Quantitative inferences of past hypolimnetic anoxia and nutrient levels from a Canadan Precambrian Shield lake

Paleolimnological analyses were used to infer limnological changes during t he past similar to 300 yrs in the west basin of Peninsula Lake, a small (85 3 ha) Precambrian Shield lake in Ontario, Canada, that has been subjected t o moderate cultural disturbances (forest clearance, cottage and resort deve lopment). This study represents a pioneering attempt to use sedimentary chi ronomid assemblages and weighted-averaging models to quantify past hypolimn etic anoxia (expressed as the anoxic factor, AF). Impacts of forest clearan ce and human land-use on deepwater oxygen availability and surface water qu ality were assessed by comparing chironomid-inferred AF and diatom-inferred total phosphorus concentration ([TP]) to changes in terrestrial pollen and historical data. This study also discusses the ability of chironomids to q uantitatively infer changes in AF. Pre-disturbance chironomid assemblages were stable and dominated by taxa in dicative of oxygen-rich hypolimnetic conditions (e.g., Protanypus, Heterotr issocladius, Micropsectra type), while diatoms indicated oligotrophic lake status (diatom inferred [TP] = 5-7 mu g . l(-1)). Chironomids characteristi c of lower oxygen availability (e.g., Chironomus, Procladius) increased fol lowing land-clearance, road construction, establishment of a grist mill and lakeshore development beginning ca. 1870. Increased abundances of Tanytars us s. lat., a multigeneric group of mainly littoral chironomids, since 1900 , indicated that littoral chironomids may have comprised a greater proporti on of fossil assemblages during periods of eutrophication and prolonged ano xia. Abundances of meso-eutrophic diatom taxa (e.g., Fragilaria crotonensis , Asterionella formosa, Aulacoseira ambigua, A. subarctica) increased concu rrent with European settlement (ca. 1870) and diatom-inferred [TP] doubled (similar to 6-12 mu g . l(-1)), further indicating that naturally- oligotro phic Precambrian Shield lakes were extremely sensitive to initial land-clea rance activities. Recent increases in oligotrophic diatom taxa (e.g., Cyclotella stelligera) indicate a shift to more oligotrophic conditions since ca. mid-1960s, with greatest changes since ca. 1980. The chironomids Heterotrissocladius and Mi cropsectra type also increased at this time suggesting greater deepwater ox ygen availability. These recent water-quality improvements, possibly in res ponse to enhanced nutrient removal from detergents and sewage, climate-rela ted reductions in external phosphorus loads, and catchment (but not lake) a cidification and reforestation, suggest that habitat for commercially-valua ble cold-water fishes has improved in recent decades despite greater recrea tional lake-use. Paleolimnological assessment of trophic status changes in Peninsula Lake us ing fossil diatom and chironomid assemblages were in good agreement. Diatom inferences of [TP] and chironomid inferences of AF both suggest that Penin sula Lake was historically oligotrophic, became oligo-mesotrophic after Eur opean settlement, and returned to oligotrophy in recent yrs. Chironomid inf erences of [TP] consistently underestimated the trophic status of Peninsula Lake, possibly due to its relatively large hypolimnion. These results sugg est that AF represents a useful tool for quantitatively reconstructing the past trophic status of deeper, stratified lakes.