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

## TL;DR

This study identifies 19 species of filamentous algae in crab aquaculture ponds and finds that environmental factors like pH and nutrient levels strongly influence their growth.

## Contribution

The study provides the first detailed analysis of filamentous algal species composition and environmental drivers in Eriocheir sinensis aquaculture systems.

## Key findings

- Nineteen filamentous algal species were identified, with Rhizoclonium being the most common.
- Algal biomass peaked in May 2024 and was strongly correlated with pH, nitrogen-to-phosphorus ratio, and dissolved organic matter.
- Seasonal patterns showed Cladophora and Spirogyra dominating in spring, while Rhizoclonium dominated in summer and autumn.

## Abstract

This study investigates the blooms of filamentous algae that are frequently observed in Eriocheir sinensis aquaculture ponds. The excessive growth can form dense green mats along pond margins and at the water surface, hindering crab movement, degrading water quality, and increasing management difficulty. We monitored 19 ponds across five farms for 2 years. The filamentous algae were identified using microscopy and genetic analyses, and we tracked seasonal changes in algal coverage and biomass together with environmental factors. Nineteen species of filamentous algae belonging to four genera were recorded. Filamentous algal biomass and coverage varied markedly among ponds and years and were associated with multiple environmental factors. These findings provide a scientific basis for integrated pond management and for controlling filamentous algal blooms in the Eriocheir sinensis aquaculture systems.

Filamentous opportunistic algae, which behave as opportunistic species, are frequently observed in Eriocheir sinensis aquaculture ponds. These algae can entangle Eriocheir sinensis 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 E. sinensis 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 Chlorophyta and Charophyta were identified. Rhizoclonium was the most common genus, followed by Cladophora and Spirogyra. These genera exhibited distinct seasonal succession, with Cladophora and Spirogyra dominating in spring, while Rhizoclonium 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.