Plankton Communities of Floodplain Lakes of Samarskaya Luka National Park (Rozhdestvensko-Shelekhmetskaya Floodplain, Samara Oblast)

Abstract

The changes that have occurred in Lake Shelekhmetskoe have affected abiotic indicators and its plankton community. As compared to 1999, water temperature and pH increased; the transparency decreased; and the chlorophyll a content and phytoplankton abundance and biomass became greater. Based on the water transparency and chlorophyll a content, it may be concluded that the lake is transformed from the eutrophic to the hypertrophic state, and from an oligoalkaline to a mesoalkaline type based on the active reaction of the environment. The number of phytoplankton has increased tenfold. The composition of the dominant complex of species of all components of the plankton community has changed: there has been a transition from a polydominant complex of phytoplankton to a monodominant one with the predominance of Planktothrix agardhii (Gomont 1892) Anagnostidis and Komarek 1988. Changes in the quantitative development of ciliates are less significant: their numbers increased by 1.2 times over a 25-year period. Changes in the composition of the dominant complex of ciliates, whose organisms attach to planktonic algae (ciliates of the genus Vorticella), began to play the leading role, and the role of tintinnids became much smaller. Changes also occurred in the trophic structure of ciliates, which were manifested in a decrease in the contribution of non-selective omnivorous ciliates to their total number and in an increase in the contribution of bacteriodetrirovorous. Differences in the plankton communities of the two lakes highlight the influence of morphometric features and hydrological regime on the structure of ecosystems. The high abundance and biomass of plankton in Lake B. Ostrovnoe are likely related to its smaller depth and more intensive water warming, which promotes the development of algae. Harsh dominance of P. agardhii in this reservoir is possibly related to higher nutrient concentrations and smaller competition by other species. The appearance of invasive species in both lakes indicates the ongoing invasion of alien species into the aquatic ecosystems of Samara Luka.