Diel changes in the near-surface biomass of zooplankton and the carbon content of vertical migrants

Zooplankton biomass and the carbon content of vertical migrants were measur ed in the NE Atlantic (36.5 degreesN, 19.2 degreesW) between 11 and 18 July 1996 as part of the Plankton Reactivity in the Marine Environment (PRIME) programme. The increase in zooplankton biomass near the surface (0-100 m) a t night compared to during the day suggested that diel vertical migration w as an important feature at this site. For three species of vertically migra nt copepods, Pleuromamma pisekii, P. gracilis and P. abdominalis, the carbo n content of individuals collected at dusk was significantly less than for individuals collected at dawn, with this reduction being 6.2, 7.3 and 14.8% , respectively. This dawn-dusk reduction in carbon content is consistent wi th the diel pattern of feeding and fasting exhibited by vertical migrants a nd supports the suggestion that migrating zooplankton will cause an active export of carbon from the surface layers. (C) 2001 Elsevier Science Ltd. Al l rights reserved.