Diel periodicity of Subantarctic copepods: relationships between vertical migration, gut fullness and gut evacuation rate

A zooplankton community in the Polar Frontal Zone north of South Georgia was sampled for 5 days in February 1994. Feeding of various copepodite stages of six copepod species was assessed by a series of gut fluorescence/gut evacuation experiments. Feeding periodicity was compared to vertical distribution and migration patterns, as revealed by Longhurst Hardy Plankton Recorder (LHPR) and ring net catches. Despite chlorophyll a levels reaching only 0.8 mg m−3 and daily carbon rations based on phytoplankton intake being low, feeding of all the copepods was restricted mainly to the 8 h night-time perid. During the daytime, the epipelagic community was vertically dispersed within the top 100 m. At night, upward migration by most species led to a convergence of almost all zooplankters in the upper half of the surface mixed layer. However, large-amplitude vertical migrations which crossed the thermocline were performed only by Metridia lucens and Pleuromamma robusta. Although feeding by both migrants and non-migrants was mainly at night, there was no diel signal in gut evacuation rate. The gut evacuation rates of the nine copepod species and stages differed significantly (5-fold) and were negatively related to the extent of their diel vertical migration. The long gut-passage times of the migratory species, M.lucens and P.robusta, would have allowed them time to defaecate some of the food eaten near the surface at depth, contributing to an active carbon transfer out of the mixed layer. However, their scarcity at this site meant that their grazing comprised only -<1.4% of total copepod ingestion, so their combined gut flux is likely to have been negligible (-0.4 mg carbon day−1).

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