Plankton patchiness in the Polar Front region of the West Spitsbergen Shelf

Plankton patchiness, a widely known but poorly studied phenomenon, was investigated concurrently with measurements of environmental conditions in the Polar Front region west of Spitsbergen in summer (June-July) 2012. Continuous high-resolution observations were made using a laser optical plankton counter, a fluorometer and CTD along several sections in the surface 50 m of the sea with additional data from zooplankton net sampling and full-depth range hydrographical measurements at stations. The observed patches differed in terms of size, number and distribution patterns. They occupied only a minor portion of the studied ocean area (2-17%), but contained plankton concentrations that were 3-17 times higher than the concentrations in the background waters. The generation and existence of patches was associated with particular conditions created by increased primary production, optimal hydrography, discontinuities in the density field and/or the presence of meanders or eddies. The distribution of small size fraction patches, consisting of the phytoplankton, inorganic particles and detritus, was closely related to specific patterns in hydrography. The medium-size zooplankton and marine snow were concentrated mainly in conjunction with discontinuities in density field. The distribution of the large copepods was independent of any oceanographic feature detected in this study, which points towards biological cues as factors regulating zooplankton aggregation processes. Diverse mechanisms of patch formation as well as their ecological significance are discussed in the context of planktonic responses to the structured environment in the region of the Polar Front.