Trophic state and metabolism in a Southeastern Piedmont reservoir
Mayhew, Mary Callie
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Lake Sidney Lanier is a valuable water resource in the rapidly developing region north of Atlanta, Georgia, USA. The reservoir has been managed by the U.S Army Corps of Engineers for multiple purposes since its completion in 1958. Since approximately 1990, Lake Lanier has been central to a series of lawsuits in the “Eastern Water Wars” between Georgia, Alabama and Florida due to its importance as a water-storage facility within the Apalachicola-Chattahoochee-Flint River Basin. Of specific importance is the need to protect lake water quality to satisfy regional water supply demands, as well as for recreational and environmental purposes. Recently, chlorophyll a levels have exceeded state water-quality standards. These exceedences have prompted the Georgia Environmental Protection Division to develop Total Maximum Daily Loads for phosphorus in Lake Lanier. While eutrophication in Southeastern Piedmont impoundments is a regional problem, nutrient cycling in these lakes does not appear to behave in a manner consistent with lakes in higher latitudes, and, hence, may not respond to nutrient-abatement strategies developed elsewhere. Although phosphorus loading to Southeastern Piedmont waterbodies is high, soluble reactive phosphorus concentrations are generally low and phosphorus exports from the reservoir are only a small fraction of input loads. The prevailing hypothesis is that ferric oxides in the iron-rich, clay soils of the Southeastern Piedmont effectively sequester phosphorus, which then settle into the lake benthos. Yet, seasonal algal blooms suggest the presence of internal cycling driven by uncertain mechanisms. This dissertation uses existing data sets and analyses of lake-eutrophication mechanisms to develop an improved understanding of nutrient cycling in Southeastern Piedmont impoundments. Water quality data for Lake Lanier between 1967 to 2001 are used to characterize the frequency, location, and magnitude of algal blooms, and to develop alternative conceptual models of nutrient cycling. The data suggest that phosphorus is not permanently sequestered in lake sediments, and is occasionally released, causing harmful algal blooms. Three mechanisms were considered that could account for sediment-phosphorus release, including: i) Anoxic release (or reduced sorption) of phosphorous from oxyhydroxides in benthic (hypolimnetic) sediments; ii) Alkaline release (or reduced sorption) of phosphorus from oxyhydroxides from epilimnetic sediments as photosynthesis causes pH to exceed 8; and iii) Biologically mediated release into the photic zone by recycling of algal biomass in the metalimnion, as algal biomass is bound to clay and settles. The clay-algal complex includes sorbed phosphorus which may be available to algae and this complex explains the characteristic, heterograde, dissolved oxygen profile in summer. This setting also explains the relatively low algal biomass, which underestimates the rate of photosynthesis, and therefore of primary productivity. These observations suggest that traditional measures of lake eutrophication (e.g., chloro- phyll a and phosphorus concentrations) should be supplemented with metabolic parameters (e.g., oxygen and carbon dioxide production and consumption) to better understand, predict, and manage biologic productivity in Lake Lanier.