Aggregations of predators and prey affect predation impact of the Arctic ctenophore Mertensia ovum

The importance of gelatinous zooplankton in marine systems is increasingly recognized, but little is known about its role in the Arctic pelagic food web or about the way patchiness affects these ecological interactions. We studied the influence of the spatial patchiness of predators and prey on the predation impact of the Arctic ctenophore Mertensia ovum, using a combination of feeding experiments, gut content analyses and net sampling, together with aggregation estimations. A nonlinear functional response was detected for M. ovum feeding on Calanus spp. Ingestion rates at low prey densities were comparable with previous studies (1 +/- 0.2 prey predator(-1) h(-1)), but increased significantly at higher prey densities (6 +/- 2.5 prey predator(-1) h(-1)). We estimated that M. ovum is capable of consuming on average 1.4% d(-1) of the Calanus spp. population in the whole water column, or 33% in the upper 20 m layer, when assuming even distributions of prey and predators. Most importantly, ingestion rates did not significantly decline at high predator aggregations; hence, predation impact increased considerably when predator and prey aggregations were considered. However, feeding saturation was observed at high prey densities, suggesting that copepods may create a refuge by forming dense patches. These are significant consumption rates given that Calanus spp. comprises an important part of the Arctic marine food web. Studying the patterns of fine-scale distribution of these gelatinous predators should be emphasized in order to adequately model prey-predator interactions.