Does zooplankton grazing affect seston size structure and areal hypolimnetic oxygen depletion in lakes?
Sehallenberg, Marc; Burns, Carolyn W.
published: Nov 29, 1999
ArtNo. ESP141014701005, Price: 29.00 €
In four New Zealand lakes of differing trophic status, we examined the ability of zooplankton grazing to alter the size structure of the seston and thereby affect the rates of areal hypolimnetic oxygen depletion (AHOD). In general, the study lakes had relatively low abundances of Daphnia, reflecting a trend for many New Zealand lakes to be copepod dominated. Seston size-structure shifted towards larger particles as the proportion of copepods in the zooplankton community increased but Daphnia had no discernible effects on the seston size-structure. An empirical model, based on North American and European lakes, that predicts areal hypolimnetic oxygen depletion from chlorophyll-a, mean depth and euphotic depth was used as a null model to generate predictions of AHOD for the study lakes. Differences between observed and predicted AHOD were used to test the effects of zooplankton community structure and particle size-structure on AHOD. The empirical model predicted AHOD in the study lakes remarkably well and only 4 % of the variation in observed AHOD remained to be explained. Analysis of the residuals of the model's fit to the data indicated that neither seston size-structure nor effects of zooplankton grazing on seston sizestructure explained significant residual variation in AHOD among the lakes. The net effects of zooplankton grazing on seston size-structure and AHOD were confounded by apparent contrasting and scale-dependent zooplankton interactions. Among lakes, zooplankton biomass was tightly linked to trophic state and, hence, was also positively related to AHOD whereas, within lakes, zooplankton grazing was negatively related to AHOD, possibly as a result of the grazers affecting the sedimentation of autochthonous organic matter.