High chlorophyll a concentration in a low nutrient context: discussions in a subtropical lake dominated by Cyanobacteria

Temporal variability in some water quality parameters can play an important role in determining the presence and abundance of primary producers, and consequently in the trophic state and other characteristics and uses of lake ecosystems. In this sense, the present study aimed at understanding temporal dynamics of some trophic relevant water quality parameters in different time scales and their correlation and influence in phytoplankton biomass (chlorophyll a) in a shallow subtropical coastal lake. Peri Lake is located in Florianópolis island in Southern Brazil and samples were taken monthly between March 2007 and February 2013. The lake showed low dissolved nutrients concentration, especially phosphorus (P) (median dissolved P: 2.0 µg.l^-1)  and high chlorophyll a (median: 20.8 µg.l^-1) concentration. Total nitrogen (TN) concentration varied broadly, with a median of 672.8 µg L^-1, and total P (TP) concentration was low (median: 13.5 µg L^-1). A seasonal pattern of variation concerning dissolved and total P and chlorophyll a concentration was observed, associated mainly with temperature and wind speeds, but no clear pattern was observed for nitrogen (N) fractions. Significant differences were observed in different years for some parameters, with higher chlorophyll a and lower N concentration in the last three years sampled. The lake was considered potentially P limited during the majority of the study period and a positive correlation was found between chlorophyll a and total and dissolved P concentration. Phytoplankton biomass (as chlorophyll a) was apparently controlled by water temperature and P availability (TN:TP ratio and dissolved P). Water transparency (as Secchi depth) was strongly and negatively influenced by chlorophyll a concentration. Cylindrospermopsis raciborskii abilities to compete for P and light seem to be important factors determining its success and dominance in this low P coastal ecosystem. The fluctuating P supply, probably associated to sediment resuspension by wind in this shallow waterbody, is an advantageous factor for cyanobacteria and has an important role in chlorophyll a dynamics. Thus, high chlorophyll a concentration in this subtropical lake seems to be related to the P-limited condition, shallowness and low water column transparency, which are probably favouring the dominance of C. raciborskii, especially in higher summer temperatures, and leading to high chlorophyll a concentration even in a low dissolved nutrient environment.