MEPS 121:99-116 (1995)  -  doi:10.3354/meps121099

The stable isotope compositions (C, N, and S) of live, senescent, and standing dead Spartina alterniflora were compared in order to determine the effects of aerial decomposition on the isotopic signature of aboveground S. alterniflora tissue entering the food chain. Aerial decomposition of aboveground S. alterniflora resulted in a 6 to 18 o/oo increase in delta³⁴S, and a 2 to 3 o/oo decrease in delta¹⁵N values; delta¹³C values were unchanged. We describe mechanisms whereby the activity of fungi and epiphytic microorganisms may contribute to the observed shifts in delta³⁴S and delta¹⁵N, respectively. The delta¹³C value of salt marsh benthic and epiphytic microalgae ranged from -13.0 o/oo in summer to -17.6 o/oo in early spring. Average delta¹⁵N values of microalgae and standing dead S. alterniflora were -0.3 and 1.7 o/oo, respectively, reflecting the activity of N₂-fixing microorganisms. The delta¹⁵N values for omnivorous and herbivorous salt marsh macrofauna ranged between 7.5 and 2.2 o/oo, and for predatory Fundulus spp. delta¹⁵N averaged 9.2 o/oo. Given a presumptive +3 o/oo trophic shift in N assimilation, these results suggest that N₂-fixing microorganisms associated with microalgal communities were an important source of N to salt marsh consumers. The delta¹³C, delta¹⁵N, and delta³⁴S values of primary producers were compared to the values of the following consumers: Fundulus spp., Uca spp., Ilyanassa obsoleta, and Littoraria irrorata. delta¹³C vs delta¹⁵N and delta¹³C vs delta³⁴S dual isotope plots demonstrated that microalgae and standing dead S. alterniflora are important food resources in the North Carolina (USA) marshes we sampled. In addition, a compilation of literature values suggests that this is true in salt marshes throughout the East and Gulf coasts of North America. Future isotope studies of marsh food webs should include detrital Spartina spp. material in analyses of trophic structure.

Spartina alterniflora . Benthic microalgae . Stable isotopes . Salt marsh . Food web