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

The role of standing dead Spartina alterniflora and benthic microalgae in salt marsh food webs: considerations based on multiple stable isotope analysis

Currin CA, Newell SY, Paerl HW

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

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