Linking environmental factors to macrozooplankton size structure in summer: Insights from an Arctic shelf.

The distribution, abundance, and size structure of macrozooplankton populations in eight common taxa (Themisto libellula, Thysanoessa inermis, Limacina helicina, Aeginopsis laurentii, Aglantha digitale, Dimophyes arctica, Mertensia ovum, Parasagitta elegans) were studied in the Barents Sea in summer 2013. Four water masses were delineated based on hydrological characteristics: Murmansk Coastal Water-MCW, Atlantic Water-AW, Arctic Water-ARW, Barents Sea Water-BSW. Total abundance of macrozooplankton taxa varied between 24 and 732 individuals m. Maximum abundances were recorded in the southern, central and eastern parts of the study region. Populations of the most common macrozooplankton species were dominated by young specimens indicating their spawning in the late spring and summer seasons. Normalised abundance size spectra (NASS) and normalised biomass size spectra (NBSS) were constructed for the whole study area and for each water mass. Linear regressions for NBSS were insignificant except for ARW and the combined dataset, while NASS parameters (slopes and intercepts) differed significantly between the four water masses. NASS slopes showed spatial variability and ranged from -0.81 to -1.05. NASS slopes were significantly steeper in ARW and AW. Relatively flatter slopes in BSW and MCW suggest that macrozooplankton productivity and trophic efficiency were higher in the regions with interacting water masses. Most of the macrozooplankton had a size of 1-4 mm for the entire study area with Mertensia ovum, Aeginopsis laurentii, and Dimophyes arctica being the most numerous. Redundancy analysis and Pearson's correlations revealed latitude/water temperature, bottom depth, and mesozooplankton biomass as the most important factors controlling macrozooplankton size variability during the study period. Our study improves our understanding of marine macrozooplankton assemblages and provides useful information for future research focused on pelagic communities during the period of warming and related ecosystem shifts in the Arctic.