Original paper

Grazer control of nitrogen fixation: phytoplankton taxonomic composition and ecosystem functioning

Hambright, K.D.; Hairston, N.G.; Schaffner, W.R.; Howarth, R.W.

Abstract

By differentially manipulating external nutrient inputs and grazer assemblages (through the presence or absence of fish that fed preferentially on daphniids) in replicate experimental ponds, we explored the potential for grazers to affect phytoplankton composition, particularly the abundance of N-fixing cyanobacteria, the concentration of fixed N, and the extent to which these effects were influenced by nutrient loading. Results indicate marked effects of both grazing and nutrient loading on phytoplankton abundance and composition, N-fixing cyanobacteria presence and N-fixing heterocyst density. When we manipulated N:P using two different N loading regimes at high P loading, grazer and N:P loading effects on cyanobacteria were similar in magnitude. In contrast when N:P loading was kept high while P loading was varied, grazer effects on cyanobacteria were small relative to nutrient loading effects. Aggregate grazer effects appeared to result exclusively from direct removal of cyanobacteria by grazing. Indirect alteration of available N:P by zooplankton was minor in magnitude compared with external and internal nutrient loading. Ecological interactions that structure zooplankton assemblages, such as fish predation, can have substantial cascading effects not only on primary producer biomass, but also on phytoplankton taxonomic composition and ecosystem functioning.

Keywords

cladoceracopepodsgrazingcyanobacteriaexperimental pondsnitrogen to phosphorus ratio