Water Quality - Jan 2020
Water clarity is recorded at each of the seven sample sites on the lake on six dates between May 1st and the end of October each year. There is an extensive database on Charleston Lake for this parameter extending back into the mid-1970s; since the year 2000 the seven basins of Charleston have been monitored for water clarity, phosphorus and calcium concentrations and periodically surface to bottom water column oxygen and temperature values. These are reported on annually. As of the writing of this article, the phosphorus and calcium analytical data for the summer of 2019 is not yet available from the Ministry of Environment, Conservation and Parks.
The 20 years of findings are summarized in the graph.
The mean water clarity value (Secchi disc depth) for the lake as a whole in 2019 is 6.415 metres (42 recordings). Since and including the year 2014, Charleston Lake has experienced a decline in water clarity when compared with values for the period 2009 to 2013 inclusive. The whole lake average water clarity for the period 2009 to 2013 inclusive was 7.56m; for the period 2014 to 2019 inclusive, the average water clarity value was 6.32m. The mean whole-lake average water clarity value for all twenty years combined (years 2000 to 2019) is 6.058 metres.
For the 20-year period of record, the poorest whole lake average water clarity (3.873 m) was recorded in 2002, the same year that the highest whole-lake mean total phosphorus value (16.857 ug/L) was recorded. More phosphorus produces more algae and results in higher turbidity and therefore poorer water clarity.
The distribution of zebra mussels (Dreissena polymorpha) is lake-wide in Charleston Lake and zebra mussel densities are highest in the littoral zone areas where the rocky shoreline provides the most and best substrate for the organisms to become attached. Zebra mussels filter particulates, algae and zooplankton out of the water and therefore increase water transparency. In the matter of little more than a decade (2002-2013 inclusive) the mean whole-lake Secchi disc depth value increased by 4.26 metres.
The year-2019 mean value of 6.567m is lower than the clearer values in the range of 8 metres recorded in the years 2010, 2012 and 2013; the mean value for the period of record 2010 to 2013 inclusive is 7.753m. The apparent decline in water clarity since 2013 may be due to a decline in the zebra mussel population; invasive zebra mussel populations are known to increase to a climax population density and then decline to a background new equilibrium level. Once established at that lower population level, a ‘new’ norm becomes established for water clarity. Zebra mussel densities have likely reached a steady state equilibrium; their influence on improved water clarity is undeniable. As a result, water clarity values will likely continue at the present level into the future.
Prior to the year 2005, Charleston Lake was classified as mesotrophic on the basis of water clarity; lakes with mean Secchi disc depths > 2.0 m and < 5.0 m are considered to be mesotrophic. However, for all years since and including the year 2005, Charleston Lake would be classified as oligotrophic on the basis of water clarity; the whole-lake mean water clarity for the years 2005 to 2019 inclusive is 6.643 m.
When the first five years (2000 to 2004 inclusive) of clarity data is compared against the last five years (2015 to 2019 inclusive) of data, the improvement is quite remarkable. The mean water clarity value for the first five years is 4.31 m (mesotrophic) while for the most recent five-year period it is 6.28 m (oligotrophic); an improvement of 1.97m.