Coastal Marshes

(Published April 2002)

Coastal marshes or wetlands can be viewed from two perspectives. They can smell bad because of the release of hydrogen sulfide (“rotten egg” gas). They often breed undesirable organisms like mosquitoes. They consume prime waterfront real estate. They release greenhouse gasses like methane and nitrous oxide.

But there is another, positive, perspective. In our tidal marshes, which occupy approximately 18% of our 451 miles of shoreline, the salt marsh cordgrass Spartina alterniflora is the most abundant plant. It is adapted to a limited tidal range. Spartina grows rapidly in full sunlight and forms the base of what biologists call a “detritus-based food chain”. Few animals actually eat the living plant. But when the grass dies and bacteria begin to decompose its woody tissue, the slimy particles provide a nutritious meal for many organisms like juvenile fish and crabs. These small organisms, in turn, provide a meal for larger organisms, like birds, and so on up the food chain. Salt marshes are one of the most productive ecosystems on Earth, producing more organic matter per unit of area than tropical coral reefs, rain forests or even modern agriculture. Their photosynthesis consumes carbon dioxide (a greenhouse gas) and liberates oxygen. The habitat provided by marshes is critical in the life cycle of numerous organisms, including many species with important commercial and/or recreational value. Not only do marshes provide abundant food, they also provide a refuge at critical times during the development of many organisms.

Plants like Spartina have an extensive “root” system (called rhizomes), which serves a variety of important functions. Nutrients like nitrate and phosphate are taken up to support the growth of the plant and thus are prevented from entering our waterways and contributing to growth of water-clouding suspended algae. The roots stabilize the substrate, retarding erosion. The marsh also traps sediment washed off the land, helping to keep the water clear and prevent the creeks from shoaling up and smothering oyster beds.

Some scientists have likened salt marshes to “nature’s kidneys”, ridding surface water and groundwater of unwanted substances. As an example, salt marshes DESTROY NITRATE! The bacteria which convert dissolved nitrate back into harmless nitrogen gas, called denitrifying bacteria, require three things: 1) a source of energy, 2) an anoxic environment (one with no oxygen gas present), and 3) nitrate. Salt marshes contain abundant organic matter, which is the source of energy. The mud in the marsh is anoxic because other bacteria quickly consume dissolved oxygen to respire (burn) the abundant organic matter. Groundwater discharge and surface runoff into the marsh provides nitrate.

Some modern wastewater treatment plants attempt to duplicate this natural process of denitrification, at considerable expense. It is a form of tertiary sewage treatment, or nutrient removal, called BNR (Biological Nitrogen Removal). In fact, artificial marshes are touted as excellent tools to destroy nitrate before it can enter waterways and contribute to over-fertilization. The construction of artificial marshes often accompanies the retrofitting of other forms of tertiary sewage treatment to existing wastewater treatment facilities and to the design of new facilities.

There can be no doubt about which perspective must prevail. The positive aspects of marshes clearly outweigh the negative ones. But our marshes are vulnerable. They are eroding away on their seaward margins. As sea level rises, the Spartina is killed when its low tide limit is exceeded. We can combat this inexorable process by addressing global warming, and by always proceeding at No Wake speed in the vicinity of marshes. As sea level rises, the marsh will try to move landward. We can assist this process by pruning away scrubs and overhanging tree limbs which produce shade and thus limit the ability of the grass to grow and expand. Marine grasses prefer full sunlight. The simple act of pruning away overhanging limbs also helps slow the tipping of trees toward the water and reduces the fall of leaves and twigs into the marsh and the water.

We must begin to care for our marshes just as we care for other parts of our property. Marsh grass can be planted where barren east- south- or west-facing inter-tidal flats are exposed at low tide and where full or nearly full sunlight penetrates for at least part of the day. During spring, sprigs can be transplanted along with a pinch of a slow release fertilizer to get the grass off to a good start.

Healthy marshes have an annual cycle and a beauty all their own. They are a very important part of our ecosystem. Improving the health of our marshes will improve the health of our local waterways and the Bay.