by Bruce Leicher ·
Thursday, September 16, 2021
With the recurrence of an algal bloom and the excessive invasive plant growth in Bare Hill Pond this year, many residents have asked why and what is being done. Several residents have questioned whether drawdowns may be responsible. Let me share what we know.
Over 15 years, habitat monitoring data has demonstrated significant benefit from annual drawdowns. Over that period, drawdowns have helped to control invasive species (milfoil and fanwort), and substantially reduced the phosphorus in the pond’s water. This is important because absent that action, the pond was at high risk for algal blooms, and invasive plant species were out-competing our native plants.
Last year (as in 2013-14 when a drawdown was skipped to see if alternate-year drawdowns might be sufficient) the pump used to lower the water level broke before the pond froze, so the drawdown had little or no impact on the invasive plants. In prior years, monitoring of plants and the habitat shows that successful freezes have significantly reduced invasive species and allowed for repopulation of native plants in the drawdown zone. This year further confirms the downside of taking a year off and the effect of the drawdown on our invasive species.
Phosphorus was a primary driver when the state Department of Environmental Protection put Bare Hill Pond on its list of endangered lakes and ponds in 1998. Phosphorus enters a pond from throughout the watershed over many years. In 1998, phosphorus in the water column—as measured from the pond’s surface to the bottom—was 0.44 units per liter (u/l), significantly higher than the acceptable level of risk for a natural lake or pond. Since commencing drawdowns, phosphorus in the water column has been reduced by more than 50%.
So why have we experienced algal blooms in the past two summers?
The short answer is climate change and a significant rise in water and air temperatures in the past decade. June and July this year were the hottest in recorded weather history in Massachusetts. The benefit of the drawdown is that it added resilience to our pond, and made room for moderate phosphorus release during a summer without experiencing algal blooms for almost 15 years.
What we learned in the last two years, as demonstrated by temperature, oxygen, and phosphorus measurements, is when the temperature rises as much as it did, the level of phosphorus climbs enough to trigger algal blooms even with the extra resiliency. At higher temperatures (the pond at the surface was more than 80 degrees starting in early June this year), the water at the bottom of the pond does not hold enough oxygen to support aerobic life, and the biology becomes anaerobic, or anoxic, at depths greater than 10 feet. Monitoring data confirms this. Anoxic water interacts with the pond bottom sediment and releases phosphorus that normally would have remained bound to the sediment and not dissolved in the water. This triggers internal loading of phosphorus into the water column and algal blooms. This is likely the major cause of the algal blooms.
It is also possible that we have achieved a “tipping point” as a result of the temperature increase and internal phosphorus loading, so that additional stormwater and watershed runoff of phosphorus from fertilizers and auto exhaust contribute to overcoming the resiliency created by drawdowns.
This is not unique to Bare Hill Pond. According to the Environmental Working Group, a nonprofit that specializes in exposing the presence of toxic chemicals in consumer products and in drinking water, 51 lakes and ponds in Massachusetts reported algal blooms this summer, and most of them occurred in July, not mid-August. It is possible that our resilience, if not preventing algal blooms, is at least delaying onset.
So what can we do to prevent algal blooms and help build more resiliency into the Bare Hill Pond watershed?
It’s not really possible to get rid of cyanobacteria that cause algal blooms. According to the EPA, cyanobacteria are everywhere. In normal conditions, they remain on the pond bottom and have limited impact, but when phosphorus is released, when temperatures increase and oxygen is limited, they rise to the surface and bloom. Cyanobacteria are highly evolved species over hundreds of millions of years. They were some of the first organisms on Earth and are responsible for converting carbon dioxide to oxygen to create our atmosphere, making life possible on the planet. They are found everywhere in spore or living form, including, according to scientists, in the Sahara waiting for the right conditions to bloom.
We can all do our part to continue to reduce the size of lawns, the use of fertilizer, and other sources of phosphorus in the Bare Hill Pond watershed. We can encourage our community to protect our forests and natural landscape. There is no dispute among scientists that development of a watershed is the primary cause of increased phosphorus pollution, and the more we can do to preserve the watershed, the more likely we increase resiliency in Bare Hill Pond.
Because of the algal bloom last year, the Bare Hill Pond Watershed Management Committee has engaged an expert advisor this year to study the deeper areas of the pond and help identify actions that could stave off anoxic phosphorus release. The data is still being collected, and it could point to steps to take in addition to drawdowns. Possible options might include aeration of anoxic areas or using the pump to remove anoxic water during the summer rather than have surface water flow over the dam.
There are no magic bullets for algal blooms, in large part due to their hundreds of millions of years of evolution, making them incredibly hardy. Another challenge is Bare Hill Pond’s size: At 350 acres, it is not a typical small pond where technologies have been found to be helpful. It’s premature to speculate ahead of the data, but expect more to come later this year.
For now, this leaves us with the one action that data indicates is helping: the drawdown. In years when there is a significant freeze, it has significantly controlled invasive species; and equally and perhaps more important, it reduces the level of phosphorus in the water column, providing a level of protection against the impact of climate change and in-lake phosphorus loading.
If you want to take a deeper dive, take a look at the Bare Hill Pond Watershed Management Committee reports to the Conservation Commission each year that show the impact of the drawdown on invasive species, repopulation of native species, and on phosphorus. Go to: https://bit.ly/3CjUsXR.
Bruce Leicher chairs the Bare Hill Pond Watershed Management Committee.