Distribution of periphytic diatoms in the rivers of the Lake Ladoga basin (Northwestern Russia)

Relationships between distribution of periphytic diatoms and environmental variables in 19 rivers of the Lake Ladoga basin (Northwestern Russia) were examined using gradient analysis. On the basis of geology and river water chemistry, the Lake Ladoga basin could be separated into twomain parts, the northern and the southern sub-basin. The rivers in the northern sub-basin are slightly acidic and low in conductivity (mean value 53 mS cm–1); the rivers in the southern sub-basin have neutral to slightly alkaline waters with higher conductivities (mean value 168 mS cm–1). A detrended correspondence analysis (DCA) defined two groups of rivers generally corresponding to the two main parts of the Lake Ladoga basin. Fragilaria capucina var. rumpens, Frustulia saxonica and Tabellaria flocculosa were the typical species for the northern sub-basin, whereas Cocconeis placentula var. euglypta, Ulnaria ulna and Gomphonema parvulum were characteristic species for the southern sub-basin. A canonical correspondence analysis (CCA) identified conductivity, pH, bicarbonate, total phosphorus and water colour as the most important environmental variables related to changes in assemblage structure. Both DCA and CCA ordination showed that conductivity related to geology was the most important variable, while concentration of total phosphorus was the second most important variable. Weighted averaging was used to infer total phosphorus from relative biomass of diatoms. The predictive ability of the inference model was sufficiently strong with r2 = 0.71 and RMSEP= 1.9 mg L–1. These results strongly support the use of a diatom-based inference phosphorus model for indicating eutrophication in the rivers of the Lake Ladoga basin.