Copepod communities along an Atlantic Meridional Transect: Abundance, size structure, and grazing rates

Large-scale variability in copepod abundance, size structure, and ingestion rates on phytoplankton was investigated during the cruise Atlantic Meridional Transect-13. The main aim of the study was to assess the relative importance of small copepods and copepod nauplii in different regions (Temperate N and S, Oligotrophic N and S, Equatorial and Mauritanian upwelling). Samples were fractionated into four size fractions (<200, 200–500, 500–1000, and >1000 μm). The only factor that significantly affected copepod biomass was chlorophyll concentration, which explained 71% of the variation. The gut fluorescence technique was used to estimate ingestion rates and experiments were performed to obtain naupliar gut evacuation rates. We found a similar relationship between nauplii gut evacuation rates and temperature as that described by Dam and Peterson [1988. The effect of temperature on the gut clearance rate constant of planktonic copepods. Journal of Experimental Marine Biology and Ecology 123, 1–14] for larger copepods. Chlorophyll ingested daily by copepods was higher in regions affected by Mauritanian and Equatorial upwellings and the South Subtropical Convergence. Copepods were found to be major grazers of phytoplankton. Grazing impact upon primary production was more important for upwelling areas, with values higher than 100% of primary production at some stations. Even in oligotrophic gyres, where the relative importance of protists increases, copepods exert substantial feeding impact on their autotrophic prey. In oligotrophic gyres, small copepods and nauplii were relatively more abundant, and accounted for a higher amount of total chlorophyll ingestion than larger ones. Thus, studies with 200 μm mesh nets in oligotrophic areas are seriously underestimating nauplii and copepod abundance and grazing impact on phytoplankton.