Referring to data for orthophosphate phosphorus and soluble inorganic nitrogen consisting of NO31− and NH41+ ions, an attempt was made to estimate the trophic status and to follow up trophic changes over 15 to 30 year periods. The PO43−-P concentrations alone put most lakes permanently in the highest trophic state (hypereutrophic level) according to the Likens (1975) scale, set for total phosphorus and nitrogen concentrations. Only the Ezerets and Shabla lakes presented an exception in the 1963-1967 period. The nitrogen concentration covered the range from higher than oligomesotrophic to hypereutrophic levels. The average values of the N/P ratio of Dourankoulak and Bourgas lakes (the more shallow lakes) varied around the optimum, and the lakes have been tending to become slightly nitrogen limited, while Ezerets, Shabla and Mandra lakes (the deeper lakes) turned from phosphorus into clearly nitrogen limited lakes. The dominance changes in phytoplankton composition of Shabla lake corresponded to the variations in the N/P ratio. The NH41+ -nitrogen/ NO31− -nitrogen ratio shows higher values and a greater increase in the more shallow lakes than in the deeper lakes, which reveals the differences of eutrophication development in lakes of different depths.

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