Nutrient Fluxes and Their Impact on the Phytoplankton in a Shallow Coastal Lake

Data on nutrient concentrations and phytoplankton growth was analyzed in Lake Gardno, representing a separate group of coastal lakes within the Polish abiotic typology according to the Water Framework Directive. The aim of this work was to identify sources of nutrient variability in lake water and consequences of this variability for phytoplankton growth. Phytoplankton composition was dominated by green algae, present in high biovolumes throughout the year, and cyanobacteria, whose elevated biovolumes were noted at temperatures above 13 degrees C. Production of phytoplankton was often light-limited throughout the vegetation period (changing on a day-to-day basis). Low N-to-P ratio, the presence of nitrogen-fixing cyanobacteria, and Carlson's trophic state indices (TSI) analysis indicated also possible nitrogen limitations of primary production during the vegetation period. No phosphorus limitation was indicated. Possible nitrogen limitation was a result of in-lake modification of the N-to-P ratio compared to external N-to-P ratio in nutrient loads discharged by the freshwater inflow. Lake Gardno was a substantial sink of nitrogen discharged by the Lupawa River, probably due to denitrification. Nutrient budget for the 2006-08 period revealed 50% loss of nitrogen in a lake, whereas the phosphorus budget did not reveal substantial phosphorus retention. From a management perspective, the study indicates the importance of reduction of phosphorus loads discharged to the lake, which could prevent further development of cyanobacterial blooms stimulated by low N-to-P ratio.