A meta-analysis reveals knowledge gaps in our understanding of the *Spartina*-*Geukensia* mutualism

Abstract

The relationship between Atlantic ribbed mussels (Geukensia demissa) and smooth cordgrass (Spartina alterniflora) represents a textbook example of a facultative mutualism. It is also foundational to wetland ecosystems along the Atlantic coast of North America. Spartina plantings play an essential role in marsh reconstruction projects, and Geukensia is rapidly gaining popularity as a tool in coastal restoration to help marshes resist erosion and to remove nutrient pollution. However, it remains unclear whether positive effects documented primarily in natural settings will translate to urban and constructed estuaries where they are most needed. Using systematic review and meta-analysis, we compared the influence of the Spartina-Geukensia mutualism on growth of the species and on nitrogen-cycling processes. Our review suggests the potential for context dependence in this mutualism, with substantial implications for coastal marsh resilience and restoration. Notably, the positive effect of the species on the growth of their mutualist found in natural marshes was absent in constructed marshes and negative in urban marshes. Encouragingly, the effect of each species on nitrogen cycling was similar regardless of context. We found that this mutualism remains severely understudied in urban and constructed marshes, and we suggest future directions to address the knowledge gaps identified in our analysis.