The diet of saltmarsh consumers

Abstract

Salt marshes are areas of high primary production that in turn support high secondary production. Macrophytes, phytoplankton and a variety of benthic algae all contribute to the high primary productivity. Each has the potential to contribute to the saltmarsh food web making it difficult to determine organic matter resources important to secondary production in salt marshes. Furthermore, our understanding of saltmarsh food webs is complicated by anthropogenic effects such as nutrient inputs and exploitation of key predators. I utilized a combination of natural abundance stable isotopes, isotope tracer additions, gut content analysis and manipulative experiments to determine the diets of saltmarsh consumers collected from tidal creeks within Plum Island Estuary, Massachusetts, USA (42¢ª44'N, 70¢ª52'W) and to address the effects of nutrient additions and species modifications on the saltmarsh food web. Results indicate detritus of the vascular marsh plants, Spartina alterniflora and S. patens, was of limited dietary importance to many consumers. Instead, microalgae, including epipelic and epiphytic diatoms and phytoplankton were dominant basal resources in the saltmarsh food web. However, Spartina dietary contributions increased in the high-marsh intertidal later in the growing season. Furthermore, the addition of nutrients altered the relative contribution of basal resources to the diet of infauna. Specifically, nutrient additions facilitated a change in the diet of an oligochaete, Cernosvitosviella immota, from macrophyte detritus to epiphytic algae and a switch within algal resources from tidal resources (e.g., phytoplankton) to local sediment associated algae (e.g., epipelic and epiphytic microalgae) for two polychaetes, Nereis diversicolor and Manayunkia aestuarina. This strongly suggests that anthropogenic nutrient inputs have the potential to increase the relative importance of algae to primary consumers and ultimately higher trophic levels. My research further suggests the diet of an important intermediate omnivore, Palaemonetes pugio, was altered with the reduction in abundance of a top omnivore, Fundulus heteroclitus. Some P. pugio became more carnivorous when F. heteroclitus where reduced indicating top-down control of infauna by P. pugio mediated through behavior. In the long-term, anthropogenic effects could fundamentally alter food web structure by changing saltmarsh species composition and linkages between primary producers and higher trophic levels.