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Updated: August 05, 2015
Lake water residence times vary greatly, ranging from a few days to tens of, even hundreds, of years.
For lakes having longer residence times (a year or more), long-term average pollutant loadings become more important to overall lake water quality.
As an ideal example, a lake having a water residence time of one year will still retain 50% of its original water after a year of average inflow. Following the second year (after two flushings), 25% of the original water will still remain. In the third year, the lake will have 12.5% of the original water, and so on. This characteristic requires that the longer the water residence time, the longer the time frame needed for in-lake observations to detect any response to loading reduction.
Moreover, because lakes are not always completely mixed reactors and because pollutants are not always conservative substances, evaluations may be even more tenuous. Vollenweider (1976) discusses the concept of phosphorus residence times and how they differ from water residence times.
Phosphorus is just one element among an array of stressors!
In an Irish lake, Lough Ennell, an important factor in delaying recovery was the buffering capability of the lake sediments that acted as both a sink and a source of phosphorus. In other instances, where the mass of labile sediment-associated phosphorus is low, then a lake might improve more quickly than predicted by water residence times. Such a situation could be observed in a lake that received a brief, one-time loading, such as from a fertilizer spill.
In shallow and/or dystrophic lakes, such as those common to Nova Scotia, the standard limnetic (i.e., open-water or pelagic) trophic parameters may not represent the true trophic status of the lake. In such lakes, the limnetic parameters may not reflect changes in the watershed unless those changes are significant in relationship to the overall size of the watershed and/or in its prevailing inputs!
To reliably ascertain incremental inputs from new developments, outflows of every storm pipe outlet, of in-situ devices (e.g., CDS, Stormceptor or Vortechnics), of constructed wetlands outlets, and others have to be monitored almost on an hourly basis during runoff, and pollutographs have to be developed.
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