Water Quality Research: 2016-2019

One of the primary controls on algal growth is the concentration of phosphorus in the water. It does not take much increase in phosphorus from low levels of around 10 micrograms per litre (mg/L or parts per billion) to change a lake from being in an Oligotrophic state (4-10 mg/L) with low primary production, and thus lacking algal blooms, to mid (Mesotrophic, 10-20 mg/L) or high (Eutrophic, 30-100 mg/L) states that would commonly lead to blooms. Numerous lakes in North America, including Nova Scotia, now experience algal blooms that did not have them before, due to increased concentrations of phosphorus.

The MLSA wanted to establish if higher phosphorus contributed to the algal blooms of 2014-16 in Mattatall Lake and if so, to investigate what might have caused the increase. In pursuing this interest, we have been fortunate to join forces with two research groups at Dalhousie University: the Biofluids and Biosystems Modelling Lab (BBML) headed by Dr. Tri Nguyen-Quang and Dr. Rob Jamieson’s group in the Centre for Water Resources Studies (CWRS).

Initial results by Nguyen-Quang et al. (2016) indicated very high but quite variable levels of phosphorus in the lake particularly during the fall 2015. Similar though somewhat lower levels were observed again in 2016. However, no specific causes for these patterns were found. To further investigate potential sources of phosphorus, Dr. R. Jamieson conducted a detailed study in 2017 (see Executive Summary from CWRS, 2017). However, these results have indicated no source of higher phosphorus from the watershed. Their predicted phosphorus values of around 10 mg/L were consistent with measurements in the lake and the absence of an algal bloom.

Measurements by the MLSA focused on the various small streams that flow into the lake from the surrounding watershed and the single outlet stream in the NE. Our results show higher values of phosphorus in 2016 that reduced in 2017, except for two streams on the western side which remained much higher than the others. Measurements from the outlet are consistent with the CWRS measurements in the lake, and indicate a significant reduction during fall 2016. But we still do not understand the reasons for the higher values.

One indication of higher levels of phosphorus within the lake in the CWRS study comes from the deepest part of the southern basin below about 8 m or 26 ft (see Lake Depth Project). This increase is caused by uptake from higher phosphorus in the sediment into water that lacks oxygen because it does not mix with the shallower water as the lake warms during the summer. The deeper water with higher phosphorus is released in the fall by overturning when surface waters cool again. Studies on many other lakes have shown that it is an important part of phosphorus uptake.

Measurements during 2018 & 2019 have studied this process in the area with water depths over 9m in the southern part of the lake. The results show the development of stratified layers during the period from early July to late August. The deep water during this period does not mix with the surface and thus becomes anoxic (i.e. no oxygen present). The absence of oxygen allows the water to chemically leach phosphorus from the bottom sediment, as shown by the higher values sampled in August and September. Lower values of phosphorus remain in the upper layers. In early Sept, the stratification decays after the upper water starts to cool. This eventually leads to the overturn of the deep layer at which time the water with the higher phosphorus is released into the lake. Details vary from year to year as temperatures and mixing due to surface winds change. High winds during Hurricane Dorian in 2019 produced a particularly unusual double overturning event.

The timing of the overturning matches the mid-September timing of the algae blooms in 2014 & 2015 but not the earlier bloom in August 2016. The beginning of the bloom in the SW part of the lake also agrees with the input of excess phosphorus from the deep basin at this location. We note, however, that the region with water depths > 9m is restricted to a very small area that represents only about 0.2% of the total volume of water in the lake. In addition, this process occurs every year even when algae blooms do not occur and thus it is not sufficient to cause the algae blooms by itself. Our conclusion is that the overturning might cause the blooms in certain years when the overlying water throughout the lake has higher concentrations of phosphorus (i.e. ~20 mg/L) rather than its typical value of ~10 mg/L.

This highlights the importance of keeping the lake at low values of phosphorus to avoid future algae blooms.