The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m d) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.

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Primary production is nitrogen limited in most salt marshes with the possible exception of those impacted by high anthropogenic inputs of nitrogen. It is hypothesized that mature salt marshes which receive only small inputs of 'new' nitrogen from the atmosphere, surface water runoff, groundwater, tidal creek, and nitrogen-fixation will have a conservative nitrogen cycle. We have developed a process-based N mass balance model for a short-form Spartina alterniflora marsh in Virginia, USA. Data for the model included rates of gross mineralization, nitrification, denitrification, nitrogen fixation, above-and belowground macrophyte production, and benthic microalgal production. The annual balance between sources (mineralization, nitrogen fixation, tidal creek flux, atmospheric deposition, and sediment input) and sinks (above-and belowground macrophyte uptake, sedlment microalgal uptake, sediment burial, microbial immobilization, denitnfication, and nitrificat~on) of dissolved inorganic nitrogen (DIN) was determined for both interior S. alterniflora-vegetated sites and unvegetated creek bank sites. S e d i m e n t h a t e r exchanges of DIN species, predicted by results of the mass balance analysis, were compared to measured exchanges. Annually, sources and sinks of DIN in the vegetated marsh were in close balance. The vegetated marsh imported DIN from the adjacent creek during most of the year; the unvegetated creek bank exported NH,-to overlying tidal water during July and imported NH,' during other seasons. The net flux of DIN wds 5 7 g N m-' yr-l from overlying water into the marsh; however, this flux was small relative to rates of internal N-cycling processes. The sediment NH,+ pool turned over rapidly as a result of the high rate of gross mineralization (84 g N m-'yr.'). Other microbial N-cycling rates were low (0 6 to 4 g N m-2 yr-l). The NH,+ supplied by mineralization was more than sufficient to support both macrophyte (33 g N m-2 yr-l) and benthic microalgal (5 g N m-' yr.') uptake. We propose that in order to maintain steady state in the system approximately half of the DIN mineralized is immobilized into a readily remineralizable particulate organic N pool. Since rnineralization and macrophyte uptake are temporally out of phase, the labile organic N pool may serve to temporarily sequester NH4' until it is required for plant uptake.

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