Grazing impact of Oikopleuradioica and copepods on an autumn plankton community

Abstract

Copepods and appendicularians are major grazers in the pelagic environment. They have different retention efficiencies for prey and may therefore exert a variable grazing pressure on the spectrum of pico- to micro-plankton. We determined clearance rates of both groups at one station during 24 h in the Gullmar fjord, west Sweden, in autumn 1999. Total potential prey biomass ranged from 75 µg C l⁻¹ at the surface to 14 µg C l⁻¹ at 30 m with a dominance of larger dinoflagellates (10–25 µm athecate species and Gymnodinium /Gyrodinium sp.) and the pennate diatom Pseudo-nitzschia sp. Grazer biomass was dominated by copepods (Acartia clausi, Paracalanus parvus) and appendicularians (Oikopleura dioica). O. dioica showed non-selective clearance rates of 0.7–1.8 ml µg C⁻¹ h⁻¹ on most diatoms, flagellates and ciliates, whereas Pseudo-nitzschia sp. and dinoflagellates and ciliates >25 µm were not removed by O. dioica. Appendicularian grazing impact was 0.06% d⁻¹ on the phytoplankton and 0.4% d⁻¹ on bacterial biomass. Despite a seven-fold higher biomass, the grazing impact of copepods on phytoplankton biomass was only 0.28% d⁻¹ indicating that O. dioica had a proportionally greater impact and, in contrast to copepods, also utilised bacteria. The low observed grazing impact was due to a low grazer biomass and a prey community largely unavailable to the investigated grazers.

Keywords:

• Grazing
• Oikopleura dioica
• copepods

This study was supported by EC-TMR Large Scale Facility program (contract ERB FMGE CT980119) to Kristineberg Marine Research Station and EC-MAST3 project KEYCOP (MAS3-CT97-0148). T.G.N. was supported by the Danish National Research Council, P.T. and K.T. by the Carl Trygger Foundation, A. S. by a post-doc fellowship provided by Munkedals Bruk and C.V. by the Swedish Institute.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Additional information

Notes on contributors

Cristian Vargas*

Permanent address: Department of Oceanography, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile