The Ecology of Tidal Freshwater Marshes of the - USGS National ...

In general the river systems of themid-At1 antic and south At1 antic CoastalPi ai ns tend to be more numerous, more extensive,and fed by greater quantities ofrunoff. For these reasons, the tidalfreshwater marshes in f,laryl and, Virginia,Georgia, and South Carolina are muchvaster than the states north of NewJersey.For a variety of reasons (e.g.,sl ower decomposition rates, freezing ofmarsh surfaces during the winter, anddifferences in vegetation), most NewEngland coastal marshes tend to have morepeat than southern coastal marshes (Freyand Basan 1978). Southern marshes, on theother hand, have sediments with a highercl ay and silt content.In spite of these regional differencesin geology and sediments, the recentgeological history of east coast tidalfreshwater marshes is similar. Virtuallyall conteinporary marshes are very recentin origin (Holocene). They lie in rivervall eys which were cut during Pleistoceneperiods of lowered sea level. As sealevel rose during the post-Wisconsin periodof the Holocene (5,000 to 15,000 BeforePresent [BPI), both tidal freshwater andestuarine marshes expanded rapidly as thelower stretches of drowned river systemswere inundated (El 1 i son and Nichol s 1976).There is excel lent evidence (Froomer1980a, 1980b) which suggests that coastalmarsh expansion has continued at a relativelyrapid rate to the present. Infact, Froomer (1980b) concluded that therate of coastal marsh building in themid-At1 antic reg ion has been accel eratedover the past three centuries due toincreased soil runoff associated withman's activities. He reported an averagevertical growth in marsh sediments of 27.4cmlcentury for estuarine and tidal freshwatermarshes in the northern portion ofChesapeake Bay. Because of these highrates of deposition, many tidal freshwatermarshes have started and have grown toconsiderable extent in only the last fewcenturies.The recent geological history oftidal freshwater marshes can be demonstratedby examining a vertical profile takenfrom corings through a contemporary marsh.A typical sequence through a mid-At1 anticmarsh could show (1) a hard bottom consistingof a Pleistocene erosion surface(bedrock) cut during a glacial period oflowered sea 1 evel ; (2) varying layers ofriver, estuarine, and narsh sediments; and(3) a cap of recent tidal freshwater marshsediments varying in thickness from lessthan 1 m (3 ft) to more than 10 m (30 ft).Of course, very young marshes might beunderlain by layers of sand or clay andhave only a thin 1 ayer of narsh sedimentson the top.Even though contemporary tidal freshwatermarshes are generally 1 ess than15,000 years of age and most are nuchyounger, this does not nean that this typeof wetland did not exist in earlier geologicalperiods. Certainly, during Pleistoceneperiods of reduced sea 1 evel, alltypes of coastal marshes were relativelyreduced in extent. There is ample evidencefrom coal deposits, however, whichshows that early equivalents of ourpresent-day tidal freshwater marshesexisted hundreds of mill ions of years ago.1.6 MARSH DEVELOPMENTAL STAGEIn the same way that all wet1 andspass through various stages of development,tidal freshwater marshes have certain geomorphol og i cal and ecol og i calcharacteristics which tend to ref1 ecttheir geological age. Frey and Basan(1978) have cl assif ied coastal estuarinemarshes into three categories: (1) youngmarshes which are largely low or intertidalmarsh, (2) mature marshes which area mixture of low and high marsh, and (3)old marshes which are largely high marsh.We feel that this classification system,while somewhat simp1 istic, is equallyuseful in studying tidal freshwatermarshes. Young tidal freshwater marshes,of a few hundred years of age or 1 ess, aretypically low-lying, largely intertidal,and dominated by vegetation of the lowmarsh (spatterdock, arrow-arum, and wildrice). Old marshes are generally all highmarsh (except along creek banks and arounddepressions), may not be flooded at all byneap tides, and are dominated by highmarsh vegetation (e.g., cattails, marshmallow, and iris). Mature marshes are