Havens, KE; Bierman, VJ, Jr; Flaig, EG; Hanlon, C; James, RT; Jones, BL; Smith, VH
Lake Okeechobee is shallow eutrophic lake in an agricultural watershed of South Florida. It is one of the largest lakes in the USA and is among the world's shallowest great lakes. The South Florida Water Management District has monitored basin land use, nutrient loading and lake water quality since the early 1970s. The series of six papers in this issue, concluding with this synthesis, gives a detailed description of major trends for the period 1973-1992, and documents possible responses of the lake to agricultural nutrient inputs and recent external load reductions. Phosphorus loads (normalized for runoff) increased during the 1970s when basin agriculture intensified, and there were coincident increases in lake water phosphorus concentrations. Nitrogen also increased in the lake, coincident with marginal loading increases. After 1982, there were reductions in external nitrogen and phosphorus loads, and lake water nitrogen declined. Lake water phosphorus remained high due to internal loading from the sediments. As a result, N:P ratios declined from near 30:1 to below 20:1, and lake-wide nitrogen limitation became probable. These unnatural conditions favored nitrogen-fixing cyanobacteria, and they became the dominant phytoplankton. The lake-wide frequency of algal blooms (dominated by Anabaena circinalis) increased between 1980 and 1992, and in the late 1980s, particularly extensive surface blooms were documented. Phosphorus load reductions should be continued in order to reduce in-lake phosphorus concentrations, enhance N:P ratios and restore a natural phosphorus-limited state. Under those circumstances, cyanobacterial blooms are predicted to decline. Internal phosphorus loading will delay, however, any responses of Lake Okeechobee to external load reductions.