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

Abstract

Carbon, sulfur, and nitrogen stable isotope ratios were measured in an irregularly floodedMississippi, USA, salt marsh to evaluate the relative importance of vascular plants and algae in the foodweb. Primary producers had lstinct 6I3c values: Spartina alterniflora (hereafter Spartina) (-13%),Juncus roemerianus (hereafter Juncus) (-26Y~),an d edaphic algae (-21 %o). A pure zooplanktonsample, which should closely approximate the phytoplankton, had a 6I3C value of -23%~. Values for6 3 4ra~n ged from 0 to +2 7~ for vascular plants, while those for edaphic algae and zooplankton were + 14and + l 1 %I, respectively; b I 5f~or all primary producers ranged from +S to +6%. For 49 of the 56 (88 %)consumers sampled, 613C fell w i t h a range of -22 to -18 %o; this range centered around edaphic algaeand zooplankton, but was dstlnct from the b13C of Spartina and Juncus. Values of b 3 4fo~r 48 of the 56(86 estuarine consumers ranged from +9 to +16%0, which included edaphic algae and zooplanktonbut was 8 to 15 36o more enriched in 3 4 t~ha n Spartina or Juncus. Dual isotope plots showed the fauna totightly cluster around values for edaphic algae and zooplankton; no consumer even moderatelyresembled Spartina or Juncus. This strongly suggests that the ultimate food sources for this marsh'sinvertebrate and fish fauna are the benthic and planktonic algae; direct contributions from vascularplants appear to be minor.