By Robert Bowen
Urban freshwater environments are increasingly experiencing human-induced influences that all too often create imbalances in the natural lake processes. A common example of this is the enrichment of nutrients from groundwater movement and potent fertilizer runoff. The impacts of these enrichments often lead to the onset of harmful algal blooms and widespread fluctuations in dissolved oxygen (DO) levels.
To better understand the interplay between nutrients, algae growth and DO, Diversified Scientific Solutions has been conducting surveys to measure the seasonal variability of DO, pH, oxidation-reduction potential, temperature, nitrogen and phosphorus at Swan Lake in Victoria, British Columbia. The sampling program began in 2016 and has consisted of weekly water column profiling using YSI handheld probes and water sample collections from several sites on the lake, starting each May and continuing until the end of September.
As Swan Lake is a shallow and warm lake, strong DO gradients set up mid-summer where DO levels are super-saturated at the surface, largely due to the photosynthetic production of oxygen through the cyanobacteria Aphanizomenon flos-aquae, along with other species. DO levels rapidly decline with depth to anoxic conditions only a few metres below the surface. As summer progresses, this anoxic zone begins to migrate towards the surface. It is in this period that blooms of various algae become extensive, creating significant volumes of biomass. This biomass is supported by the enriched phosphorus levels but these levels soon begin to decline, leading to a mass die-off of algae.
With this die-off, oxygen levels collapse as turbidity increases, photosynthesis decreases and bacterial decomposing consume the remaining depleting DO. In data collected in 2018, it can be seen that the lake experienced three hypoxic events (< 2 mg/l) in the late summer and early fall periods. (Figure 1)
A large fish kill was observed in 2017 and a smaller one in 2018.
In order to increase the understanding of the oxygen fluctuations just prior to and during the hypoxic events, a RINKO dissolved oxygen logger from ASL Environmental Sciences’ lease pool was installed to expand upon the weekly YSI DO probe. The logger was set to sample every 10 minutes. Fluctuations as high as 16 mg/l over a 17 hour interval were observed just before onset, with a general dampening of the oscillations during hypoxia. (See Figure 2)
This captured the diurnal cycling of photosynthesis (daylight oxygen production) and respiration (night respiration).
As Swan Lake has exhibited a general pattern of hypoxic events in the late summer, a modest intervention was installed last September to help mitigate this troubling trend. Two 15 m lengths of Aquatech O2B2 linear aeration bubbler lines were placed on the bottom at a depth of about three metres. The aeration tubing consists of a double row of fine bubbles that gently rise through the water column to promote mixing and the reduction of thermal and DO stratification. As Swan Lake is approximately nine hectares in size, these lines would provide refuge areas in times of depleted DO.
To examine the impact of these bubblers, two ASL RINKO DO loggers were installed this year. One is adjacent to the bubblers and the other is outside of the influence of the bubblers. These loggers will be recovered in October.
Another tool that was used in 2018 to give insight into the abundance and distribution of algae and fish in the water column was ASL’s Acoustic Zooplankton Fish Profiler. This instrument, in this specific configuration, had three acoustic frequency transducers (200, 769 and 1250 kHz) that continuously monitored backscatter in the water column.
This was particularly useful as the transducers were able to detect the Aphanizomenon flos-aquae species. Backscatter data showed the diurnal vertical migration of this cyanobacteria and showed the decline of this species coinciding with the collapse of oxygen.
Annual reports have been generated from this monitoring program and inter-annual time series comparisons provide the data to guide discussions for potential mitigation strategies such as the bubbler lines introduced in September of 2018. Swan Lake water quality monitoring and the annual reports generated were for Swan Lake Christmas Hill Nature Sanctuary and were funded by the Royal Bank of Canada.
Robert Bowen is with Diversified Scientific Solutions. This article appears in ES&E Magazine’s October 2019 issue.