Freshwater Ponds

Cape Cod has nearly 1,000 freshwater ponds. The Cape’s natural ponds are the result of the glaciers that left this area 18,000 years ago. Chunks of glacial ice gouged depressions into the substrate, leaving what are referred to as kettle ponds. The water that fills these ponds is groundwater that is recharged by rain and snow melt. Precipitation saturates the sandy substrate beneath us some 300 feet deep above bedrock. This is groundwater that fills the ponds and is the same water we draw from for our drinking water and irrigation. This water connection is why our ponds are called “windows on our aquifer”.

Water is not static. Groundwater moves, it flows. It passes through our ponds as it travels ultimately to the ocean. A map of the Cape’s groundwater reveals contours, changes in elevation resembling the irregular surface of the land, although it is not directly related to surface contours.

Most ponds, unaltered by development, are ringed by native woody shrubs like button bush, high bush blueberry and winterberry. These are shrubs grow in moist areas, but do not want to be submerged and therefore, the annual high water level keeps them to a defined edge. Fallen trees and overhanging shrubs give shade and help moderate the water’s temperature in summer months and provide cover for fish and sunning spots for turtles.

In the shallows of the water you will find emergent plants like pickerel weed with its spade-like leaves has with purple flower spikes, a favorite of bumble bees. There may be rushes and reeds like Woolgrass and Pipewort. Submergent plants found in deeper water may include Spatter Dock with golden globe flowers amid large flat leaves and native white water lilies. Their summer leaves also give shade and moderate temperature, minimizing algal growth. They form a micro-habitat for insects both above and below the leaves. Fish hide from great blue herons and feed on algae and smaller swimming creatures. Small birds land on the floating vegetation to grab an insect or a drink. The submerged portions of all aquatic plants provide habitat value for numerous invertebrates which are the food of fish, ducks and other wildlife. As fall temps arrive, pond vegetation dies back, falling beneath the water’s surface where the nutrients of the plant material are recycled through decomposition.

The water levels in most Cape ponds fluctuate due to changes in groundwater, precipitation, evaporation and sometimes drawdown by municipal wells. When the shoreline becomes exposed, a coastal plain pond plant community may thrive where they have not been trampled or the shoreline otherwise altered by beach fill or structures. This is a globally threatened habitat of a specialized plant community consists of Rose Tickseed, Grass-leaved Goldenrod, Plymouth Gentian, and Sandplain Agalinis. These plants are accustomed and adapted to the Cape’s kettle ponds that historically are low nutrient, acidic glacial ponds with sand or gravel bottoms.

Threats to pond health

The health of our ponds is at risk from past and current agricultural activities, untreated stormwater runoff, effluent from our septic systems that seeps down to the groundwater that feeds the ponds, invasive species, pollutants from human activities like applications of pesticides and fertilizers, careless littering and other forms of deliberate dumping.

Mercury enters our ponds from polluted air carried by westerly winds, contaminated by coal-powered and trash-burning energy plants. While there has been some reduction in the pollutant-causing activities, mercury persists. If you fish, before consuming any fish caught in a freshwater pond, refer to the MA Department of Public Health’s advisory postings.

Pond health is also at risk from the impacts of a changing climate. Extreme rainfall events can overwhelm stormwater management systems and cause erosion, dumping a slug of pollutants, nutrients and sediments into ponds. An increase in water temperatures and excess nutrients change the biology and chemistry of pond habitats in favor algal blooms, low oxygen sometimes resulting in fish kills.

Cyanobacteria, also called bluegreen algae, are ancient organisms responsible for creating the first oxygen on the earth. Cyanobacteria are commonly found in ponds. However, under certain conditions yet to be fully understood, they can produce toxins that pose health issues to animals and humans. Warmer temperatures with excessive nutrients contribute to toxic blooms that make swimming and other contact dangerous. Also called harmful algal blooms or HABs.

APCC has developed a cyanobacteria monitoring protocol with the assistance of scientists from UNH and the Cyanobacteria Monitoring Collaborative. Working with volunteers from the Brewster Ponds Coalition and Friends of Chatham Waterways and their respective town departments, data is being gathered as a means of documenting potential blooms and to contribute to better understanding of cyanobacteria. APCC is working on a rapid assessment program to better inform communities when toxicity is present so that the public can be advised about the potential health risks for contact.

Watch our freshwater ponds video.