Diurnal and vertical grazing activity of mesozooplankton during summer on the SW coast of Finland

Diurnal and vertical grazing activity of the most dominant mesozooplankton species was studied during the summer 1987 in the coastal area of the SW Finland. The samples for gut fluorescence measurements (Chl-a + phaeopigments) were taken from five depths between the surface and 35 m depth four times a day. Chl-a concentration was higher in the upper water layer of 10 m than at 20 m depth, and higher in the daytime (1300 and 1900 hrs samplings) than by night (0100 and 0700 hrs samplings). The ambient Chl-a concentration explained 34% to 90% of the variation in the gut pigment contents of Eurytemora affinis, Bosmina longispina maritima and Synchaeta spp., when the whole data was examined using the linear regression analysis. In Acartia spp. this relation was insignificant. The gut pigment contents of copepods, B. longispina maritima and Synchaeta spp. were higher in the upper water layer (0 to 10 m) than in the deeper layer (20 to 35 m), but only by night. Diurnal variation was not found within either of the water layers or vertically between the layers in the daytime. The gut pigment content of the cladocerans Podon/Pleopsis spp. and Evadne nordmanni exceeded several times that of copepods and B.longispina maritima. The estimated daily specific ingestion rate (ingestion% of body carbon) varied from 2% to 12% in Acartia spp. and 3% to 28% in E. affinis, the highest percentages being found in the upper water layer. For Pseudocalanus minutus enlongatus, which was mostly found in the deeper depths, the corresponding percentage was only 4%. Carbon originating from other sources than phytoplankton was likely to be important for the nutrition of copepods.