Wind effect on zooplankton distribution in the estuary of the Pregolya River (the Baltic Sea basin) after technogenic transformation of its riverbed

In 2014–2018, large-scale hydrotechnical works were carried out in the estuary of the Pregolya River. The structural changes in the summer zooplankton in the river mouth in 2019 were revealed in comparison with the data obtained prior the riverbed transformation. In June 2019, zooplankton total abundance and biomass were of (136 ± 111) thousand ind.·m−3 and (860 ± 840) mg·m−3, respectively. It is comparable with mean annual data of 1996–2006: (71 ± 66) thousand ind.·m−3 and (664 ± 337) mg·m−3, respectively. In the summer of 2019, for the first time, the euryhaline species Eurytemora affinis inhabiting the Vistula Lagoon was recorded in the Novaya Pregolya branch. The presence of this species in the river branches, as well as the values of water salinity, may result from an increase in the frequency or intensity of water surges into the river from the Vistula Lagoon. In this regard, the wind conditions in 1998–2006 and 2011–2019 were analyzed. In 2011–2019, there was no increase in the frequency of winds acting along the effective surge direction (southwest and west ones) compared with those in 1996–2006. However, a rise in the frequency of storms was noted, inter alia in summer. Westerly storm winds cause upstream water inflow from the Vistula Lagoon and channel. Probably, the destruction of river macrophyte communities, concreting of embankments, and a change in the channel bottom configuration affected the intensity of water inflow from the lagoon into the river branches during surges and became the main factor affecting the distribution of euryhaline species from the lagoon in the river branches.