Phytoplankton as an indicator of water quality in the rivers of the Lake Ladoga basin and its relation to environmental factors

Abstract

A comparative analysis of the phytoplankton of 21 tributaries of Lake Ladoga and the Neva River has been made on the basis of samples taken in May, July, September of 2000-2004. Species composition, values of biomass and its structure have been used to assess trophic state and level of saprobity of the rivers. The relationships between environmental variables and phytoplankton biomass and community structure in the rivers have been examined. The phytoplankton species composition of 22 rivers was found to include 635 species, the most diverse being green algae and diatoms. The maximum total species number and number of diatoms and cyanophytes were recorded in the phytoplankton of large rivers. The average seasonal phytoplankton biomass varied from 0.2 to 4.9 mg l -1 . The most productive was the phytoplankton of the Volkhov, Burnaya and Morje rivers, which may be considered eutrophic. In the Svir', Neva and middle-sized rivers, the level of phytoplankton biomass corresponds to the mesotrophic type of water, while in the majority of small rivers, to the oligotrophic type. Based their biomass, centric diatoms dominated in large rivers, while cryptophytes and raphydophytes prevailed in middle-sized and small rivers. According to the calculated values of saprobity index (S = 1.2-2.5), the saprobity level of the rivers ranged from oligosaprobic to β-α-mesosaprobic. The lowest index values, indicating oligosaprobic zone, were characteristic for the northern and northeastern tributaries. The southern rivers (the Pasha, Syas, Lava, Nazya, and Volkhov), where, as a rule, S >2, may be considered β-mesosaprobic or moderately polluted. Total phytoplankton biomass was positively correlated with catchment area and negatively with water colour, but was not related to phosphorus content. Phytoplankton species composition was much more sensitive to phosphorus concentration than its total biomass. Canonical correspondence analysis showed that size of the river and its catchment area, percentage of lakes in catchment, and phosphorus were the most important variables accounting for the changes in community structure. The relationship between phytoplankton community composition and phosphorus was sufficiently strong to develop weighted-averaging regression-calibration model for inferring phosphorus concentration.