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Where the algae come from

Geology, pollution, and groundwater all play a roll in the increasing algal blooms in Cape Cod’s kettle ponds.

Shubael Pond in Marstons Mills has been closed to swimmers and pets the result of algae bloom and cyanobacteria.John Tlumacki/Globe Staff

In addition to warming temperatures and surface runoff, it is important to recognize the role that geology, pollution, and ground water play in the increasing algal blooms in Cape Cod’s kettle ponds (”A plague of algae in Cape ponds,” Page A1, Aug. 1).

The Cape was formed as the terminus of a glacier some 16,000 years ago. Typically, glaciers shed sediments such as gravel and sand that are dumped off during their retreat, forming ridging patterns and often kettle ponds. This is how Cape Cod was formed and why it has its unique shape even today.

The downside of being a glacial dumping-off point is that the resulting terrain has minimal to nonexistent bedrock. No barriers exist to restrict groundwater flows along the entire extent of the gravel and sand. In effect, pollution of any sort, including septic, entering the ground water at a given point is able to flow and accumulate anywhere within the Cape. The resulting trapped nutrients build up mostly in the form of excess nitrogen and phosphorus, which help enable events such as the disturbing algal blooms in kettle ponds. Nutrient overloading occurs not only at interior locations but also at shorelines where seagrass and other biologically nurturing habitats are compromised.

Until the Cape can somehow significantly extend its sewering and construct enough water treatment plants, it will be difficult to reverse the flow of excess and destructive nutrients into the biosphere.


Jim Brown


The writer was an analyst at the Massachusetts Department of Environmental Protection from 1995 to 2017.

What to do about it

The “climate justice” movement emphasizes that the effects of climate change will do the most harm to those who have done the least to cause it — less developed countries, low-income people, communities of color. Unfortunately, this means that those best positioned to act against climate change — those with privilege and power — have often been less motivated to do so.


But perhaps this is changing. As ponds on Cape Cod become unswimmable due to toxic algae caused by warming waters, climate change is beginning to interfere with the lives of more people. Skiers having to look harder for skiable slopes is not the same as drought-induced starvation, but it may get people’s attention.

And what behavior changes can we hope for as awareness of the crisis becomes more personal? Maybe flying less. Maybe more restraint with the thermostat.

Let’s all do what we can.

Susan Donaldson