LAUSR

Abstract Estuaries of large Siberian rivers, Yenisei and Ob, play a pivotal role in transformation and sedimentation of the suspended matter from the riverine runoff. The mesozooplankton community is one of the key players in estuarine functioning. Most of the studies in the Yenisei region were performed at the end of the productive season (August–September) and concentrated on the structure of zooplankton community. The main goal of this study was to characterize structure and grazing of the zooplankton community in the Yenisei estuarine and plume-affected shelf region in early summer soon after the discharge maximum. More specifically, we assessed zooplankton biomass and analyzed the distribution patterns of the dominant species in July; assessed herbivorous feeding rates; and grazing pressure of these species during the period of high phytoplankton production. Zooplankton were collected and feeding rates were studied along a quasi-latitudinal transect in the Yenisei Estuary and the adjacent shelf in July 2016. Freshwater zone (FWZ), frontal zone of the Yenisei plume (YFZ) and shelf zone outside the plume (MSZ) were distinguished according to salinity and temperature. Highest biomass (12–101 mg DW m−3) was recorded in the YFZ. Several specific aspects of distribution and grazing were distinguished. Cladocera were highly abundant in the FWZ and grazed up to 14% of phytoplankton there. Species which dominated zooplankton in the YFZ avoided the upper freshened layer, and maximum abundance of both marine and brackish species coincided with the gradient layer. The brackish Limnocalanus macrurus penetrated far into the north where this species was supposedly exported earlier with the maximal runoff. Low ingestion (0.1–0.9% body carbon) and demographic structure of L. macrurus suggest termination of seasonal development of the population. Herbivorous feeding of the marine copepods exceeded their metabolic demands in the estuarine zone and was insufficient in the marine shelf zone. The highest daily grazing pressure in the YFZ (up to 21% phytoplankton biomass) was exerted by Calanus glacialis. Analysis of our results together with the published data implies that timing of ice retreat is among the main factors controlling seasonal development of the dominant zooplankton populations.