FISH KILLS AND DEOXYGENATION
By John Hicks
The probable cause for the increasing numbers of dead fish (fish kills)
in Lake Simcoe’s bays and canals this year (and the Maskinonge River
last year), is likely due to the process of Thermal Stratification in combination
with the infiltration of excess nutrients. (See illustration below.)
The surface layer of the lake (called the epilimnion) experiences wind and wave action
which encourages oxygen in the air to dissolve and be dispersed throughout the water
column of the lake. The spring sun gradually warms the surface water and thermal
layers begin to form in the lake. Immediately below the top layer (epilimnion), a separate layer of water exists (called the thermocline) where the most rapid drop in temperature
occurs and where oxygen begins to deplete. With just a few degrees Celsius between
the two layers, the natural mixing of water stops, and de-oxygenation occurs.
The organisms in the lower layers begin to consume what little oxygen is left in the water, which now cannot be replenished without circulation.
If the temperature layering continues long enough, coupled with excess nutrients entering the lake (from sewage treatment plant outfalls, farm run-off, septic beds, soil loss from subdivision re-grading, fertilized lawns etc,) toxin-producing algae, coupled with the lack of oxygen can result in a massive fish kill. In this situation, the microscopic plants and animals (called phytoplankton and zooplankton) begin to die along with the larger organisms which feed upon them (i.e. fish), ultimately producing a fish kill. Foul odours and floating dead fish are a sure give-away to thermal stratification and deoxygenation.
The situation is worse in the canals and swampy bays of the lake, where another, deeper poisonous layer exists, (the benthal layer) on the bottom. The benthal layer is the result of decaying vegetation and anaerobic bacteria (bacteria not requiring oxygen), which form an “ooze” or sludge on the bottom. An event known as the “Spring Turn-Over”, which is caused by periods of strong winds and persistent rainfall, can occur which mixes the benthal layer with the upper layers of the lake spreading its decaying material and deoxygenating the whole water column above. Fish gasping for air on the surface are often indicative of this situation. In early years on the lake, it was rare to see a fish kill like we see today, and it’s not because of thermal stratification alone, but aggravated by the continuous infiltration of excess nutrients. The lake will surely express its stress many more times severe than we now see it unless we stop this infiltration. Development pressures and poor land use practices are the root problems which provoke this phenomena. We need to take charge of this situation quickly or the lake will deteriorate.
Already hosting taste and odour algae, its south portion is losing its appeal for swimming.
An extract from my new book covering the planning, design and management of small
lakes and ponds to be released this summer by Fitzhenry & Whiteside, Toronto.