The influence of subsurface hydrology on nutrient supply and smooth cordgrass (Spartina alterniflora) production in a developing barrier island marsh

The supply of nutrients from surface and subsurface water flow into the root zone was measured in a developing barrier island marsh in Virginia. We hypothesize that high production of tall-form Spartina alterniflora in the lower intertidal zone is due to a greater nitrogen input supplied by a larger subsurface flux. Individual nitrogen inputs to the tall-form and short-form S. alterniflora root zones were calculated from water flow rates into the root zone and the nutrient concentration corresponding to the source of the flow. Total dissolved inorganic nitrogen (DIN) input las ammonium and nitrate) was then calculated using a summation of the hourly nutrient inputs to the root zone over the entire tidal cycle based on hydrologic and nutrient data collected throughout the growing season (April-August) of 1993 and 1994. Additionally, horizontal water flow into the lower intertidal marsh was reduced experimentally to determine its effects on nutrient input and plant growth. Total ammonium (NTH,I) input to the tall-form S. alterniflora root zone (168 mu moles 6 h(-1)) was significantly greater relative to the short-form (45 mu moles 6 h(-1)) during flood tide. Total NH4+ input was not significantly different between growth forms during ebb tide, and total nitrate (NO3-) and total DIN input were not significantly different between growth forms during either tidal stage. During tidal flooding, vertical flow from below the root zone accounted for 71% and horizontal flow from the adjacent mudflat accounted for 19% of the total NH4+ input to the tall-form S. alterniflora root zone. Infiltration of flooding water accounted for 15% more of the total NO3- input relative to the total NH4+ input at both zones on flood tide. During ebb tide, vertical flow from below the root zone still accounted for the majority of NH4+ and NO3- input to both growth forms. After vertical flow, horizontal subsurface flow from upgradient accounted for the next largest percentages of NH4+ and NO3- input to both growth forms during ebb tide. After 2 yr of interrupted subsurface horizontal flow to the tall-form S. alterniflora root zone, height and nitrogen content of leaf tissue of treatment plants were only slightly, but significantly, lower than control plants. The results suggest that a dynamic supply of DIN las influenced by subsurface water flows) is a more accurate depiction of nutrient supply to macrophytes in this developing marsh, relative to standing stock nutrient concentrations. The dynamic subsurface supply of DIN may play a role in spatial patterns of aboveground S. alterniflora production, but determination of additional nitrogen inputs and the role of belowground production on nitrogen demand need to also be considered.