Abstract Coastal ecosystems like salt marshes are exposed to anthropogenic nutrient loading from sources including urban run‐off. Although fertilization often drives declines in marsh below‐ground production, outcomes vary according to marsh health. On the U.S. East Coast, cordgrass ( Spartina alterniflora ) forms an important mutualism with ribbed mussels ( Geukensia demissa ), through which mussels benefit cordgrass by delivering nitrogen, among other mechanisms. Despite the ecological importance of this mutualism, scientists have not rigorously examined the interaction between fertilization and mussel mutualists. To disentangle potential interacting effects of fertilization and mussel presence in marshes, we performed a two‐year manipulative field study in a deteriorating Florida salt marsh, examining vegetative growth, porewater, leaf tissue and soil nutrients, and the abundance of resident fauna. We found that fertilization, regardless of mussel addition, significantly increased above‐ and below‐ground production of short‐form cordgrass (370% and 160% respectively). Mussels in unfertilized plots also enhanced above‐ground biomass, stem heights and stem density but less so than fertilization. We further explored impacts of fertilization on cordgrass by manually uprooting stems, finding that fertilizer—not mussel presence—increased resistance to dislodgment. Lastly, we assessed marsh recovery by removing all above‐ground vegetation and found that fertilization led to 2–3.5x more regrowth. Apparent antagonism between our treatments reduced regrowth in plots with mussels and fertilizer, while mussels improved regrowth in unfertilized plots. Multivariate analyses of nutrient metrics and fauna indicate that, absent mussels, fertilized and unfertilized plots were highly dissimilar. Mussel addition resulted in increased similarity between fertilized and unfertilized plots. Synthesis and applications . Contrary to our predictions, fertilizer addition promoted below‐ground production, while also increasing regrowth and resistance to uprooting. While the impacts of mussel addition appeared minimal relative to those of fertilization, mussel presence tended to reduce the variability of several metrics (stem density, stem height, faunal community), while fertilizer alone increased it (above‐ground biomass, faunal community). Our findings demonstrate that fertilization may help degrading cordgrass marshes overcome growth‐limiting conditions, triggering a positive feedback loop of production. As such, we suggest that strategic, short‐term fertilization may promote survival of degrading marshes while they await large‐scale restoration.
Wellman et al. (Wed,) studied this question.