Lake eutrophication and plankton food webs

Abstract

The plankton food web comprises microscopic organisms that transport carbon and energy from producers to consumers in the water column of aquatic ecosystems. In lakes, the food web includes bacteria, phytoplankton, protozoa, micro- and macrozooplankton, and the trophic links between them. Structure of the plankton food web in lakes is affected in a predictable manner by eutrophication: phytoplankton biomass becomes relatively greater, while macrozooplankton biomass declines. The relative biomass of protozoa also may increase. Food web function also is affected: ecological transfer efficiency is lowest in ultraoligotrophic lakes and hypereutrophic lakes and highest in mesotrophic lakes. The relationship between the size of producers and grazers drives this unimodal pattern. In ultraoligotrophic lakes, cooccurrence of picoplankton with copepods that cannot directly graze such small particles results in long, energetically inefficient food webs with multiple steps including protozoa and microzooplankton. In hypereutrophic lakes, similar food webs occur, but owing to cooccurrence of cyanobacteria and small macrozooplankton that cannot handle those large and sometimes toxic phytoplankton. The dominance of large cyanobacteria in hypereutrophic lakes can be directly linked to enrichment with nutrients, particularly nitrogen and phosphorus, while the small size of zooplankton is attributed most often to intense fish predation. Long-term studies indicate that the changes in plankton food web structure and function can be reversed if nutrient loads are substantially reduced.