Journal of Plankton Research Vol.25 no.8 pp.917-926, 2003
© Oxford University Press 2003
Is sedimentation of copepod faecal pellets determined by cyclopoids? Evidence from enclosed ecosystems
Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø and 1 Department of Fisheries and Marine Biology, University of Bergen, Hib, PO Box 7800, N-5020 Bergen, Norway
* Corresponding Author: camillas{at}nfh.uit.no
Vertical flux of faecal pellets was compared in 26 vertically stratified 27 m3 (diameter 2 m, depth 9.3 m) in situ seawater enclosures deriving from four separate experiments on the Norwegian west coast. Sediment traps were mounted in the non-mixed lower layer at 8 m depth. The zooplankton community composition was natural in three of the experiments, while manipulated to include four concentrations of Calanus finmarchicus in one. Calanoid copepods such as C. finmarchicus, Paracalanus spp., Pseudocalanus spp. and Microcalanus spp. dominated the zooplankton biomass in all mesocosms, except in eight of the enclosures where the cyclopoid copepod Oithona spp. occupied up to 40% of the biomass. Vertical flux of faecal pellet carbon (FPC) showed a significant negative correlation with Oithona biomass. In order to determine the retention potential of Oithona, measured sedimented faecal pellet carbon (FPCsed) was compared with estimated maximum and minimum egestion rates. FPCsed decreased with increased biomass of Oithona. When the contribution of Oithona to the total copepod biomass was high, FPCsed was reduced to a few per cent of the maximum calanoid egestion rate (Emax) and was significantly less than the expected minimum calanoid egestion rate (Emin) in four of the mesocosms. On the other hand, FPCsed increased towards Emax when the fraction of calanoid copepods increased towards 100% of the total copepod biomass. The results were obtained in experiments characterized by an extensive range of physical and biological processes. We suggest that the biomass ratio between pellet-producing (calanoids) and pellet-reworking copepods (Oithona) may be used to predict relative pellet retention and/or sedimentation rates of calanoid faecal pellets in natural plankton.
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