| Abstract: | The chemical composition of watershed waters supplying 13 mesotrophic lakes (in N.E. Poland) including as the deepest lake L. Hańcza, z = 108.5 m (summer total phosphorus TP] content ≤0.050 mg · 1?1, chlorophyll a ≤5μg · 1?1, SD≥2.5 m) in a typical postglacial lake district (Suwalski Landscape Park) as well as surface and bottom waters of the lakes were studied in summer. Although the underestimated (i.e. including only surface runoff, river inflows and precipitation) yearly TP loading is equal to or higher than the permissible value, the lakes have maintained their mesotrophic features for 20 to 30 years. P sorption to the allochthonous inorganic material as well as decalcification processes in the lakes are probably responsible for this situation, as there is a strong difference between the chemical content of supplying waters and lake waters and as there is a considerable enrichment of P on sestonic particles. As a consequence of the low bio-availability of P, the midsummer amount of chlorophyll a is lower than predicted from the “TP—chlorophyll-a” relation found for harmoniously eutrophicating (i.e. P-limited) lakes. |
