POREWATER DRAINAGE AND DISSOLVED ORGANIC-CARBON AND NUTRIENT LOSSES THROUGH THE INTERTIDAL CREEKBANKS OF A NEW-ENGLAND SALT-MARSH

Drainage of interstitial water from creekbanks vegetated by tall Spartina alterniflora was directly measured using an in situ chamber technique over complete tidal cycles throughout the year in the Great Sippewissett Salt Marsh, Massachusetts, USA. Estimates of porewater seepage into chambers compared favorably with water exchange calculated from parallel measurements of water table excursion and sediment-specific yield when plants were inactive. Upon low tide exposure of the marsh surface, there was an initial large loss of porewater, then a continuously declining discharge which paralleled the pattern of water table drop in the adjacent creekbank sediments. Seepage losses were related to the vertical height of the creekbank with banks ranging from 0.5 to 1.0 m showing more than a 3-fold greater discharge than those only 0.25 to 0.5 m high on a linear basis and nearly 2-fold greater on an areal basis. Strong seasonal variations in seepage volume and extent of low tide water table excursion occurred in the tall S. alterniflora zone. In summer when water removal was through both drainage and evapotranspiration, the water table fell at almost twice the rate as in winter when drainage predominated. The annually averaged volume of creekbank seepage was 15.2 l m(-2) or 8.9 l m(-1) per tide. The magnitude of the seepage pathway for organic matter and nutrient export from marsh sediments was assessed from simultaneous measurements of seepage volume and concentrations of DOC, NH4+, NO3-+NO2-, and PO43- in both seepage and tidal waters. Sediments underlying tall S. alterniflora showed a net export of DOC of 1080 mmol C m(-2) yr(-1) (544 mmol m(-1) yr(-1)), NH4+ 115 mmol m(-2) yr(-1) (57 mmol m(-1) yr(-1)), and NO3-+NO2- 6.5 mmol m(-2) yr(-1) (4.7 mmol m(-1) yr(-1)), while PO43- was imported from tidal waters. Measured dissolved inorganic nitrogen losses in drainage waters represent only about 3% of the inorganic nitrogen export from this marsh. While porewater drainage was a major factor in the water budget of creekbank areas and represents a potential aeration subsidy to sediment oxidation, it was not found to be a significant pathway for the transfer of organic carbon or inorganic nutrients from the Spartina root zone to tidal waters.