Woods Hole Research Center

Coastal Plain Ponds

The Coastal Plain Ponds of Cape Cod are mostly deep ponds formed during the last Ice Age by large chunks of ice breaking off the retreating glaciers and causing depressions in the ground called "kettle holes." If the hole was deep enough to reach the water table, then the hole filled with water and became a kettle pond. Seasonal changes in groundwater level are mirrored by changes in the level of these ponds. The fluctuating water levels alternately flood and expose the shore, and this determines the plant types that can live in these habitats. A rich diversity of wildflowers, grasses, sedges, and rushes inhabit these shorelines, with small, sheltered ponds, and sand/peat soil mixtures having higher diversity. The sandy soils are nutrient poor, so it is not uncommon to find insectivorous plants along the shores.

Global distribution of coastal plain ponds is extremely limited, and therefore so are the plants which grow abundantly on their shores, such as Plymouth gentian (Sabatia kennedyana) and slender arrowhead (Sagittaria teres), which are mostly confined to Plymouth and Barnstable counties. The densest populations of rare species found in Massachusetts are in the Mary Dunn Pond area of Hyannis. Rare wildlife is also found around coastal plain ponds, occasionally even bald eagles (Haliaeetus leucocephalus), but they avoid ponds surrounded by homes. Coastal plain ponds are threatened by recreational vehicle traffic, development, littering, water pollution and water table draw-down by nearby municipal wells.

Vernal Pools

Vernal pools on Cape Cod form in small, shallow depressions that intersect seasonally high water tables, and generally only exist during the wet spring months. During the dry months, the water table drops and the level of the pool lowers too, until it eventually dries up. Because of the seasonal drying, the wide variety of organisms that live in these pools are free from fish predation.

Many organisms breed in vernal pools, and some amphibians such as wood frogs (Rana sylvatica), marbled salamanders (Ambystoma opacum) and spotted salamanders (Ambystoma maculatum), breed here exclusively.

If a vernal pool is destroyed, the pool-dependent species living there will be unable to find alternate breeding pools. Vernal pools also seem to be important to migrating animals, or those fleeing disturbed habitats.

Acid rain, contaminated runoff, and changes in runoff amounts can cause severe damage to vernal pool habitats, and often people fill pools because they mistakenly think the waters are lifeless. In order to be legally protected, vernal pools must be mapped and certified by the Natural Heritage Program, but most are not currently mapped. Additionally, only those certified pools within lands subject to flooding and bordering vegetated wetlands can actually be protected under Massachusetts' Wetlands Protection Act.

Anadromous Fish

Anadromous Fish are fish that hatch in freshwater, spend their adult lives in saltwater, and return to freshwater to spawn. The fish migrate by fish runs, streams connecting bodies of marine and freshwater. The most significant native species of anadromous fish on Cape Cod are alewives (Alosa pseudobarengus), blueback herring (Alosa aestivalis), and white perch (Morone americana).

Anadromous fish generally return to spawn by the same route that they followed to the ocean as juveniles, and they return to the same location in which they hatched. Anadromous fish are important links in both marine and freshwater food webs, and they are an important renewable resource to humans. They are recreationally, aesthetically, and commercially important.

Overfishing of anadromous fish is a concern, as are human activities that pollute, obstruct, or otherwise alter the runs.

Salt Marshes

Salt marshes are coastal wetlands that form in areas protected from erosion, typically behind a barrier beach or tidal flat, and extend from the mid-tide line to the highest spring tide line. Salt marshes are dominated by grasses, and characterized by dense peat layers beneath the lush surfaces.

Lower marsh vegetation is dominated by salt water cord-grass (Spartina alterniflora), and upper marsh by salt meadow-grass (Spartina patens). These form the base of very complex marine and estuarine food webs.

The vegetation is outstandingly productive. In fact salt marshes are among the most productive of all ecosystems on the planet. The grasses take up nutrients (sometimes from pollution), and the anaerobic bacteria that live in the ecosystem break down nitrates so efficiently that water quality is often improved by flowing through a marsh. The grasses also slow down the tidal currents and capture sediments, which gradually raises the level of the marsh until only high tides flood it.

The dense vegetation of a salt marsh acts to stabilize sediments, and provides protection from destructive wave energy. Salt marshes provide haven for many different organisms, with two thirds of all commercially important fish and shellfish spending at least part of their lives in a salt marsh.

Salt marshes are destroyed by draining, filling and the dumping of dredge spoils, which typically occurs with development. Heavy metals, pesticides and other contaminants may enter the marsh system, become concentrated in the sediments, and eventually work their way up the food web.

Shellfish Habitats

All of Cape Cod's shoreline, except for a few areas that are frequently subjected to heavy wave action, are potentially rich shellfish habitats.

There are six commercially important species of shellfish in the Cape Cod waters: soft shelled clams (Mya arenaria), surf clams (Spisula solidissima), blue mussels (Mytilus edulis), oysters (Crassostrea virginica), quahogs (Mercenaria mercenaria), and bay scallops (Argopecten irradians). Optimal sediment types and salinities are different for each species, so as natural conditions change the location and types of shellfish beds may change as well.

Another key factor that controls development of shellfish beds is successful larval recruitment, or having a large batch of larval shellfish land in a suitable area to settle and grow into adults. It is believed that these three factors may all contribute to the natural fluctuation in shellfish populations from year to year, with larval recruitment being the primary controlling factor. Shellfish are important to marine and estuarine food webs, but they also have very important roles in Cape Cod's economy, culture and ecology.

Shellfish beds are very susceptible to damage from human activities, ranging from physical destruction of the beds, to bacterial and viral contamination associated with fecal coliform. Such contamination poses serious health risks and affected beds must be closed to harvesting.

Coastal Salt Ponds

A coastal salt pond is a shallow marine embayment that receives freshwater inflow from groundwater entering the head of the pond, and saltwater inflow through an inlet from the sea. The inlet is periodically opened and closed by the shifting of barrier sands.

Tidal and groundwater inflows mix to form a salinity gradient in the pond. Much of the pond's plant and animal life is segregated along this gradient, because different organisms prefer different salinities. However, conditions will vary greatly with the opening and closing of the inlet, and some organisms will not survive the sudden changes in salinity.

When the inlet is closed for a long period of time there will be large-scale die offs among the plant and animal communities, and massive decay will give off unpleasant odors and make the pond unsightly. When the pond is reopened by a breach in the barrier beach the organisms adapted to the fresher water conditions will then die off due to the increasing salinity levels.

On Cape Cod, salt ponds are only found in Falmouth, and many of these are no longer true salt ponds because they are kept permanently open to the sea by dredging or the use of jetties. Rare natural habitats are being lost because nearby residents do not want the ponds undergoing the natural cycles of decay.

Due to limited tidal flushing, salt ponds are very prone to degradation by pollution from a variety of sources, such as improper septic systems and road runoff. Dredging and filling may physically destroy communities, or it may increase the tidal flow, thereby affecting plants and shellfish.