PHOSPHORUS CYCLE OF SILVER CARP FEEDING ON DIFFERENT ALGAE AND ITS EFFECT ON THE WATER NUTRIENT CYCLE

In order to explore the impact of filter-feeding fish on the geochemical cycle of water nutrients, an indoor experiment was conducted to study the filter-feeding efficiency of silver carp (Hypophthalmichthys molitrix) on algae, and to measure the absorption and excretion of phosphorus by silver carp during the filter-feeding process, also we analyzed the influence of excretion on nitrogen and phosphorus in water bodies. The results showed that (1) silver carp reduced the population density of cyanobacteria, green algae, and diatoms through filter feeding, with comparable effectiveness against cyanobacteria as against green algae and diatoms; (2) silver carp filter-feeds on algae by ingesting particulate phosphorus (PP) and excreting soluble phosphorus (SRP) into the water. In the presence of sufficient food supply, the total phosphorus (TP) of the water body remained relatively stable. However, when the food was insufficient, the silver carp consumed its own nutrients, continuing to release SRP into the water, contributing to phosphorus raise; (3) Following food consumption, silver carp reintroduced inorganic nutrients to the water environment, leading to a high nitrogen-to-phosphorus (N﹕P) ratio. Most nitrogen and a small part of phosphorus from the food were excreted as inorganic nutrients, resulting in significantly higher N﹕P ratios in the water environment of the filter-feeding group compared to the control group. Furthermore, the N﹕P ratio in the water environment of the filter-feeding cyanobacteria group was significantly higher than that of the diatom and green algae groups, suggesting that silver carp’s filter feeding on algae promotes the shift toward phosphorus limitation in the water body, and in particular, filter feeding cyanobacteria may strengthen this trend. In conclusion, silver carp might not only reduce the density of algae through feeding pressure but also affect the growth of algae with high N﹕P excretion and redistribution of nutrients. The role of silver carp in the nutrient cycle of water body deserves more attention.