Ingestion rates of phytoplankton by copepod size fractions on a bloom associated with an off-shelf front off NW Spain

Herbivory by copepods was studied from the coast towards theocean, during a bloom in May 1994 off NW Spain.Ingestion rateswere estimated by the gut chlorophyll content method in threesize fractions. The chlorophyll content displayed significantdaily cycles. Three different water bodies were described: coastal,shelf break and oceanic; the latter two zones separated by athermo haline frontal structure. Marked differences in planktonspecies composition, vertical distribution and biological rateswere found between zones. Theiiighest phytoplankton biomass,dominated by chain- forming diatoms, occurred in the oceaniczone associated with low primary production rates. Copepod feedinghad a low effect on oceanic phytoplankton; up to 0.2% of carbonstock and<3% of carbon production was consumed daily. Incontrast, medium-sized and large copepods removed 3% of carbonstock and 12% of primary production daily near the coast, wherephytoplankton were dominated by small flagellates in activegrowth. The highest variability in both plankton compositionand ingestionrates was found in the shelf-break zone, probablydue to displacements of the front. Copepods exerted a moderatepredation pressure on phytoplankton in coastal waters. Meanwhile,the impact of copepods on the offshore bloom was negligibleand the fate of the accumulated particulate carbon would bemostly determined by sedimentationand water dynamics.