Copepod grazing in the oceanic northeast Atlantic during a 6 week drifting station: the contribution of size classes and vertical migrants

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Abstract
Copepod grazing and abundance were measured during a 6 week time series at a Lagrangian drifting station in temperate waters of the NE Atlantic (46–50°N, 15–19°W) during spring 1990. Copepod assemblages, in the mesozooplankton size range, were fractionated into three size classes (200–500, 500–1000 and 1000–2000 μm) and dominant groups were analysed separately. Gut fluorescence was used to investigate did feeding and to estimate ingestion rates. Diel feeding cycles were relatively constant during the study. Copepods in the large and medium fractions (mainly Pleuromammaand Metridia spp.) displayed strong vertical migration, appearing in the surface 100 m only at night. A trend of increasing night-time gut pigment content was observed for these size fractions which, together with estimates of evacuation rate constant and defaecation rate, suggested that these migrants may transport most of the food consumed in the surface layer to depth. Ingestion rates were higher during the first half of the sampling period (1–18 May), in the large and medium fractions, while those of the small fraction were, on average, constant throughout the study (1 May– 15 June). Variations in consumption rates were mainly caused by changes in copepod abundance. The small copepod fraction usually made the greatest contribution to the total grazing pressure on phytoptankton. However, copepod grazing on average represented only <10% of the daily primary production. These results are discussed in the context of comparable estimates of grazing pressure in other marine systems together with aspects that demand further analysis in order to increase the accuracy of such estimates.

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