Ecosystem functions of tidal fresh, brackish, and salt marshes

Background/Question/Methods

Few studies have compared ecosystem functions of tidal fresh, brackish and salt marshes, despite speculation by Odum (1988) that these functions might vary across the salinity gradient within an estuary. We examined patterns of habitat function (plant diversity), productivity (plant biomass, total C) and nutrient stocks (N, P in aboveground plant biomass) in tidal marshes of the Satilla (Sa), Altamaha (A) and Ogeechee (O) Rivers in Georgia. We worked at two sites within each salinity zone (fresh, F; brackish, B; saline, S) in each estuary. Twelve 0.5x0.5m and fifty 1x1m plots were located at each site along a transect from the creekbank to the marsh platform. Species richness (1x1m plots) and percent cover, standing crop biomass, and standing stocks of C, N and P (0.5x0.5m plots) were measured with standard methods.

Results/Conclusions

In total 109 plant species were found. Site-level species richness decreased across the salinity gradient (average F=21.3, B=11.0, S=4.2) and did not differ among rivers (Sa=11.3, A=13.0, O= 12.2). Plot-level richness showed a similar pattern. Standing biomass was greatest at brackish sites (B=1715 g/m², F=1424 g/m², S=994 g/m²) and in both brackish and fresh sites tended to decrease away from the creekbank, whereas in the salt marsh sites biomass was greatest in the midmarsh. Total Carbon stocks paralleled patterns of biomass in that they were greatest at the brackish sites and lowest at the salt marsh sites (B= 732 g/m², F=609 g/m², S=420 g/m²). Nitrogen stocks decreased across sites as salinity increased (F=25.6 g/m², B=22.7 g/m², S=14.8 g/m²) and were greatest in the creekbank zone. Phosphorus stocks did not differ between fresh and brackish sites (~2 g/m²) but were lowest at salty sites (1.24 g/m²). These results generally support Odum’s (1988) speculations, and emphasize that ecosystem function in tidal wetlands changes sharply across the relatively short horizontal distance of the estuary. Changes in plant distribution patterns driven by global changes such as sea level rise or fresh water withdrawal are likely to have strong impacts on a variety of wetland functions.