A palaeolimnological record of human disturbance from Harvey's Lake, Vermont: geochemistry, pigments and diatoms

Stratigraphic analyses of inorganic geochemistry, pigments and fossil diatoms in a 0.7 m core of profundal sediments are used to reconstruct the limnological history of Harvey's Lake, Vermont, over the last 1000 years. The lake is moderately productive, deep (44 m) and clear, and the phytoplankton today is dominated by the blue‐green alga, Oscillatoria rubescens. Sedimentary pigments unique to blue‐green algae, oscillaxanthin and myxoxanthophyll, provide a detailed history of changes in the O. rubescens population. Accurate sediment chronology is derived from ²¹⁰Pb, ¹³⁷Cs and ¹⁴C dating and from the stratigraphy of pollen and sawmill wastes. Primary production increased in Harvey's Lake in 1780 following European settlement and again after 1945, as shown by greater accumulation of sedimentary pigments and diatom frustules, and changes in fossil algal assemblages. Blue‐green algae first appeared in abundance about 1945, indicating nutrient enrichment from dairy wastes and shoreline development. Increased deposition of elements associated with classic minerals also suggests greater soil erosion during both of these intervals. Two episodes of increased sedimentary anoxia (1820–1920 and 1945–present) are marked in the sedimentary record by enhanced pigment preservation, changes in authigenic Fe and Mn stratigraphy,’and the development of laminated sediments. The earlier episode of oxygens depletion is correlated with the discharge of sawmill wastes into the lake, and the later episode is associated with increased primary production. Based on these data a new model for Fe and Mn sediment stratigraphy is proposed for lakes that do not undergo complete hypolimnetic anoxia. Fine‐scale resolution of recent diatom and oscillaxanthin stratigraphy provides historical evidence for a long‐term negative interaction between diatom and blue‐green algal populations in Harvey's Lake.