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We quantified the fate and transport of watershed-derived ammonium in a tidal freshwater marsh fringing the nutrientrich Scheldt River in a whole-ecosystem [sup.15]N labeling experiment. [sup.15]N-N[H.sup.4.sub.4] was added to the floodwater entering a 3,477 [m.sup.2] tidal marsh area, and marsh ammonium processing and retention were traced in six subsequent tide cycles. We present data for the water phase components of the marsh system, in which changes in concentration and isotopic enrichment of N[O.sup.-.sub.3] N[O.sup.-.sub.2], [N.sub.2]O, [N.sub.2] N[H.sup.4.sub.4], and suspended particulate nitrogen (SPN) were measured in concert with a mass balance study. Simultaneous addition of a conservative tracer (NaBr) confirmed that tracer was evenly distributed, and the Br budget was almost closed (115% recovery). All analyzed dissolved and suspended N pools were labeled, and 31% of added [sup.15]N-N[H.sup.+.sub.4] was retained or transformed. Nitrate was the most important pool for [sup.15]N, with nitrification accounting for 30% of [sup.15]N-transformation. In situ whole-ecosystem nitrification rates were four to nine times higher than those in the water column alone, implying a crucial role for the large reactive marsh surface area in N-transformation. Under conditions of low oxygen concentrations and high ammonium availability, nitrifiers produced [N.sub.2]O. Our results show that tidal freshwater marshes function not only as nutrient sinks but also as nutrient transformers.

A research was conducted to examine the vertical distribution in the sediment and inhibition by ammonium of methane oxidation in intertidal marshes utilizing slurry incubation methods. A Carlo-Erba high-resolution MEGA 5340 gas chromatograph was utilized to characterize methane. Results suggested that the vertical distribution of methane oxidation in intertidal marshes may be determined by hydrological conditions that change with distance from a tidal river rather than by the dominant type of present vegetation.

We tested a hypothesis that the high densities of some passerines breeding in North American marshes result from greater safety of this habitat from predators as compared to upland habitats. We examined the relative importance of water depth and distance from the marsh edge in lowering predation rates on experimental nests with Blue-breasted Quail (Coturnix chinensis) eggs. In addition, using cameras we studied the role of water depth in determining the predator community. Our results showed that: (1) predation was lower in the marsh than in the adjacent upland; (2) predation rates decreased with increasing water depth in the marsh; (3) for the marsh nests, the distance to the marsh edge was relatively unimportant; (4) the diversity of nest predators decreased with increasing water depth; and (5) in the deep marsh areas, there was only one major predator, the Marsh Wren (Cistothorus palustris). We propose that the greater safety of deep-water marsh areas, the reduced complexity of the predator community, and the type of predators allowing effective nest defense by nest owners have played the key role in the evolution of reproductive strategies of marsh-nesting passerines. Received 10 October 1991, accepted 15 June 1992.

Current theories differ in their predictions concerning the effects of interspecific interactions on species growth and distribution along environmental gradients. In this study, we examined the influence of competition on species composition across a salinity gradient. This work involved three common fresh and brackish marsh species. The three species, Spartina patens, Sagittaria lancifolia, and Panicum hemitomon, differ in salt tolerances but are commonly found in overlapping zones across coastal marsh gradients in the southeastern United States. Plants were grown in a greenhouse under four salinity treatments (0, 2, 4, and 8 g/kg), in monocultures and in three-species mixtures. Data from the monocultures and mixtures were used to examine the importance of competition in regulating community composition and species' growth. Results for individual species were significantly different when grown in monoculture vs. the three-species mixture. While with increasing salinity community dominance shifted toward the most salt-tolerant species, Spartina patens, competition was found to alter community composition equally over all salinities. However, the responses of individual species demonstrated a very different pattern; the relative importance of competition in influencing individual growth differed substantially depending on the salinity tolerances of species. These results support the contention that, as abiotic stress increases, competition becomes less of a limiting factor in regulating the abundance of a species. However, the role of competition at the community level is the sum of the individual species' responses and may or may not vary with the level of abiotic stress. Key words: competition; Panicum hemitomon; Sagittaria lancifolia; salinity gradient; Spartina patens; wetland.