Phytoplankton of Estonian rivers in midsummer

Abstract

The material for the present phytoplankton study was collected from 427 reaches of 162 Estonian rivers during the period of 1991–1997. Sampling was carried out at the end of June and in July. A total of 402 taxa of algae were identified in phytoplankton. The most numerous were diatoms (180), followed by green algae (125) and cyanobacteria (45). The phytoplankton of the Estonian rivers is sparse and heterogenous: only 13 species of algae were recorded in more than 50% of streams. Pennate diatoms dominate most frequently in phytoplankton biomass, while tiny flagellates dominate in phytoplankton abundance. The nonparametric means (medians) of phytoplankton biomass, abundance and number of taxa per sample in the lower reaches of the Estonian streams were 0.2 mg l−1, 0.4×106 cells l−1 and 19, respectively; the highest mean values (0.3 mg l−1, 0.6×106 cells l−1 and 23, respectively) were observed in the rivers of the drainage basin of Peipsi-Vortsjarv and the lowest ones (0.1 mg l−1, 0.2×106 cells l−1 and 13, respectively), in the streams of Hiiumaa. Phytoplankton biomass was correlated with content of chlorophyll a according to Lorenzen (L chl a) (Spearman r=0.72) and Jeffrey & Humphrey (JH chl a) (Spearman r=0.70) at the confidence level P<0.001. The Spearman correlation coefficients between phytoplankton abundance, and L chl a and JH chl a were 0.63 (P<0.001) and 0.64 (P<0.001), respectively. A low correlation was observed between phytoplankton biomass and BOD5 (Spearman r=0.43; P<0.001). Phytoplankton parameters were not significantly related to flow velocity, water temperature, pH, dissolved oxygen and chemical properties of water. Estonian streams are short. In many places, river banks are bordered with trees and bushes, and nutrients cannot be assimilated by phytoplankton due to deficiency of light. Only two rivers – the Vaike Emajogi and the Elva with their tributaries – are relatively rich in phytoplankton.