Stable Isotopes Suggest Limited Role of Wetland Macrophyte Production Supporting Aquatic Food Webs Across a Mangrove-Salt Marsh Ecotone

Abstract

Mangroves and salt marshes are considered important habitats for a diversity of fishery and other species. One of the key presumed values of these systems is their role in fuelling aquatic food webs that support many fisheries species. The mangrove-salt marsh ecotone along the NE Atlantic coast of Florida provides a natural experiment to examine the relative importance of mangrove and marsh production in supporting aquatic food webs. Representative food web components comprising key functional groups were sampled across the ecotone to test the prediction that species or functional groups that rely on the dominant wetland producers should show large differences in carbon stable isotope values across sites that span C4-dominated Spartina marshes in the north to C3-dominated mangrove wetlands in the south. Source contributions were estimated for fishery species (blue crabs, penaeid shrimp), wetland specialists (cyprinodontiformes fishes, grass shrimp, fiddler crabs), zoobenthivores (pinfish, spot), phytodetritivores (mullet) and planktivores (menhaden, anchovy, silversides). The range of wetland macrophyte source contributions had minima < 5% for most groups at most sites, despite the shift from C3- to C4-dominated wetlands, suggesting that Spartina and mangroves contributed minimally to the trophic support of these groups. Spatial correlations between sources and individual consumer species that spanned the ecotone showed the strongest correlations with micro-algal sources (microphytobenthos/phytoplankton), weak correlations with a ‘wetland’ source reflecting the transition from Spartina to mangroves across the ecotone, and no correlation with either mangrove or Spartina sources. These findings suggest a limited role of mangrove or Spartina production in supporting the aquatic consumers examined.