How anthropogenic darkening of Lake Apopka induced benthic light limitation and forced the shift from macrophyte to phytoplankton dominance

Controversy exists about the historic shift of primary producer community structure (PPCS) in Lake Apopka, a shallow, 125-km(2) lake in central Florida, U. S. A. The controversial questions are: was a lake-wide shift from macrophyte to phytoplankton dominance triggered by a 1947 hurricane or tornadoes spawned by the hurricane within a few weeks or months, or was the shift forced by progressive cultural eutrophication over several years? In 1947, fringing wetlands (35 km(2)) were being diked and drained for agricultural purposes changing their function from nutrient sinks to nutrient point sources to the lake. We examined whether anthropogenic darkening of the water column induced the PPCS shift during a 0.6-m lake-stage rise. Darkening was defined as lake-bottom photosynthetically available radiation (PAR) <= 1.0% of incident PAR. We simulated darkening from changes in the light extinction coefficient (K-d) due to water color, chlorophyll a, and rising lake stage. Simulations predicted that darkening was restricted to 80% of the lake deeper than 2.16 m, a finding consistent with persistence of shallow-water submersed macrophytes after 1947. Anthropogenic wetland draining increased water color, lake stage, and phytoplankton blooms stimulated by phosphorus enrichment, causing a progressive shift to lake-wide phytoplankton dominance over at least several years. The relationship between the 1947 hurricane and the PPCS shift was not causal, but only a temporal coincidence. Our darkening model can be applied to other systems with known bathymetry to predict submersed aquatic vegetation (SAV) coverage from SAV light requirements and water-column light attenuation.