The Role of a Faunal Engineer, Geukensia demissa, in Modifying Carbon and Nitrogen Regulation Services in Salt Marshes.

Interest in leveraging suspension feeders, such as marine bivalves, to exert top‐down control on organic matter (OM) loading in estuaries is gaining momentum. Not only can these faunal engineers alleviate the consequences of nutrient pollution, but they may also bolster the critical blue carbon services provided by coastal ecosystems—a potential dual, mitigating effect on cultural eutrophication and climate change. Ribbed mussels, Geukensia demissa, offer a useful model for assessing faunally driven carbon (C) and nitrogen (N) processes in these systems and their relationships with faunal density. Combining bulk geochemical analyses with Bayesian stable isotope mixing model frameworks (MixSIAR), we quantified the effect of mussels on the source and amount of organic C and N deposited to the benthic floor (i.e., sedimentation), accumulated in surface sediments, and stored in aboveground Spartina alterniflora in Georgia salt marshes. Relative to areas without mussels, mussel presence shifted the source of deposited and accumulated OM to a more allochthonous makeup; amplified the amount of deposited, but not accumulated, allochthonous and autochthonous OM; and enhanced aboveground storage of C and N. Both sources of OM accumulated in sediments as well as standing stocks of C and N were highly and positively correlated with local mussel density (ind. m−2) but unrelated to neighboring mussel density (ind. ∼25 m−2) in adjacent, non‐mussel areas. This work provides new evidence that suspension feeders, through their faunal engineering activities, can interact powerfully and synergistically with primary producers to enhance the blue carbon services of marshes and counteract coastal eutrophication. Plain Language Summary: Coastal ecosystems regulate carbon and nitrogen generated both inside and outside of their physical extents—important ecosystem services that can help combat the consequences of pollution and climate change. Suspension‐feeding animals living in these ecosystems, like mussels, may enhance this regulation by feeding on algae suspended in the overlying tidewater and depositing their waste to the habitat bottom. There, these deposits can accumulate in sediments or be taken up by plants. We studied how ribbed mussel presence and density influence carbon and nitrogen regulation processes in Georgia, USA salt marshes. By comparing deposits in marsh areas with and without mussel aggregations with elemental and stable isotope analyses, we found that mussels amplify the reclamation and import of marsh‐derived and marine‐derived carbon and nitrogen, respectively. We took a similar approach to sediment and found that concentrations of both sources of carbon and nitrogen increase as mussel density increases within their aggregations. Finally, we found that mussel presence and density boosted cordgrass biomass and, in turn, carbon and nitrogen storage. Mussel populations are disproportionately augmenting the role of salt marshes in regulating carbon and nitrogen pollution and should be incorporated into coastal management decisions related to these challenges. Key Points: Through suspension feeding and biodeposition, mussels augment sedimentation of organic matter (OM) produced inside and outside of salt marshesOM accumulation in sediments and aboveground storage increases with increasing mussel densityMussels bolster the capacity of salt marshes to counteract coastal eutrophication and provide blue carbon services [ABSTRACT FROM AUTHOR]