The structure of the mesoplankton community in the area of the continental slope of the St. Anna Trough (Kara Sea)

Abstract

Zooplankton samples and concomitant hydrophysical data have been obtained in the outer Kara shelf over the continental slope and adjacent deepwater region of the western spur of the St. Anna Trough in the last ten days of September in 2007 and 2011. Mesoplankton biomass in the examined regions in 2007, the warmest year of the last three decades, was 1.5–2 times higher than the relatively cold year of 2011. A frontal zone, distinct in temperature, salinity, and chlorophyll fluorescence in the surface sea layer was located over the continental slope. The temperature gradient in the frontal zone reached 0.25–0.67°C/km, and its salinity gradient reached 1.6–4.7 psu/km. An increase in mesoplankton biomass was associated with the frontal zone, which was especially pronounced in the upper layers of the water column. The average biomass content in the upper 50 m in the frontal maximum amounted to 1210 mg/m3 in 2007 and 972 mg/m3 in 2011, being two orders of magnitude higher than the outer shelf and the deepwater domain of the basin. The pteropod Limacina helicina was dominant at the slope maximum, accounting for up to 80% of mesoplankton biomass. The frontal zone over the slope also represented a distinct boundary separating the shelf mesoplankton community from the deepwater community, which drastically differed in composition and biomass.