Subtidal food webs in a georgia estuary: δ13C analysis

Abstract

Stable carbon isotope compositions were analyzed for estuarine fauna collected in tidal creeks draining salt marshes dominated by either Spartina alterniflora Loisel. ( δ 13 C of −12.5%.) or Juncus roemerianus Scheele ( δ 13 C of −26%.). Mean δ 13 C values of subtidal invertebrates and fishes were −17.8%. and −17.5%. in the Spartina creek and −20.1%. and −20.0%. respectively in the two Juncus creeks. The intermediate δ 13 C values of the fauna implied an alternate carbon input to the food web, most probably benthic algae ( δ 13 C of −16 to −18%.) and phytoplankton ( δ 13 C of −20 to −22%.). In the Spartina marsh creek, animals with δ 13 C values less negative than −17%., e.g. mummichogs, mullet, and mud crabs, were most closely linked to the Spartina detritus/benthic algal food web, while species with δ 13 C values more negative than −19%., e.g. brown shrimp, menhaden, and oysters, were apparently more dependent on the phytoplankton food web. The data suggested that 1. (1) marsh plant carbon is a component of subtidal food webs, 2. (2) phytoplankton carbon is a more important food source for subtidal animals than for intertidal marsh fauna, 3. (3) the subtidal food web is structured such that individual invertebrate and fish species show varying degrees of dependence on the detritus/benthic algae-based food web of the marsh vs. the phytoplankton-based food web of the tidal creek.