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

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