Emilia Trudnowska

Visual prey availability and distribution of foraging little auks (Alle alle) in the shelf waters of West Spitsbergen

As diving seabirds use vision underwater, it is presumed they should preferentially select sites where their preferred food items are not only abundant but also clearly visible. To test this, we studied the optical properties of the seawater in the West Spitsbergen Shelf, in combination with zooplankton abundance in the feeding grounds of the planktivorous little auks from the nearby colonies in Hornsund. We estimated the relative attractiveness of the foraging sites using a novel parameter—visual prey availability (VPAv), which relates density and proportion of the preferred food item (Calanus glacialis) of the little auk, in total zooplankton, to the optical properties of the seawater. We found a significant positive correlation between the density of foraging little auks and VPAv values. Birds chose areas where C. glacialis was both abundant and clearly visible, because of the clarity of the water and low proportion of other zooplankton species. The birds avoided foraging over the warmer Atlantic-type waters, characterised by a high abundance of zooplankton taxa mostly ignored by birds and where VPAv values were low. VPAv values could potentially also be applied to other visual planktivores for which prey preference and visual acuity are known.

Variations in the structural and functional diversity of zooplankton over vertical and horizontal environmental gradients en route to the Arctic Ocean through the Fram Strait

A multi-scale approach was used to evaluate which spatial gradient of environmental variability is the most important in structuring zooplankton diversity in the West Spitsbergen Current (WSC). The WSC is the main conveyor of warm and biologically rich Atlantic water to the Arctic Ocean through the Fram Strait. The data set included 85 stratified vertical zooplankton samples (obtained from depths up to 1000 metres) covering two latitudinal sections (76°30’N and 79°N) located across the multi-path WSC system. The results indicate that the most important environmental variables shaping the zooplankton structural and functional diversity and standing stock variability are those associated with depth, whereas variables acting in the horizontal dimension are of lesser importance. Multivariate analysis of the zooplankton assemblages, together with different univariate descriptors of zooplankton diversity, clearly illustrated the segregation of zooplankton taxa in the vertical plane. The epipelagic zone (upper 200 m) hosted plentiful, Oithona similis-dominated assemblages with a high proportion of filter-feeding zooplankton. Although total zooplankton abundance declined in the mesopelagic zone (200–1000 m), zooplankton assemblages in that zone were more diverse and more evenly distributed, with high contributions from both herbivorous and carnivorous taxa. The vertical distribution of integrated biomass (mg DW m-2) indicated that the total zooplankton biomass in the epipelagic and mesopelagic zones was comparable. Environmental gradients acting in the horizontal plane, such as the ones associated with different ice cover and timing of the spring bloom, were reflected in the latitudinal variability in protist community structure and probably caused differences in succession in the zooplankton community. High abundances of Calanus finmarchicus in the WSC core branch suggest the existence of mechanisms advantageous for higher productivity or/and responsible for physical concentration of zooplankton. Our results indicate that regional hydrography plays a primary role in shaping zooplankton variability in the WSC on the way to the Arctic Ocean, with additional effects caused by biological factors related to seasonality in pelagic ecosystem development, resulting in regional differences in food availability or biological production between the continental slope and the deep ocean regions.

Large versus small zooplankton in relation to temperature in the Arctic shelf region

ABSTRACT Climate change results in the alteration of the size structure of plankton, which consequently may affect higher trophic levels, such as planktivorous seabirds. In this study Laser Optical Plankton Counter measurements were performed over seven summer seasons (2010–2016) to test the ratio of large versus small zooplankton in relation to environmental conditions. Investigated transects were repeated during the same time of the year (July/August) in different zones of the West Spitsbergen Shelf crossing the Arctic front. The plankton particles were grouped into two size fractions: “Calanus”, potentially consisting of a majority of the high-energetic, older life stages of the preferred prey for little auk (Alle alle) and the “small” fraction including less preferred items. The vertical availability of the Calanus fraction was tested on the background of usually abundant smaller zooplankton, which may hinder the detection of larger zooplankters by little auk. Larger zooplankton were found closer to the coast, in the upper 20-m depth layer in years characterized by significantly lower mean temperatures. Potential availability of prey for the little auk thus could be higher in colder years than in warmer years. Additionally, our study indicated the tendency of the small plankton fraction to concentrate near the locations of the highest chlorophyll fluorescence, in the 20–30-m water layer. The high spatial and temporal resolution of the data indicated a variation in the proportion of large versus small zooplankton, and thus in the availability of Calanus to little auk with respect to temperature.