Algal Nutrients and Other Factors that Promote Algal Growth

Water quality monitoring of Kahshe and Bass Lakes by the DMM and the MECP has not identified any major water chemistry issues or trends in terms of algal friendly nutrients – phosphorus and nitrogen. Although both lakes have been classed as ‘Vulnerable’ under the updated Muskoka Official Plan (MOP), Bass Lake is expected to be removed due the findings of a DMM-funded Causation Study that identified natural causes as the reason for the elevated (>20 µg/L) levels of phosphorus, which is now one of three ‘Water Quality Indicators’ under the updated MOP.  Kahshe Lake remains on the ‘Vulnerable’ list due to the documented presence of harmful blue-green algal blooms (HAB) in both 2020 and 2021, although none have been recorded since then. While Bass Lake is slated to be removed from the ‘Vulnerable’ listing, it may be re-listed, as there was an HAB in 2023 and it remains on the Simcoe-Muskoka Health Unit’s active list.

Trends in water temperature, which can accelerate algal growth also were examined and there was no evidence of a warming trend in either lake based on water temperature measurements dating back to the early 1980s. Water clarity also has remained fairly steady over this four decade period, although clarity in Bass Lake is noticeably lower than in Kahshe.

And finally, it should be noted that the trend towards increasing numbers of HABs in Muskoka lakes continued in 2023, with 13 water bodies with documented HABs. This is cause for concern as the levels of algal-friendly phosphorus in Muskoka lakes have not increased over this time period. As such, changes in other factors that are known to accelerate algal growth and bloom development are happening. And, as we discovered in our Near-Shore Sampling Project in 2021, it is also possible that the historical sampling in mid-lake, deep water locations by the DMM and MECP is giving an excellent historical record of nutrient levels but not reflecting the elevated levels that have been documented in near-shore areas where algal blooms typically form.

Calcium Depletion

Decreasing lake water calcium concentration is an emerging concern for lakes on the Precambrian Shield in Ontario due to its impact on the reproduction and survival of zooplankton and other aquatic species that are important components of the aquatic food chain. The 2023 data confirm that there is no detectable trend towards decreasing levels of calcium in either Kahshe or Bass Lakes. However, as levels of calcium are fairly close to the aquatic growth limiting threshold of 1.5 mg/L and, as this threshold value would not be protective of all aquatic organisms, continued vigilance is necessary.

This is underscored by late season population explosions of Holopedium in Kahshe Lake since 2021. These pea sized, jelly-like orbs are not known to be toxic, but are an uncomfortable nuisance as they can be felt against your skin while swimming.

Lake Acidification

The waters of Kahshe and Bass Lake have acidity (pH) levels that are within a normal range and there is no evidence of an increasing or decreasing trend in acidity over the two decade monitoring period.

Metals and Other Chemical Contamination

The analysis of over 30 additional anions, cations and other chemicals by the DMM in 2023 has, with one possible exception, identified no trends or aquatic toxicity issues. While there were some minor exceedances of chronic (long term) health protection benchmarks established by the MECP and other agencies in the early years, most of these exceedances were likely due to sampling or laboratory artifacts, as more recent sampling has shown concentrations that are in the expected range for non-impacted surface water bodies in Ontario.

In the case of aluminum, the aquatic benchmark has been exceeded in Bass Lake, with the highest concentration in 2023; however, the benchmark consists of a range in values and must be evaluated based on the pH and DOC concentrations in lake water. Based on these findings, the levels of aluminum need to be followed carefully in future monitoring to ensure that the waters of Kahshe and Bass Lakes are safe from an aquatic perspective.

The other finding from this data set is that there are numerous parameters that, like total phosphorus, are higher in Bass Lake compared to Kahshe Lake. There are also a couple of parameters (silicon and N-NO3) where levels in Kahshe are higher than in Bass. The reason for these differences in the two lakes is unknown and was not evaluated in the DMM-funded Causation Study on Bass Lake.

Dissolved Oxygen Depletion

Dissolved oxygen (DO) in lake water is important, as it is essential for the survival of all aquatic organisms. A lack of oxygen in the lower layers of the lake (referred to as being anoxic) also can cause mobilization (release) of phosphorus from sediments.

Dissolved oxygen is influenced by seasonal temperature changes that factor into lake stratification, the process whereby lake water is turned over (mixed) in the late fall and again following the winter ice melt and then begins to stratify through the spring, summer and early fall as water temperature increases at the surface and DO levels decrease with increasing depth.

As expected, the spring measurement of DO at increasing water depths in both Kahshe and Bass Lakes for 2023 revealed that with only one exception (Bass Lake >4m), DO was adequate for aquatic organism survival down to lake bottoms. However, by early August 2023, as in all previous decades, DO levels in Kahshe Lake fell below the aquatic health-based Provincial Water Quality Objective (PWQO) at around 4-5m depth and remain that way down to lake bottom (20m). In Bass Lake, by mid-August, the 2023 DO levels steeply declined beyond a depth of 2m and fell below the aquatic PWQO at depths greater than around 2.5m.

From a biological perspective, unlike Kahshe Lake, there appears to be a trend towards DO levels in Bass Lake falling below the PWQO at more shallow depths (just below 2m) than back in the 1980s and 1990s when DO crossed below the PWQO at depths almost at lake bottom (just below 3m).

Benthic Health

The presence of benthic organisms indicates the health of both the riparian and littoral zones, as these areas can be impacted by snowmelt, runoff, sedimentation and other shoreline development activities. Collected benthos are grouped into seven different categories based on their typical response to environmental contamination and are then compared to the Muskoka average from locations known to be remote from any shoreline development sources.

The 2023 assessment at two sites on Kahshe Lake confirmed that the benthic community is comparable to the Muskoka average for most family groupings, with the exception of % Chironimids and % EOT, where lower and higher benthic counts, respectively, are desirable outcomes. And, based on the benthic monitoring over the period from 2006 through 2023, there appears to be no increasing or decreasing trend in the various families of benthic organisms in Kahshe Lake.

Although the KLRA has not provided volunteer benthic counting support to the DMM for Bass Lake, we have been provided with the analysis results for 2023 and they revealed that the average (all years) index values for two sites on Bass Lake for %Chironimids are slightly lower than the Muskoka Average, and this is a good sign, as the species in this grouping are pollution tolerant. In the case of %EOT, the lower values relative to the Muskoka Average are not a desirable outcome, as these benthos are sensitive to pollutants. However, it was noted that in the case of %EOT, the 2023 findings do indicate a healthier outcome for this grouping that was masked in the averaging approach using data from all years. An evaluation of trends in benthic family groupings at both sites over time revealed no evidence of any trend in the health of these organisms over the period from 2016 to 2023.