Anthropogenic activities on coastal watersheds increase nutrient concentrations of groundwater. As groundwater travels downslope it transports these nutrients toward the adjoining coastal water. The resulting nutrient loading rates can be significant because nutrient concentrations in coastal groundwaters may be several orders of magnitude greater than those of receiving coastal waters. Groundwater-borne nutrients are most subject to active biogeochemical transformations as they course through the upper 1 m or so of bottom sediments. There conditions favor anaerobic processes such as denitrification, as well as other mechanisms that either sequester or release nutrients. The relative importance of advective vs. regenerative pathways of nutrient supply may result in widely different rates of release of nutrients from sediments. The relative activity of denitrifiers also may alter the ratio of N to P released to overlying waters, and hence affect which nutrient limits growth of producers. The consequences of nutrient (particularly nitrate) loading include somewhat elevated nutrient concentrations in the watercolumn, increased growth of macroalgae and phytoplankton, reduction of seagrass beds, and reductions of the associated fauna. The decline in animals occurs because of habitat changes and because of the increased frequency of anoxic events prompted by the characteristically high respiration rates found in enriched waters. Woods Hole Oceanographic Institution Contribution Number 7418.

Spurtina alterniflora oxidizes the sediments in which it grows through both passive oxygen release and active metabolic processes. Eh is higher in the root zone of this grass than in the sediment below the root zone or in unvegetated sediments. Sediments underlying the tall form of S. aZterniJorcl are more oxidized than those under the short form, and sediment redox condition and S. aZterni$oru production are related through a positive feedback loop. Reducing conditions inhibit aboveground grass production. But also, more productive plants have a greater capacity for sediment oxidation, as shown by the increased Eh in fertilized plots. Waterlogged sediments inhibit plan growth by decreasing passive oxygen release and thereby lowering Eh.

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Downward movement of the water table during both day and night in the short grass zone of intertidal salt marshes is due not to drainage but to water uptake by roots. Removal of water from the sediment results in the entry of air into the sediment, suggesting a feedback between plant growth, water uptake, and sediment oxidation. The water balance of Spartina alterniflora appears to influence the internal morphology of its roots, potentially giving rise to a new mechanism for the mass flow of gas in plants.