Holocene Trophic State History Of A Subtropical Blackwater Lake, South Georgia, Usa

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Journal Of Paleolimnology




Organic-rich sediment from Lake Louise, a dystrophic sinkhole lake in south Georgia, displays variations in C, N, P, C/N, delta (super 13) C, delta (super 15) N, biogenic silica (BSi) and diatom flora that document changes in trophic state over the past approximately 9,500 years. The lake initially was oligotrophic and moderately productive, but by the middle Holocene a rising regional water table, driven by eustatic sea level rise, caused expansion of wetlands around the lake and a shift to humic waters. Low rates of sediment accumulation, low C contents, rising C/N, and light delta (super 13) C and delta (super 15) N indicate this was a time of low productivity, more anoxic bottom waters and extensive recycling of littoral organic matter. These conditions persisted until approximately 1800 AD when a physical disturbance to the watershed, probably the Great Hurricane of 1780, resulted in a dramatic increase in productivity that has continued to the present day. We attribute this shift, recorded by a >tenfold increase in sediment accumulation rate, higher C, P, and delta (super 15) N, and lower BSi, to establishment of an inflow stream that increased nutrient delivery to the lake, raised water level, and expanded the wetland area around the lake. Since approximately 1930, logging, farming, and highway construction have impacted the lake, further accelerating biological productivity as well as the delivery of terrigenous sediment. Results of this study illustrate the potential of a single, catastrophic event to permanently alter the hydrology and chemistry of a lacustrine system and confirm that dystrophic lakes can be highly productive and therefore promising targets for paleolimnological study.