Timing of Ice Retreat Determines Summer State of Zooplankton Community in the Ob Estuary (the Kara Sea, Siberian Arctic)

In the estuaries of large Siberian rivers, ice coverage and the timing of ice retreat have varied in recent decades under the ongoing climate change. The seasonal development and functioning of the mesozooplankton community depend to a great extent on the timing of ice retreat. In the Arctic estuaries, the response of zooplankton to the timing of ice melt remains unclear. An earlier ice retreat was suggested to result in an advanced seasonal development of zooplankton, and higher biomass and feeding rates. Zooplankton composition, biomass, demography and grazing (assessed with the gut fluorescent approach) were studied in the Ob Estuary in July 2019 ("typical" ice retreat time). The obtained results were compared with the published data for July 2016 (ice retreat three weeks earlier). Zooplankton biomass in 2019 was considerably lower than in 2016, while species composition was similar; dominant populations were at an earlier stage of development. Herbivorous feeding of the dominant copepod, Limnocalanus macrurus, was also lower in 2019. The consequences of an earlier ice melt and increased temperature on seasonal population dynamics of the dominant brackish-water species are discussed. Our findings demonstrate that zooplankton communities in the Arctic estuaries are highly sensitive to the environmental changes associated with early sea-ice reduction.