ABSTRACT-The stable isotope compositions (C, N, and S) of hve, senescent, and standing dead Spartina alterniflora were compared in order to determine the effects of aerial decomposition on the isotopic signature of aboveground S. dlterniflora tissue entering the food chain. Aerial decomposition of aboveground S. alterniflora resulted in a 6 to 18% increase in P S , and a 2 to 3 % decrease in 615N values; 613c values were unchanged. We describe mechanisms whereby the activity of fungi and epiphytic microorganisms may contribute to the observed shlfts in and 6I5N, respectively. The 613c value of salt marsh benthic and eplphytic microalgae ranged from -13.0% in summer to -17.6%0 in early spring. Average 6'" values of microalgae and standing dead S. altemiflora were -0 3 and l.?%, respectively, reflecting the activity of NZ-fixing microorganisms. The 615N values for omnivorous and herbivorous salt marsh macrofauna ranged between 7.5 and 2.2% and for predatory Fundulus spp. averaged 9.2%. Given a presumptive + 3 % trophic shift in N assimilation, these results suggest that N2-fixing microorganisms associdted with microalgal communities were an important source of N to salt marsh consumers. The 6% 6'"N, and 6"' s values of primary producers were compared to the values of the following consumers: Fundulus spp., Uca spp., Ilyanassa obsoleta, and Littoraria irrorata. 6I3C VS 6 1 5~ and 613C vs 63JS dual isotope plots demonstrated that microalgae and standing dead S. alternjflora 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.

ABSTRACT-We examined the net production, decomposition, and microbial utilization of the seagrass Halophila decipiens during a 6.5 d period in May 1985 in the Salt River Canyon, St Croix, US Virgin Islands. H. decipienscovered 37 % of the Canyon floor between depths of 14 and 32 m with a biomass of 9.15 g dry \vt m-'; its net productivity was ca 0.145 g C m-' d -' Turnover time, estimated by 2 independent methods, was 10.7 d. After 6.5 d H decjpiens incubated in lltterbags buried in the sediment lost 5 6 % of their original ash free dry weight (AFDW) while litterbags incubated on the sediment surface lost only 28 % of their original AFDW. Bacteria grew rapidly on the detritus, doubling in 3.1 d in the surface bags and 3.7 d in the buried bags. Per-cell thymidine incorporation rates peaked within the first 13 h in both treatments but declined thereafter. Final incorporation rates were highest in surface bags. Mean bacterial cell size and bacterial abundance associated with degrading H. decipiens were larger in the buried litterbags. Bacterial biomass, however, was only 29.3 mg cell C g -' AFDW in buried bags and 17.5 mg C g-' AFDVV in surface bags Using bacterial production averaged for the 6.5 d, we estimate that only about 0.26 "10 of the daily detntal input from H. declplens is converted daily into bacterial biomass attached to the degrading plant material. We conclude that, unless the bacterial community on H. decipiens detritus were to use the organic matter more efficiently and were heavily grazed upon, attached bacteria would not make a significant contribution to a deposit-feeding detritivore's energy demands.