Plankton Food Web Responses to Experimental Nutrient Additions in a Subtropical Lake

Karl E Havens,Therese L East

doi:10.1100/tsw.2006.176

Abstract

During two controlled enclosure experiments using water from a subtropical lake, the plankton food web displayed a highly variable response to combined addition of nitrogen and phosphorus. In July, the nutrients stimulated growth of Cylindrospermopsis raciborskii, and the biomass of macrozooplankton and microbial food web components did not increase. In October, the same addition of nutrients stimulated growth of small edible Lyngbya spp., and there were coincident increases in biomass of macrozooplankton and components of the microbial web. Past generalizations that cyanobacteria blooms inhibit growth of other food web components may not always hold true.

Highlights

More than 35 years ago, Gliwicz[1] postulated that biomass partitioning in the plankton is substantively different in highly eutrophic lakes than in their less-enriched counterparts
The results indicate that responses of subtropical plankton to nutrient enrichment can vary considerably, depending on genus-level responses in the cyanobacteria assemblage
When lakes experience increased nutrient enrichment and associated symptoms, including cyanobacterial blooms, observational data indicate a tendency for the biomass of plankton to become more focused in net phytoplankton[17], and for the ratio of zooplankton to phytoplankton biomass to decline due to a predominance of large inedible algae[18]

Summary

INTRODUCTION

More than 35 years ago, Gliwicz[1] postulated that biomass partitioning in the plankton is substantively different in highly eutrophic lakes than in their less-enriched counterparts He presented a conceptual diagram indicating that in eutrophic lakes, a considerable portion of phytoplankton biomass accrues in large inedible taxa, and that the major basal resource for higher trophic levels is bacteria. Empirical data support the view that with increasing eutrophication, there is an increase in the relative importance of microbial pathways, an increase in the number of links in food webs connecting basal resources with macrozooplankton, and reduced food web efficiency[4] This situation is acute in eutrophic subtropical lakes, where cooccurrence of small zooplankton and large phytoplankton may result in very low transfer efficiency in phytoplankton-based food webs and relatively greater carbon transfer via microbial webs[5,6,7]. This study expands on earlier observational work by experimentally testing the hypothesis that nutrient enrichment in a subtropical lake has predictable effects on plankton structure and function, including increased biomass of inedible net phytoplankton and a decline in macrozooplankton biomass

METHODS

Anabaena Diaptomus dorsalis

RESULTS

DISCUSSION