The phosphorus mass balance of lake okeechobee, Florida: Implications for eutrophication management

Abstract

A phosphorus (P) mass balance was developed for Lake Okeechobee, Florida (USA) for the period 1973 to 2002. During those 30 years, yearly P inputs averaged 499 metric tons, outputs averaged 174 metric tons, and there was a 297 metric ton ·y−1 net sink, attributed to sediment accrual. Yearly P loads were highly variable, ranging from <200 to >900 metric tons, and there was a strong positive relationship between yearly load and inflow volume (r2 = 0.73). There was no long-term trend in P loading to the lake, but the total P concentration of inflow water displayed a historic trend that was independent of inflow volume, perhaps related to P control programs in the watershed. Inflow total P increased from 150 to >200 μg·L−1 in the 1970s when land use was intensified but management programs generally were absent, but declined in the 1980s and 1990s (to ~150 μg·L−1), coincident with various P control programs. Inflow total P concentrations displayed no trend after the early 1990s, although there was considerable year-to-year variability. Lake water total P concentrations increased during the 1970s, stabilized in the 1980s and early 1990s, and increased in recent years, possibly due to sediment saturation with P. There was no significant relationship between lake water total P concentration and mean depth, contradicting earlier reports based on shorter (10 and 20-y) periods of record. The difference between annual average inflow and lake water total P declined from near 150 μg·L−1 in the 1970s to <50 μg·L−1 in recent years, and the net sedimentation coefficient declined from near 2.0 to just above zero. These trends indicate a decreasing capacity of the lake to assimilate P. Two features noted here–(1) potential of the watershed to export large amounts of P rich water in certain years, and (2) lack of P assimilative capacity of the lake–indicate that the ecosystem is at risk for continued and perhaps worsened eutrophication symptoms under current P loading conditions. © 2005 Taylor & Francis Group, LLC.