Phytoplankton of the reservoir “Dospat” (Rodopi Mts, Bulgaria) as indicator of negative trend in reservoir development due to long-term cage fish farming

Abstract

The paper presents the results of the recent (2011) investigation of phytoplankton composition and abundance, with its seasonal and spatial dynamics in the reservoir “Dospat” (the first in Bulgaria used for cage fish farming) and shows the changes in its waters after 30 years of exploitation. Totally 55 taxa from 7 divisions have been identified: Cyanoprokaryota (8), Pyrrhophyta (3), Euglenophyta (4), Cryptophyta (1). Ochrophyta (Bacillariophyceae - 26; Synurophyceae - 2), Chlorophyta (9) and Streptophyta (2), Cyanoprokaryotes have been recorded for first time in the reservoir and their harmful species Planktothrix rubescens and Aphanziomenon flos-aquae were found among the dominants and subdominants. The highest number of species was detected in June (42), the phytoplankton communities near to the dam were more species-rich and with higher abundance in comparison with those in the tailwaters, most probably due to the effect of the fish cages situated in the uppermost part of the reservoir. According to the average phytoplankton biomass (1,39 mg l-1) in 2011 the reservoir “Dospat” could be classified as mesotrophic. However, it has to be noted that this value is twofold higher
in comparison with 1972-1975 period, when the biomass was 0.7 mg l-1. Most probably, the changes in the species composition, the increase in the phytoplankton biomass and of the reservoir trophic status are related with the effects from the cage fish farming.

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