Multi-year study of phytoplankton population in the Dnistrovskiy estuary in summer (2003-2018)

Abstract

Purpose. Comparison of multi-year data on species composition, biodiversity and quantitative variables of phytoplankton that developed in the Dnistrovskiy Estuary every summer. Particular attention was focused on the development of potentially dangerous (toxic) species, which can deteriorate quality of water and threaten hydrobionts’ existence. Methods. The data used in the phytoplankton study have been collected from 22 stations in the Dnistrovskiy Estuary (456 samples). Phytoplankton sampling and analyses were carried out using standard methodologies. Nageotta и Sedgewick cameras (0,05 and 1 ml) were applied to count cell number. The samples were preserved using formalin. HUND-H600 and OLIMPUS-BH2 microscopes were used for sample analyses. Classification of microalgae and Cyanobacteria was done according to the standards of the international database. Results. The total number of species registered during 16 years exceeded 445 (13 taxa) out of which 54 are generally classified as potentially harmful species (genera Dolichospermum, Aphanizomenon, Microcystis, Cyclotella, Pseudo-nitzschia, Peridinium, Prorocentrum etc.). Minimum record of species number within 4 main taxa was in 2004 and 2016. Maximum number of species of Chlorophyta (75) was observed in 2012, and maximum of Bacillariophyta (53), Dinophyta (19) and Cyanobacteria (33) was in 2014. Northern and middle part of the estuary had the highest species diversity (Shannon index = 4.6 – 4.4 bit∙cells-1) i.e. the areas where flora of various geneses mixed. Over the years, number of species and α-diversity of phytoplankton tend to increase in the north and in the centre of the estuary. The range of changes of maximum values of microalgae biomass during 16-year period was from 2.5 g·m–3 (2010) to 231.2 g·m–3 (2018). Algal blooms were recorded with Bacillariophyceae and Cyanobacteria as dominants. Conclusions. There is a certain 12-year periodicity in gradual changes in the number of species and Shannon index within the main phytoplankton taxa. Recent years’ trends toward increase in the total biomass of microalgae and Cyanobacteria have been traced as well, the consequence of such an increase being deterioration of water quality which has downgraded to "polluted" or "extremely polluted" in those regions (as per national standards).