Trophic Complexity Between Producers and Invertebrate Consumers in Salt Marshes

Abstract

Salt marshes on the Atlantic coast of North America are characterized by having a high biomass of smooth cordgrass, Spartina alterniflora. Because of the refractory nature of the lignocellulosic structure of this angiosperm, invertebrates utilize C from these plants with very low efficiency, if at all. This is true for both living cordgrass and post-senescent plant detritus. To balance their C demands, invertebrate consumers living in salt marshes must utilize a wide variety of other resources, including microheterotrophs (bacteria and bacterivorous flagellates) either associated with detritus or free in the water column, fungi colonizing decaying vascular plants, surface-associated algae (e.g., microphytobenthic diatoms and cyanobacteria, epiphytes, surface film algae) and phytoplankton. This high degree of trophic complexity is likely to be an important source of community stability. As an example, we estimate that ribbed mussels, Geukensia demissa, in a Delaware marsh must rely on a variety of different food resources since no single food type can meet their nutritional demands for either C or N. To balance their C demands, mussels appear to rely mainly on microheterotrophs, followed by phytoplankton > microphytobenthos > cellulosic detritus. Non-detrital foods are even more important for maintaining positive N balance in G. demissa. Previous and emerging evidence from other studies suggests that other important marsh consumers have a similar general diet. Although cordgrass may dominate overall rates of primary production and detritus from cordgrass contributes significantly to secondary production, we challenge the paradigm that salt marshes have a “detritus-based food web.” Further research is needed to deduce the importance of microphytobenthos and microheterotrophs as sources of C and N for dominant animal consumers in these marsh systems.