If you’ve spent any time on Clary Lake this summer you’ve seen those green “cotton candy” clouds of algae floating around near the shore. These blooms of algae are most commonly found in shallow (littoral) areas of lakes and are comprised of free-floating masses of filamentous green algae (primarily Spirogyra, Gonium, and Zygnema) and is referred to collectively as Metaphyton. Because they’re unattached (free-floating) they tend to move around with wind and water currents so one day you might not see any on your shore line and the next the bottom could be covered with it. Metaphyton masses will often get entangled with sticks, rocks, mooring lines, and other pond plants and then they don’t move around but remain in one place. Scientists are still learning about what influences the growth of metahphyton and its significance and potential impact on water quality besides the obvious “yuck” factor. Metaphyton can form dense mats that trap gasses given off by plant decomposition, causing them float to the surface where wind and waves cause them to break up and dissipate.
It is normal to find small amounts of metaphyton growing in Maine lakes and I’ve seen it off and on over the years in Clary and have actually written about it here (see: 17 August 2013: Metaphyton growth on the rise?) but over the past couple of years, metaphyton growth has increased markedly and this summer, as the picture at left shows, there’s an alarming amount of it growing in shallow areas around the lake. I took this photograph last week over on the south shore of the lake in front of Francis Moulton’s place, next to my property in water that was between 1′ to 3′ deep. So to answer my initial question, yes, I do think metaphyton growth is a cause for concern. The VLMP has developed a process to allow volunteers to identify, document and track the growth of metaphyton in Maine lakes and ponds. We may add this regimen to our water monitoring activities next year.
There is little question in my mind that the extreme low water conditions we’ve experienced in recent years are has had a deleterious effect on water quality in general and specifically has contributed to this explosion in metaphyton growth, in the growth of other plant species such as elodea, and in the frequency, duration, and severity of algal blooms. Conditions are near perfect for algae growth. Filamentous green algae thrives in nutrient-rich warm shallow water where sunlight can reach the bottom, and as everyone knows all too well, there’s a lot of warm shallow water around the lake now. Furthermore phosphorus levels are on the rise (see chart at left). Phosphorus is the primary “plant food” that fuels algae growth. Phosphorus that doesn’t flush out of the lake can be bound-up in bottom sediments and as long as it stays there, it doesn’t cause much of a problem. However, large expanses of these sediments around the shore are now exposed to weathering and disturbance from rain and wave action, and this leads to the release of phosphorus into the lake water.
Phosphorus can also increase under conditions of low oxygen saturation. In the spring the lake water is pretty well mixed with a uniform dissolved oxygen level around 10 to 11 milligrams per liter (mg/L). Later is the season however, as the water warms up and stratifies, dissolved oxygen levels below the thermocline and near the bottom of the lake drop to near-zero (the current reading at the lake bottom is 0.1 mg/L) and this causes phosphorus bound up in bottom sediments to go back into solution. Our last phosphorus test taken on July 24th yielded the second highest phosphorus level we’ve ever observed in Clary Lake, 32 ug/L (micro grams per liter). Extremely low lake levels exacerbate the problem in another way: with the reduced lake volume (about 55% of normal), nutrients in the lake are more concentrated. Finally, Clary Lake has become stagnant due to the lack of rain and runoff this year. The “flushing rate” (how often the lake water is changed due to runoff in one year) which is normally about 1.8 is currently ZERO.