Response of Anabaena and Synechococcus to Manipulation of Nitrogen: Phosphorus Ratios in a Lake Fertilization Experiment

Additions of inorganic nitrogen and phosphorus to selected areas of oligotrophic Kennedy Lake began in 1978 and resulted in development of late-summer blooms of the nitrogen-fixing cyanobacteria Anabaena circinalis in 1981 and 1982. In subsequent years the blooms were successfully eliminated by increasing the molar N:P ratio of added nutrients to 35:1. The nutrients additions also resulted in dramatic increases in both autotrophic (predominantly Synechococcus sp.) and heterotrophic (bacteria) picoplankton numbers, with respective maxima of $4.0 !times 10^5 ml^-1$ and $>4.0 !times 10^6 ml^-1 !cdot$ Chlorophyll concentrations were generally $< 2 !mu liter^ 1$ during untreated conditions and increased to as much as $50 !mu liter^-1$ during blooms. In the years following the Anabaena blooms, hypolimnetic nitrate concentrations were significantly higher in the treated basin of the lake than they were in an adjacent untreated basin or than they were before the blooms. Development of nitrogen-fixing cyanobacteria blooms is dependent on both a low N:P supply ratio and a sufficient phosphorus supply.