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

摘要

Bachmann et al. (1999) postulated that wind energy initiated, and has maintained, high turbidity in hypertrophic (mean cholrophyll #alpha#=92 mug l~(-1) Lake Apopka, Florida (mean depth =1.6 m; area=12 500 ha). They asserted that the turbid condition was initiated by a hurricane in late 1947 that destroyed subemersed plant beds an that high turbidity has since been maintained by wind-driven resuspension of fluid sediments. In their view, there has been sufficient light for re-estabilshment of submmersed plants over about 38% of the lake bottom, but plant growth has been precluded by the fluid character of the sediments. They concluded that the restoration program of the St. Johns River Water Management District, which includes reduction of the phosphorus (P) loading rate, will not restore water clarity or submersed vegetation. An alternative explanation for Lake Apopka's turbid state is that is was initiated, and has been maintained, by excessive P loading that led to algal blooms and elimination of submersed vegetation through light limitation. The transition to the turbid state was contemporaneous with drainage of 7300 ha of the floodplain wetland to create polders for farming, beginning in the early 1940s. lake P budgets indicate that drainage of the farms caused a seven-fold increasw 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 and increase in the P loading rate in mid-century, concomitant with the decline in submersed vegetation and the increase in phytoplankton abundance. After the increase in P loading, wind disturbance may have accelerated 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 hurricanes and 41 tropical storms from 1881 to 1946. Measurements of photosynthetically active radiation (PAR) indicate that light limitation has inhibited submersed plant growth except on the shallowest 5% of the lake bottom. Furtherm the correlation between the diffuse attentuation coefficient (K_PAR) and chlorophyll a (CHLA) indicates that light limitation would be removed over about 82% of the lake bottom with a reduction in CHLA from 92 #mug l~(-1). Recently, following a 40% reduction in the P loading rate, the mean total P (TP) concentration, mean CHLA, and total suspended solids fell by about 30% while mean Secchi depth increased by more than 20%. Submersed plant beds appeared in areas devoid of macrophytes for nearly 50 years. These improvements, during a period with no change in mean wind speeds measured at Lake Apopka, provide the strongest evidence that the turbid state has been maintained by excessive P loading and that the current restoration program, which combines P load reduction with planting and removal of plaktivorous fish, will be effective.