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Updated: September 05, 2015Acknowledgements
(Also see our web pages on Shallow Lakes; Multiple stressors; Lake Carrying Capacities; Indicator thresholds for anthropogenic stressors of freshwater lakes in Nova Scotia; and submissions to the WRMS [Water Resource Management Strategy] of the Nova Scotia Environment Department [NSE].)
Diatom-based total phosphorus (TP) inferences suggest that [TP] has declined in some lakes, despite increased inputs from anthropogenic sources. When coupled with siliceous algae-based pH inferences that suggest landscape-scale long-term acidification, these results indicate that the ecological effects of anthropogenic P inputs are being masked by the cumulative effects of multiple stressors. Detailed stratigraphic analyses of lakes that have experienced severe anthropogenic disturbances indicate that despite measured and inferred recovery in epilimnetic [TP] to predisturbance concentrations, pelagic communities have not recovered to predisturbance community composition; profundal communities and the variables associated with water quality (e.g., hypolimnetic oxygen) have also not recorded recovery.
The observation that several lakes have exhibited substantial inferred declines in both pH and TP suggests that the acidification of catchments may have contributed to reduced P export to lakes, possibly because of changes in soil-P binding or increased formation of P-Al precipitates in acidified catchments.
Recent scientific reports (2002-2007) also confirm the long term impacts of acid deposition on Nova Scotia and recovery may not occur even by the end of the 21st century!
Hall and Smol (1996) also suggested that increased P uptake in aggrading successional forests (versus old-growth forests in predisturbance conditions) and other anthropogenic disturbances such as food web changes from fish stocking may have led to decreased TP concentrations in lakes.
Decreased internal P loading may also decrease TP concentrations in relatively shallow lakes where hypolimnetic oxygen concentrations have improved and anoxic conditions have declined because of lake deepening from outlet dam construction.
Recent climate changes measured as recent decreases in precipitation and surface water runoff have reduced P export from watersheds into lakes, with declines in lake water [TP] over the past three decades.
Paleolimnological top-bottom analyses and detailed stratigraphic analyses suggest that lake TP concentrations recently have declined to below historical, predisturbance levels, despite increased P loading due to anthropogenic P inputs.
Recent declines in TP concentrations have widespread ecological implications, as algal biomass may decline, with resultant impacts on water clarity, which may alter lake thermal regimes and increase exposure of epilimnetic communities to incoming ultraviolet (UV) radiation. Potential changes in algal biomass, thermal regimes, and UV radiation exposure affect higher trophic levels such as zooplankton and fish.
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