Freshwater lake ecosystem health assessment and its response to pollution stresses based on planktonic index of biotic integrity.

Based on the planktonic data monitored in Lake Balihe, an ecosystem health assessment system referring to planktonic index of biotic integrity (P-IBI) was constructed to evaluate the lake health status and study the response mechanism of P-IBI to environmental factors. The results showed that a total of 7 phylum 59 species of phytoplankton and 3 phylum 28 species of zooplankton were identified in the lake, and both the plankton density and biomass varied significantly in time and space scales. The significant variation of Protozoa density should be responsible to the inconsistency between the distributions of zooplankton density and biomass, as well as the lowest value of P-IBI in summer. The P-IBI values and therefore the health levels can be seasonally ranked as winter > autumn > spring > summer and found spatially increased along the flow direction. Based on the relationships between P-IBI and the environmental factors, ammonia nitrogen = 0.46 mg/L and Secchi depth = 63 cm were found as the environmental protection thresholds of planktonic biotic integrity for this freshwater lake ecosystem. The findings of the research may provide some guidance to the ecological monitoring and protection of freshwater lake.