Plankton community structure and carbon cycling on the western coast of Greenland during the stratified summer situation. III. Mesozooplankton

Abstract

The mesozooplankton comn~unity in Disko Bay, West Greenland, was examined by stratified sampling with a WP-2 net (200 pm), as well as a Hensen (45 pm) net, during the period of stratification in late summer 1994 Copepod biomass in the upper 50 m decl~ned over the interval from 20 mg C m-3 to virtually nothing due to the downward seasonal migration by Calanus spp. The ontogenetic vertical distribution of larger species, Calanus spp., revealed that the juvenile stages predominantly were associated with upper water strata and remained there until September. The decline in Calanus dominance was followed by increasing abundance of smaller copepods (Pseudocalanus, Oithona and Acartia) and of meroplanktonic larvae, a general restructuring of the grazer food chain in the euphotic zone. The large Calanus spp. had finished their spawning season before late August, while small copepods produced eggs throughout late August to mid-September. According to a statistical path analysis the specific egg production of Acartia longiremis was associated primarily with phytoplankton biomass and secondarily with protozooplankton biomass. Daily egg production per female declined gradually from 10 d-' on August 27 to 1.6 d-I on September 27. The biomass of A. longiremis was rather modest, indicating that it was not a dominant grazer in the euphotic zone. Weight specific egg production of A. longiremis declined from 0.11 to 0.02 d-', and was used as an index for secondary production; grazing by mesozooplankton was subsequently estimated as 3 X secondary production. Grazing impact was thereby roughly calculated for the entire copepod standing stock and compared with data from parallel studies on phytoplankton, bacteria and protozooplankton into a comprehensive pelagic carbon budget for the euphotic zone, the upper 30 m. The estimated total grazlng upon phytoplankton was 3 times the primary production. It IS proposed that phytoplankton was primarily grazed by ciliates and secondarily by ciliated meroplankton and copepods. Additionally, a large fraction of the ciliate production was presumably grazed by heterotrophic dinoflagellates.The protozooplankton were key grazers in the pelagic carbon cycling of Disko Bay.