The restoration of Lake Apopka in relation to alternative stable states: an alternative view to that of Bachmann et al. (1999)

Bachmann et al. (1999) postulated that wind energy initiated, and has maint ained, high turbidity in hypertrophic (mean chlorophyll a = 92 mug l(-1)) L ake Apopka, Florida (mean depth = 1.6 m; area = 12 500 ha). They asserted t hat the turbid condition was initiated by a hurricane in late 1947 that des troyed submersed plant beds and that high turbidity has since been maintain ed by wind-driven resuspension of fluid sediments. In their view, there has been sufficient light for re-establishment of submersed plants over about 38% of the lake bottom, but plant growth has been precluded by the fluid ch aracter of the sediments. They concluded that the restoration program of th e St. Johns River Water Management District, which includes reduction of th e phosphorus (P) loading rate, will not restore water clarity or submersed vegetation. An alternative explanation for Lake Apopka's turbid state is th at it was initiated, and has been maintained, by excessive P loading that l ed to algal blooms and elimination of submersed vegetation through light li mitation. The transition to the turbid state was contemporaneous with drain age of 7300 ha of the floodplain wetland to create polders for farming, beg inning in the early 1940s. Lake P budgets indicate that drainage of the far ms caused a seven-fold increase in the P loading rate (0.08 g TP m(-2) yr(- 1) to 0.55 g TP m(-2) yr(-1)). Paleolimnological analysis of lake sediments also indicates an increase in the P loading rate in mid-century, concomita nt with the decline in submersed vegetation and the increase in phytoplankt on abundance. After the increase in P loading, wind disturbance may have ac celerated the transition to the turbid state; but, before the increase in P loading, wind disturbance was insufficient to elicit the turbid state, as evidenced by the stability of the clear-water state in the face of 14 hurri canes and 41 tropical storms from 1881 to 1946. Measurements of photosynthe tically active radiation (PAR) indicate that light limitation has inhibited submersed plant growth except on the shallowest 5% of the lake bottom. Fur ther, the correlation between the diffuse attentuation coefficient (K PAR) and chlorophyll a (CHLA) indicates that light limitation would be removed o ver about 82% of the lake bottom with a reduction in CHLA from 92 mug l(-1) to 25 mug l(-1). Recently, following a 40% reduction in the P loading rate , the mean total P (TP) concentration, mean CHLA, and total suspended solid s fell by about 30% while mean Secchi depth increased by more than 20%. Sub mersed plant beds appeared in areas devoid of macrophytes for nearly 50 yea rs. These improvements, during a period with no change in mean wind speeds measured at Lake Apopka, provide the strongest evidence that the turbid sta te has been maintained by excessive P loading and that the current restorat ion program, which combines P load reduction with planting and removal of p lanktivorous fish, will be effective.