Impacts of Bottom-Up and Top-Down Effects on Phytoplankton Communities in Caohai Ecological Restoration Area

To investigate the effects of bottom-up and top-down interactions on phytoplankton communities in restoration areas, this study systematically monitored the restored and control areas of Caohai Lake from August 2023 to January 2024. Redundancy analysis and variance decomposition were employed to assess the regulatory effects of environmental factors and zooplankton on phytoplankton communities. The results indicated that nutrient concentrations, including total nitrogen and total phosphorus, were significantly lower in the restored areas compared to the control areas, with improved water transparency. Phytoplankton from seven phyla, representing 156 species, were identified in the restored areas, with a density of (3.63±2.40) ×10⁶ cells/L and a balanced community structure. In contrast, the control areas harbored 147 species from seven phyla, with a density of (6.20±2.12)×10⁶ cells/L, where the relative density of the Cyanobacterial phylum was notably higher. Fifty zooplankton species were recorded in the restoration area, with a density of (224.85±60.36) ind./L, predominantly consisting of rotifers and copepods. In the control area, 42 species were detected, with a density of (163.15±158.22) ind./L. Biodiversity indices were generally higher in the restoration area than in the control area. The RDA and variance decomposition results revealed that phytoplankton in the restoration area was primarily influenced by environmental factors, such as nutrients, which individually accounted for 47.9% of the total phytoplankton population. Upstream factors predominated, exhibiting weaker downstream effects, likely due to the small size of the zooplankton (Tardigrades, Copepods) and limited grazing pressure. The control area showed a significant interaction between the environment and zooplankton, with a shared explanation of 59.3%, indicating a stronger downstream effect. Enhanced bottom-up effects and high nutrient levels promoting phytoplankton growth were closely associated with the tolerance and selective feeding capacity of large zooplankton, such as Cladocerans, on Cyanobacteria. This study illustrates that ecological restoration projects, by improving the water environment, reshaped the regulatory mechanisms of phytoplankton communities, transitioning from a multi-factor synergy under natural conditions to one dominated by environmental factors. This provides important scientific evidence for the restoration and management of aquatic ecosystems.