Effects of food availability and predation on a marine zooplankton community - A study on copepods in the Baltic Sea

The importance of food resources versus predatory effects in determining th e abundance pattern of zooplankton varies over time and space. Here, we eva luate the population dynamics of dominant calanoid copepods (Acartia spp. b ifilosalA, longiremis and Eurytemora affinis) in a reference area and a eut rophied area in the northern Baltic Sea Proper. We base the paper on a comb ination of statistical analyses of a long term (12 years) data set on popul ation biomass dynamics, laboratory experiments on the feeding biology of th ese species and literature data of their susceptibility to predation. Overa ll, copepods were equally abundant at both sites, but Eurytemora was more a bundant in the eutrophied area while Acartia spp. dominated in the non-eutr ophied reference area. There was, however, no significant difference betwee n the two areas in biomass dynamics (biomass specific rate of change) of th e two groups of copepods. However, in spring and early summer the populatio n biomass increased faster for Eurytemora as compared to Acartia spp. This is consistent with our findings from feeding and respiration experiments. E urytemora exhibited generally higher ingestion rates and had a higher growt h efficiency (ratio of ingestion to respiration rates), which suggests an a dvantage over Acartia spp. when food availability increases. Both species g enerally showed a rapid population decline starting in late summer, most li kely caused by intensive predation by fish and mysid shrimps. In late autum n and winter, biomasses of both genera continued to decrease, but the decli ne was fastest for Eurytemora. During this period of the year, when the foo d availability is sparse, Acartia spp. may have an advantage over Eurytemor a, as we found that the former ingests a broader size range of food particl es and it has also been reported to be able to switch from filter feeding t o raptorial feeding. This study shows that dominant copepods in the net-zoo plankton community in our study area were structured both by food availabil ity and predation. The relative importance of these effects varied seasonal ly.