Abundance, distribution and size structure of zooplankton along a 20° west meridional transect of the northeast Atlantic Ocean in July

Zooplankton distribution, abundance and size structure were characterised on leg 2 of the PRIME cruise in the northeast Atlantic during July 1996. Latitudinal sampling was carried out from 60°N to 37°N along a meridian at approximately 20°W. Zooplankton were characterised using an Optical Plankton Counter (OPC), and from traditional depth-integrated net hauls for taxonomy and carbon. The former method allows zooplankton to be described in terms of biovolume and size. There was good agreement between the two methods for abundance and a good correlation between OPC biovolume and carbon analysis. These data also showed compatibility with concurrent acoustic measurements of zooplankton. Results for the cruise compared to recent studies in the region, show that, overall abundance was generally low, although most of the previous studies were closer to spring bloom conditions. The size structure over latitude shows that mean size fluctuated, but there was a general trend of decreasing mean size from the north of the transect to about 47°N. Further statistical treatment of the OPC data, using principal component analysis, showed that zooplankton size structure changed at five positions along the transect. Three of these coincided with changes in physical structure, one each side of the front at 48°N and the third on the outer edge of an eddy. The other two occurred in regions of high-surface chlorophyll concentration.