A patch of Calanus finmarchicus in the Lofoten-Vesterålen Region. Characteristics and determining factors

Abstract

Zooplankton patchiness has been documented in many shelf areas and is of vital importance for understanding predator-prey relationships in pelagic marine systems. By combining hydrographic, acoustic and net data collected in spring 2017, we present a detailed understanding of the extent of this phenomenon in the Lofoten-Vesterålen region. Such patches are of potential interest not only for zooplankton harvesting, but also for commercially harvested species such as Atlantic Cod, which are crucial for the region’s economy. We hereby report evidences of a large surface patch (>1000 km2) of Calanus finmarchicus extending longitudinally over the whole continental shelf. The aggregation was closely tied to the water mass distribution in the area together with the timing of the bloom and its associated ascent of the overwintering copepod population. Although most organisms concentrated at surface, species-specific depth preferences resulted in statistically definite communities at diverse depths. Despite the low chlorophyll a values, high nutrient concentrations proved optimum conditions for phytoplankton development and significantly matched with the zooplankton distribution. We thus hypothesize that top-down control via intensive grazing pressure plays a significant role in regulating both the bloom’s size and the related surface aggregation of zooplankton. This paper will assess the dimentions, morphology and composition together with possible physical and biological drivers of zooplankton patches to provide a highly comprehensive view of the distribution of this copepod, its aggregating behaviour and its ecological importance.