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Tidal Freshwater Marshes Underappreciated Part of the Estuary

By John S. Nichols, Habitat Conservation Division, Northeast Region, Annapolis, Maryland

Arrow-arum. Adjacent to Choptank River, Denton, MD.  Photo credit: NOAA John Nichols


Freshwater tidal marsh adjacent to Choptank river, Denton, MD.  Photo Credit: NOAA, John Nichols

NOAA Fisheries Service Habitat Program protects a diverse range of habitats critical to the life cycles of the natural resources we manage through federal regulations.  In mid-Atlantic estuaries, such as the Chesapeake Bay, protection of tidal freshwater marshes, which enhance important spawning grounds for migratory fish like striped bass, alewife, blueback herring, and American shad; and, estuarine fish like spot and croaker, is key.

Typically when we think of estuarine marshes, we picture a vast stretch of grass such as cordgrass, and other salt-tolerant plant species, such as salt grass and marsh elder.  However, estuaries, where freshwater rivers meet and blend with coastal saltwater are a diverse continuum of habitats, extending from a saline mouth, to freshwater habitats in the upper branches of rivers or tributaries. Migratory anadromous fish that spend portions of their lives in both fresh and salt water, and estuarine dependent species which are spawned in coastal waters, make use of the entire continuum of habitats for completing their life cycles. This includes tidal freshwater marshes found in the upper riverine corridors. Because of their geographic location, and their role in fueling riverine food webs, tidal freshwater marshes may actually be one of the more important parts of the estuary.

Yellow iris or yellow flag.  Adjacent to Choptank river, Denton, MD.  Photo credit: NOAA John Nichols

Spatterdock.  Narrow-leaf cattail and pickerel reed in the background.  Photo credit: NOAA John Nichols

Tidal freshwater marshes are particularly common in the mid-Atlantic coastal region. In the Chesapeake Bay, these systems are wide-spread along lower Maryland's Eastern Shore including Choptank, Nanticoke, Wicomico, and Pocomoke Rivers, and Maryland's Patuxent and Potomac Rivers.  In Virginia, they occur along large parts of the Pamunkey and Mattaponi Rivers, and in the Potomac, Rappahannock, Chickahominy, and James Rivers.

Unlike saline marshes, where the diversity of plant life is limited to those species that can tolerate higher concentrations of salt, freshwater tidal marshes are very diverse; a botanists' dream. Here plant diversity increases from lower to higher tidally influenced elevations. In other words, more species are adapted to the less tidally influenced areas of the marsh.  Freshwater tidal marshes are also highly productive systems, producing above-ground biomass that equals or exceeds that of saline marshes.

In the Chesapeake Bay tidal freshwater marshes, common plant species include broad-leaf herbaceous types with showy flowers, weedy species with thorns and sticky seeds, and a variety of grasses and rushes, including wild rice. Many of the perennial species, such as arrow arum, spatterdock, and cattail also have massive tubers, and swollen roots and rhizomes. Rhizomes are thickened underground stems that grow in a horizontal direction and sprout new sections as they grow. Underground tubers and rhizomes retain carbon, nitrogen, and other nutrients for subsequent growing seasons. Consequently, freshwater tidal marshes assist humans by trapping carbon emissions, holding fine-grain sediments in place, and mitigating nutrient levels in the estuary.

More importantly, tidal freshwater marshes play a vital role in supporting the reproduction of commercially and recreationally important fish through timely stimulating of riverine food webs in areas where fish spawn and nursery. Fish that benefit from tidal freshwater marshes include anadromous species, such as striped bass, alewife, blueback herring, American shad, hickory shad, white perch, and yellow perch; and, estuarine species, such as spot, croaker, weakfish, and drum. Over time, these species have selected spawning and nursery grounds in river areas contiguous to or near areas of maximum tidal freshwater marsh development.

How do tidal freshwater marshes assist fish reproduction? Many of the plant species occurring in tidal freshwater marshes decompose rapidly at the close of the growing season, releasing to the river large amounts of particulate detritus, dissolved organic matter, and nutrients. These products of decomposition are readily used by bacteria, fungi, and other detritus eating creatures; as well as phytoplankton, which are  small algae or plants that float in the water column. Secondary producers, such as copepods, amphipods, rotifers, and insect larvae, in turn, increase their abundance during late winter and early spring by consuming the bacteria, fungi, and phytoplankton.  Consequently, when anadromous and estuarine fish spawn and nursery in freshwater areas of rivers during late winter and spring, there is an abundance of food to support the survival of their young. 

There is growing interest in scientific study and restoration of tidal freshwater marshes. For instance, in the heavily urbanized Anacostia River in the District of Columbia, dredge material from the Anacostia River Federal Navigation Project has been used by the Corps of Engineers to establish tidal freshwater marshes at Kenilworth Gardens and Kingman Lake. While the restored marshes are not considered "pristine," they still benefit migratory fish because some of the habitat and productivity in these areas has been successfully restored.