Lake Partner Program
Nepahwin Lake Watershed Stewardship Group has signed on with the Lake Partner Program (LPP), a volunteer-based water-quality monitoring program for Ontario inland lakes. The LPP is jointly coordinated by the Ontario Ministry of Environment, Conservation and Parks from the Dorset Environmental Science Centre and The Federation of Ontario Cottagers’ Association. We are joining over 550 other partner lakes.
As part of the LPP 2021 season, NLWSG will be sampling 3 lake sites (see map below). At spring turnover, about 2 weeks after the lake becomes ice free, our intrepid volunteers, Bruce Holson and Phil Thwaite, will be sampling for phosphorus, calcium and chloride and also taking Secchi depth (water clarity) and temperature readings.
For the duration of the open water season, 2 other volunteers will divvy up the sites with Bruce. Secchi depth and temperature readings will be taken at each of the three sites about every ~2 weeks between May and October.
Secchi Depth (Water Clarity)
Secchi depth is a measure of lake water clarity or transparency and is measured with a black-and-white disk called a Secchi disk .
A Secchi depth (SD) of > 4 meters (m) is considered high transparency, a SD of 2-4 m moderately transparent and a SD of 0-2 m low transparency.
The City of Greater Sudbury Lake Water Quality Program has measured SD over the past years. Lower water clarity was recorded in 2020 compared to the previous 2 years, ranging from the 2.7m in June 2020 to 3.8m in August 2020[i]. Historic (1999-2018) readings from a variety of sources throughout the season ranged from 2.3m to 7m, with an average of 4.8 m[ii].
Fig. 1 Nepahwin Lake Sampling Sites
Fig. 2 Secchi Disk & measurement technique
Secchi Disk Readings are taken by multiple NLWSG volunteers on a bi-weekly basis in order to track changes in water clarity.
CURRENT LAKE DATA
WHAT DOES SECCHI DEPTH MEAN?
Secchi depth is a measure of water clarity (or transparency) and is affected by the amount of sunlight that can penetrate into the lake. Thus, water clarity is impacted by fluctuations in algae, detritus, dissolved organic carbon, and other suspended solids in a lake. These, in turn, may be influenced by shoreline development, climate change, acid rain and invasive species, such as zebra mussels. These factors have different effects in every lake.
TEMPORAL TRENDS OF SECCHI DEPTH IN LAKE PARTNER PROGRAM LAKES(iii)
The LPP examined trends in Secchi depths (SD) for 410 volunteer-monitored lake sampling sites, using data between the years 2000 and 2014. It was discovered that the SD of the majority (65%) of lakes studied had not changed significantly over this time period. Of the lakes that have changed, 14% showed an increase and 21% showed a decline in SD over time. The majority of the lakes that showed a decline (less clear) over time were located within the Canadian Shield.
On the Canadian Shield, dissolved organic carbon (DOC) is an important determinant of water clarity. As DOC concentrations increase, water clarity (and in turn, SD) may show a corresponding decrease. There is evidence that DOC concentrations are increasing in recent years in a subset of lakes on the Canadian Shield (MOECC, unpublished data).
The reasons for this DOC increase and SD decline are hypothesized to be twofold:
1. Climate change causing warmer temperatures, which enhances microbiological activity in lake watershed soils; this results in a higher amount of organic carbon entering the lakes; and
2. Changes in soil chemistry resulting from the recovery from acid rain, causing the soil to retain less organic carbon.
Following recommendations of the CGS commissioned Nepahwin Lake Causation Study (July 2020), CGS is sampling Nepahwin Lake for DOC. This will add to the understanding of the influence of DOC and Secchi depth.
The above emphasizes the importance of the LPP and the role of citizen science volunteers. Regular Secchi depth readings over a number of seasons can help signal changes that will require further investigation and lead to potential understanding of cause and appropriate management.
LPP lake data can be found at: https://www.ontario.ca/environment-and-energy/map-lake-partner
[iii] R. Rossi. Secchi Trends Across the Province, Federation of Ontario Cottagers’ Association, September 2017