Battoe, LE; Coveney, MF; Lowe, EF; Stites, DL
Lake Apopka is a large (125 km(2)), shallow ((Z) over bar = 1.6 m) lake in central Florida made hypereutrophic by 50 years of agricultural stormwater discharges from farms on 80 km(2) of drained littoral marshes. The lake is characterized by high nutrient levels, high turbidity caused by algae and resuspended sediments, and almost no remaining submersed or emergent macrophytic vegetation. Phosphorus loading to Lake Apopka is being reduced through the purchase of the riparian farms and restoration to aquatic habitat. Additional management activities to accelerate recovery of the lake are creation of a treatment wetland to remove nutrients and suspended solids from lake water, removal of gizzard shad(Dorosoma cepedianum), and replanting of littoral vegetation. Because of concerns that reduction in phosphorus (P) loading will be ineffective in restoration of Lake Apopka, we have reexamined the empirical and theoretical basis for P load reduction as a lake restoration technique.
Case studies of control of P loading show that a proportional improvement in lake trophic condition often is obtained when a significant reduction in P loading is effected. Restoration of hypereutrophic, shallow, turbid lakes requires reduction in P loading to lower the stability of phytoplankton dominance and increase the stability of macrophyte dominance. Several characteristics of Lake Apopka increase the probability that the lake will respond to reduction in P loading from adjacent farms. First, P loading (approx. 0.55 g P m(-2) yr(-1)) to the lake during the last 30 years has been elevated about seven-fold compared to prefarming levels, and P loading is dominated by farm discharges. Second, biogeochemical processes will dampen internal loading from P-rich sediments after external loading is reduced. The majority of P enters the lake as soluble reactive P. However, as a result of chemical and biological processes, almost 80% of the P in surficial sediments is in Ca-Mg-bound (33%) or organic (46%) forms resistant to rapid biological uptake.
Since summer 1995, trophic indicators (TP, TSS, Chi, Secchi depth) in Lake Apopka have improved significantly based on an Ii-year data set. These changes are consistent with modest reductions in P loading achieved since 1993 through regulatory actions. Recently, patches of submersed vegetation (e.g., Vallisneria, Chara) have established naturally at more than 20 sites around the lake.
Reddy, KR; O'Connor, GA; Schelske, CL
