Tg. Nielsen et al., STRUCTURE OF PLANKTON COMMUNITIES IN THE DOGGER BANK AREA (NORTH-SEA)DURING A STRATIFIED SITUATION, Marine ecology. Progress series, 95(1-2), 1993, pp. 115-131
Plankton dynamics and hydrography were investigated in the Dogger Bank
area, southern North Sea, in May 1990 after the onset of stratificati
on. Greatest phytoplankton biomass and production rates were observed
within a subsurface maximum around the pycnocline. At all 51 stations
investigated, fluorescence maxima were found at depths >15 m and often
at depths >30 m. Small flagellates (5 to 7 mum) dominated the phytopl
ankton. The vertical distributions of the smaller heterotrophs (bacter
ia, heterotrophic nanoflagellates and ciliates) were closely related t
o the fluorescence distribution. Carbon budgets established for the so
uthern North Sea suggest that, on and south of the Dogger Bank, ca 15
% of phytoplankton production was channelled directly into the larger
zooplankton (copepods), while north of the Dogger Bank, ca 30 % was in
gested by copepods. The production of the phytoplankton fraction >11 m
um (i.e. potential copepod prey items) could not alone account for the
daily carbon demand of the copepods. The carbon budgets suggest that
cihates could potentially have been of nutritional importance to the c
opepod population. Calculations showed that copepods possessed the abi
lity to clear a substantial fraction of ciliate production. The high b
iomass of bacteria, heterotrophic flagellates and ciliates, as well as
the fact that primary production of the larger (i.e. grazable) phytop
lankton did not appear to be sufficient to meet copepod energy require
ments, suggest the importance of the microbial loop in carbon cycling
in the North Sea during the stratified period.