THE ROLE OF STANDING DEAD SPARTINA-ALTERNIFLORA AND BENTHIC MICROALGAE IN SALT-MARSH FOOD WEBS - CONSIDERATIONS BASED ON MULTIPLE STABLE-ISOTOPE ANALYSIS

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 parts per thousand increase in delta(34)S and a 2 to 3 parts per thousand decrease in delta(15)N values; delta(13)C values were unchanged. We describe mechanisms whereby the activity of fungi and epiphytic microorganisms may contribute to the observed shifts in delta(34)S and delta(15)N, respectively. The delta(13)C value of salt marsh benthic and epiphytic microalgae ranged from -13.0 parts per thousand in summer to -17.6 parts per thousand in early spring. Average delta(15)N values of microalgae and standing dead S. alterniflora were -0.3 and 1.7 parts per thousand, respectively, reflecting the activity of N-2-fixing microorganisms. The delta(15)N values for omnivorous and herbivorous salt marsh macrofauna ranged between 7.5 and 2.2 parts per thousand, and for predatory Fundulus spp. delta(15)N averaged 9.2 parts per thousand. Given a presumptive +3 parts per thousand trophic shift in N assimilation, these results suggest that N-2-fixing microorganisms associated with microalgal communities were an important source of N to salt marsh consumers. The delta(13)C, delta(15)N, and delta(34)S values of primary producers were compared to the values of the following consumers: Fundulus spp., Uca spp., Illyanassa obsoleta, and Littoraria irrorata. delta(13)C VS delta(15)N and delta(13)C vs delta(34)S 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.