Restoration trajectories and food web linkages in San Francisco Bay’s estuarine marshes: a manipulative translocation experiment

Abstract

We measured δ 13 C, δ 15 N and δ 34 S signatures of natural and translocated mussels Ischa- dium demissum to identify food web source differences among estuarine marshes displaying various stages of restorative development. We hypothesized that mussels inhabiting younger marshes would be more dependent on allochthonous organic matter sources, while those inhabiting mature marshes would depend on autochthonous sources. Mussels collected from an undisturbed (reference) marsh located within the Napa River estuarine complex in San Francisco Bay were translocated to a series of restoring marsh sites located within the same river system. The isotopic composition of naturally growing mussels was compared with translocated mussels, which were incubated in restoring sites for 5 and 7 mo. Measurements of δ 13 C, δ 15 N, and δ 34 S indicated differences in food web sources sup- porting I. demissum among the 4 marsh sites. A strong cage effect was detected during the initial 5 mo collection interval, indicating that translocated mussels had yet to equilibrate with their new environments. Multiple source mixing model analysis indicated that C3 emergent vascular plants and brackish phytoplankton contributed most of the organic matter consumed by I. demissum over both time periods, but that mussels collected from the downstream sites exhibited higher dependence upon vascular plant detritus. Bay produced phytoplankton contributed very little to I. demissum diets, suggesting that the pelagic waters of San Francisco Bay have less influence on marsh food web dynamics than previously anticipated. The results of this experiment show that food web pathways are strongest at intermediate scales; they can be relatively short and unique to specific marshes along the estuarine gradient, but similarities in mussel diets among marshes in close proximity to one another suggests inter-marsh exchange of organic matter. It is, therefore, likely that food webs in young restoration sites depend upon organic matter subsidies from neighboring marshes, rather than from San Francisco Bay.