Edaphic algae are an important component of salt marsh food-webs: evidence from multiple stable isotope analyses

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Abstract
Carbon, sulfur, and nitrogen stable isotope ratios were measured in an irregularly flooded Mississippi, USA, salt marsh to evaluate the relative importance of vascular plants and algae in the food web. Primary producers had distinct .delta.13C values: Spartina alterniflora (hereafter Spartina) (-13.permill.), Juncus roemerianus (hereafter Juncus) (-26.permill.), and edaphic algae (-21.permill.). A pure zooplankton sample, which should closely approximate the phytoplankton, had a .delta.13C value of -23.permill.. Values for .delta.34S ranged from 0 to +2.permill. for vascular plants, while those for edaphic algae and zooplankton were +14 and + 11.permill., respectively; .delta.15N for all primary producers ranged from +5 to +6.permill.. For 49 of the 56 (88%) consumers sampled, .delta.13C fell within a range of -22 to -18.permill.; this range centered around edaphic algae and zooplankton, but was distinct from the .delta.13C of Spartina and Juncus. Vaues of .delta.34 S for 48 of the 56 (86%) estuarine consumers ranged from +9 to +16.permill., which included edaphic algae and zooplankton but was 8 to 15.permill. more enriched in 34S than Spartina or Juncus. Dual isotope plots showed the fauna to tightly cluster around values for edaphic algae and zooplankton; no consumer even moderately resembled Spartina or Juncus. This strongly suggests that the ultimate food sources for this marsh''s invertebrate and fish fauna are the benthic and planktonic algae; direct contributions from vascular plants appear to be minor.