Natural abundance stable isotopes and dual isotope tracer additions help to resolve resources supporting a saltmarsh food web

We used a combination of natural abundance stable isotopes (NASI) and multiple isotope additions to examine the importance of local and tidally imported basal resources to infauna in a tidal, saltmarsh creek within the Plum Island Estuary, Massachusetts, U.S.A. NASI analysis was conducted on primary producers and infauna prior to isotope additions. While algal resources as a group had distinct NASI values compared to Spartina spp. resources, various algal resources [sediment-associated algae (local) and phytoplankton (tidally imported)] had similar NASI leading to low dietary resolution with mixing models. To separate primary producer isotope values, we conducted a large-scale, 42 day 15N-addition to the whole saltmarsh creek, which enriched tidally imported (i.e., phytoplankton) and local algae resources. We also conducted small-scale additions of enriched 13C to 1-m2 plots within the creek receiving the 15N addition to separately label local resources. Enriched 15N and 13C were taken up by filamentous algae and associated epiphytic diatoms and benthic microalgae creating distinct isotope values in local algal resources. In addition, changes in 15N-tracer uptake by phytoplankton during the first week of the 15N-addition provided a unique enrichment pattern that was followed through the food web. Both NASIs and isotope additions indicated that Spartina spp. was not an important basal food resource to infauna, lending credence to the notion that saltmarsh food webs don't often rely on macrophyte detritus. Isotope additions indicated that various types of algae, both local and tidally imported, were important to the diets of infauna, including polychaetes, tanaids and harpacticoid copepods. Isotope additions increased diet resolution and revealed less accurate basal resource contributions determined with NASI alone. While many food web studies rely on NASI, we feel that the combination of NASI and isotope additions is needed in systems with similar primary producer values to more accurately determine resource contributions, to increase resolution of dietary contributions and to determine the dietary importance of local basal resources.

► Spartina spp. was not important to the diets of most infauna.
► The importance of algal resources could not be determined using NASI.
► Dual isotope additions created distinct isotope values in algal resources.
► Algal resources, both local and imported with the tide, were important to the diets.
► Diets did not reflect the availability of resources indicating selective feeding.