LIPID BIOMARKERS - LINKING THE UTILIZATION OF FRONTAL PLANKTON BIOMASS TO ENHANCED CONDITION OF JUVENILE NORTH-SEA COD

The effects of physical mixing processes on phytoplankton production i n the marine environment are well established. However, the effects of these processes on growth and condition of zooplankton and larval fis h are at present poorly understood. In this study, we utilized phytopl ankton group-specific fatty acid content to trace the phytoplankton gr oup and mixing regime contributing to the condition of individual juve nile North Sea cod. In order to establish a relationship between lipid tracer content and algal utilization, post yolk-sac larval North Sea cod were reared in the laboratory on food chains based on monocultures of either the diatom Skeletonema costatum or the dinoflagellate Heter ocapsa triquetra (algae dominating in the mixed and stratified regions of the North Sea). In the laboratory, these algae were fed to culture s of adult Acartia tonsa, the copepod eggs were collected, hatched and the N1 nauplii from these different feeding regimes fed to post yolk- sac larval North Sea cod. Post yolk-sac larval cod required 8 d on eit her a Heterocapsa- or Skeletonema-based food chain before tracer Lipid signals (the ratio of the lipids 16:1 omega 7 to 16:0) in the larvae began to change from their original values to those similar to the alg ae at the base of their respective food chains. The cod larvae display ed a lipid tracer content similar to that of their algal food source a fter 13 d on their respective feeding regimes. During a cruise in May 1992 to examine the distribution of larval and juvenile North Sea cod, a subsample of 100 juvenile cod from the stratified, mixed and fronta l regimes of the northeastern North Sea were examined for their conten t of lipid biomarkers and condition (as determined by the ratio of tot al lipid content to total length). Juvenile cod displaying a lipid tra cer content indicating utilization of diatom-basea food webs (found in proximity to regions of frontal mixing) were observed to be in signif icantly better condition (p less than or equal to 0.05) than those con taining a lipid signal indicative of utilization of flagellate-based f ood webs (found in stratified regions of the North Sea).