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

and Powers (1978) measured the cation exchangecapacity of sediments from a JamesRiver marsh and found values ranging from39.6 to 67.3 meg x 100 g dry weight. Thisis a relatively high value compared tocoastal plain and piedmont soils, but typicalfor highly organic, high clay wetlandsediments (personal observation). Thesehigh values of cation exchange capacitya1 so indicate a young, sl ightly weatheredsediment with high nutrient avail abil ity.A range of sediment nutrient concentrationsis shown in Table 2. It shouldbe noted that these limited data come frompolluted sites. It is possible that unpollutedsediments might have lower nutrientconcentrations.As in other wet1 and sediments, ammoniumis the most abundant form of inorganicnitrogen (Table 2). Nitrate and nitriteusually do not accumulate in anaerobicsoils. This is because nitrificationproceeds slowly while deni trification proceedsrapidly. Bowden (1982), who workedin the tidal freshwater marshes of theNorth River, Massachusetts, found that theamount of ammonium that is free in theinterstitial water is often less than theamount of ammonium adsorbed loosely ontosediment particles. Consequently, thepool of available ammonium is probablymuch greater than the pool of free ammonium.Bowden also presents evidence thatthe amount of ammonium in tidal freshwatermarsh sediments may be highest in midsummerand lowest in late spring, coincidentwith heavy demands for nitrogen from newvegetation. He found the highest concentrationsnear the surface (3.7 mg/l).Lower concentrations were found at 20-cm(8-inch) depth (0.9 mg/l), and an increase(2.0 mg/l ) was found at 60-cm (24-inch)depth.In summary, from 1 imi ted informationit appears that the sediments of tidalfreshwater marshes typically have (1) ahigh organic content, (2) a pH in the 6.0to 6.5 range, (3) moderate to strongreducing conditions, (4) a high cationexchange capacity, and (5) inters ti tialnutrient concentrations which are high inaiamonium and low in nitrate and nitrite.Sediment and water nutrients are discussedfurther in Section 3.3.Table 2. Concentrations of chemicals in the soils and interstitial water of threetidal freshwater marshes. na = not available, + = one standard deviation.Soi 1Interstitial waterLocation and Total N Total P NH NO + NO Total dissolved Preference (% dry wt.) (% dry wt.) (g/l) (9/1) (9/1)Herring Creek Marsh,James River, Va. (Lunz 1.5r0.8 0.7k0.4 1600t1200 look50 180k190et a1 . 1978)North River Marsh,Mass. (Bowden 1982) 1.6r1.9 0.1-0.3 900-3700 0-170 n aHami 1 ton and WoodburyCreek marshes. Delaware 0.5-1.0 0.04-0.2 na naRiver (Simpson et al.1981)Hami 1 ton marshes ,Delaware River(Whi gham et a1 . 1980)1.03-1.64 0.12-0.35 n a na