Determination of copepod grazing on natural plankton communities: correcting for trophic cascade effects

C Klaas,Pg Verity,S Schultes

doi:10.3354/meps07262

Abstract

Grazing rates and grazing selectivity of Antarctic copepods (Rhincalanus gigas, Calanus simillimus and copepods 0.8 to 2 mm in size) were determined using incubation experiments compar- ing plankton community development with and without added grazers. Estimates of grazing rates were corrected for trophic cascade effects using a simple ecosystem model composed of 5 compart- ments: (1) nanophytoplankton, (2) nanozooplankton, (3) microphytoplankton, (4) microzooplankton (mainly protozoans) and (5) copepods. Copepod grazing rates on the different planktonic compart- ments were derived by solving for model parameters as constrained by microscopy enumeration of nano- and microphytoplankton and nano- and microprotozooplankton in treatments with and with- out added copepods. Simulations based on different parameter values and model configurations sup- port the notion that grazing estimates were robust and between 18 and 30% higher than uncorrected estimates. Furthermore, model analyses suggest the presence of an additional trophic cascade effect due to grazing of R. gigas on small copepods and copepod developmental stages present in incuba- tion bottles.

Highlights

Estimating grazing rates of metazooplankton is key to understanding plankton dynamics in the ocean
The positive feedback of R. gigas on nanozooplankton and nanophytoplankton leads to a significant underestimate of copepod grazing on phytoplankton and zooplankton. These results indicate that grazing estimates are generally not sensitive to the number of model compartments as long as no positive feedbacks occur on species or groups within the compartments considered
The addition of R. gigas had a significant positive feedback on the nanophytoplankton and nanozooplankton, and grazing estimates were lower when these groups were not resolved by the model compartments

Summary

Introduction

Estimating grazing rates of metazooplankton is key to understanding plankton dynamics in the ocean. Several methods exist to estimate the effect of in situ metazooplankton grazing on natural plankton communities. The most common are measurements of respiration rates (O2 changes in closed bottles), the gut fluorescence method, and comparisons of incubations using natural communities with and without added metazooplankton. When used simultaneously, these different methods are known to yield widely differing results (Peterson et al 1990, Atkinson 1994, 1996, Zeldis 2001).

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