Recent trophic state changes of selected Florida lakes inferred from bulk sediment geochemical variables and biomarkers

Abstract

Short sediment cores (80–100 cm) from three Florida (USA) lakes (Sheelar, Wauberg, and Apopka) that range in trophic status, were analyzed for total organic carbon (TOC), total nitrogen, stable isotopes of organic carbon (δ13CTOC) and biomarkers (n-alkanes and fatty acids), to identify the sources of organic matter in the lake deposits, and to link changes in primary productivity to anthropogenic activities during the last ~ 150 years. Relatively small (0.07 km2), ultra-oligotrophic Lake Sheelar is located in a state park, and sediment analyses indicate stable trophic status in the water body since at least the middle of the nineteenth century. Algal biomarkers are in low abundance throughout the core and terrestrial lipids and δ13CTOC values suggest that vascular plants were the primary source of TOC to the lake sediments during the period of record. In larger, eutrophic Lakes Wauberg (1.5 km2) and Apopka (125 km2), algal-derived biomarkers increase in recent sediments, whereas δ13CTOC values and concentrations of terrestrial biomarkers decrease toward the sediment surface. Increasing dominance through time of autochthonous carbon sources in the Lake Wauberg and Apopka sediment records coincides with specific anthropogenic activities in the respective watersheds. Submersed macrophytes in Lake Wauberg were replaced by algal communities in the mid-1980s, following expansion of residential development in the watershed. Biomarker data from the Lake Apopka core show there was an abrupt transition in the lake in the late 1940s, from domination by vascular plants to domination by algae, which has been documented by other paleolimnological studies that used alternative trophic state indicators. The trophic state shift in Lake Apopka corresponds to a period of increased nutrient input to the lake, associated with extensive farming along the north shore. Florida lakes have experienced different trophic state trajectories over the last ~ 150 years, driven by specific human activities in their respective watersheds.