Fine-scale distribution of zooplankton is linked to phytoplankton species composition and abundance in a North Norwegian fjord system

We studied the influence of dense layers of phytoplankton and aggregates on shaping the vertical distribution of zooplankton in a North Norwegian fjord using a Video Plankton Recorder (VPR). This instrument provided fine-scale vertical distribution (cm-m scale) of planktonic organisms as well as aggregates of marine snow in relation to environmental conditions.

At the height – later stage of the spring phytoplankton bloom in May, the outer part of the fjord was dominated by Phaeocystis pouchetii, while diatoms (Chaetoceros spp.) were dominating in the innermost basin. Small copepods species like Pseudocalanus spp., Microsetella norvegica, and Oithona spp. prevailed over larger copepod species in the inner part of the fjord whereas the outer part was dominated by large copepods like Calanus finmarchicus.

While the zooplankton where spread out over the water column during the early stage of the bloom, in May they were linked to the phytoplankton vertical distribution and in the winter situation they were found in deeper waters. Herbivorous zooplankton species were affected by phytoplankton species composition; C. finmarchicus and Pseudocalanus spp. avoided the dense layer of P. pouchetii while herbivorous zooplankton matched the distribution of the diatom-dominated bloom.

Small, omnivorous copepod species like Microsetella sp., Oithona sp. and Pseudocalanus sp. were often associated with dense layers of snow aggregates. This distribution may provide a shelter from predators as well as a food source. Natural or anthropogenic-induced changes in phytoplankton composition and aggregate distribution may thus influence food-web interactions.