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

Abstract

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.

Impact of water depth and sediment type on root morphology of the submerged plantVallisneria natans

“…Generally, light availability can be severely attenuated by water depth, which is the main factor restricting the growth of submerged plants (Voesenek et al 2006). The decrease of light availability in water is also considered to be the main reason for the extinction of submerged plants (Schelske et al 2010). Light attenuation in water is affected by not only the absorption of water itself but also the absorption, reflection and scattering of suspended matter, which is related to water transparency.…”

Stable isotope (δ13C and δ15N) values of sediment organic matter in subtropical lakes of different trophic status

“…Once a clear-water, macrophyte-dominated lake, it changed in 1947 to a turbid, algal-dominated water body (Clugston 1963), following nutrient input from several sources, including agricultural drainage from adjacent vegetable farms (Schelske et al 2005(Schelske et al , 2010. Although these inputs were controlled and regulated to some degree, the eutrophication process continued and Lake Apopka is now considered hypereutrophic.…”

Cyanobacterial dynamics in shallow Lake Apopka (Florida, U.S.A.) before and after the shift from a macrophyte‐dominated to a phytoplankton‐dominated state

“…The lake bottom is relatively flat, with most of the lake area lying between depths of 1.0 and 2.0 m (Schelske et al, 2010). At present, it has a mean depth of 1.62 m, surface area of 125 km 2 , a mean hydraulic residence time of 2-3 years and a relatively small drainage area (c. 480 km 2 ).…”

“…At present, it has a mean depth of 1.62 m, surface area of 125 km 2 , a mean hydraulic residence time of 2-3 years and a relatively small drainage area (c. 480 km 2 ). They include land-use changes , nutrient enrichment , wind/storms (Bachmann, Hoyer & Canfield, 1999) and reduced light penetration as a consequence of water darkening caused by increased dissolved organic carbon (DOC) input and higher lake stage (Schelske et al, 2010). The catchment was disturbed severely in the early 1940s when levees were constructed along the north shore so that extensive, shallow sawgrass (Cladium sp.)…”