum biomass. These findings provide a scientific basis for the ecological control and targeted management of filamentous algae in aquaculture systems.