The goal of this study was to characterize the seasonal changes and relationships between total and dissolved organic carbon (TOC, DOC) and nitrogen (N) and phosphorus (P) levels in the lakes Majcz Wielki (area – 163.5 ha, max. depth – 16.4 m) and Mikolajskie (area – 497.9 ha, max. depth – 25.9 m), located in the Mazurian Lake district of northeastern Poland. The lakes were varied in morphometric properties, degree of pollution, and watershed development. Limnological samples were collected seven times throughout 2007 in the surface and bottom water layers and in the metalimnion during summer stratification. Additionally, we determined the quality of organic matter by using SUVA (specific UV absorbance = Abs 260 * 1000/DOC) and lake productivity by using chlorophyll concentrations.

Results/Conclusions

Lake Majcz Wielki had lower TP concentrations (range 0.04-0.13 mg L^-1) than Lake Mikolajskie (range 0.10-0.29 mg L^-1).  In spring, TP in both lakes was dominated by organic P (approx. 80%), while at the end of summer stagnation dissolved orthophosphate (DRP) was 75% of TP in Lake Mikolajskie and 55% in Lake Majcz Wielki.  While mean TN concentrations were similar (1.4 mg L^-1) in both lakes, Lake Majcz Wielki had a wider range of TN concentrations throughout the study period.  High concentrations of ammonium N and DRP associated with low concentrations of oxygen in the bottom waters at the end of summer suggested that the source of nutrients was from the underlying sediments, with nutrient concentrations being highest in Lake Mikolajskie. The TN/TP ratio in Lake Majcz Wielki ranged from 13 to 41, while in Lake Mikolajskie the TP/TN ratio was 8-17.  These ratios indicated algal growth was probably limited by P in Lake Majcz Wielki and N in Lake Mikolajskie.  TOC, DOC and particulate organic C were about 20% higher in Lake Mikolajskie than in Lake Majcz Wielki throughout 2007.  In both lakes, DOC was highest in May, then decreased in the summer and increased again in fall whereas SUVA values exhibited the opposite seasonal trends.  According to Carlson’s (1977) trophic state index (TSI), both lakes are eutrophic when the index is based on TP, while Lake Majcz Wielki is mesotrophic and Lake Mikolajskie is moderately eutrophic when based on Secchi disc and chlorophyll.