Greenhouse gas response and Spartina alterniflora resilience to macroalgal exposure

Climate change can alter salt marsh plant communities and their associated carbon flux dynamics via several mechanisms. Due to warming waters and sea level rise, macroalgal wrack accumulation rates in salt marshes are expected to increase. The smothering and shading effects of macroalgal wrack can have adverse effects on salt marsh vegetation. Most studies have focused on the impacts of high-density accumulation or future accumulation projections, but the impacts of current accumulations of macroalgae on Spartina alterniflora (S. alterniflora) are unclear. We investigated how current, relatively low-density ephemeral (Ulva spp.) and perennial (Fucus spp.) algal wrack coverage impacts S. alterniflora survival and associated greenhouse gas fluxes. We created mesocosms with S. alterniflora-vegetated soil cores using a 2 × 2 factorial design with Fucus and Ulva present or absent. After one month, S. alterniflora cores with Ulva (but no Fucus) had a significantly lower stem density, but they later recovered, attesting to S. alterniflora's resilience to low-density macroalgal coverage. Our results also suggest that Fucus can alter the interaction between Ulva and S. alterniflora by potentially mitigating Ulva's impacts. Macroalgal presence did not significantly influence greenhouse gas fluxes, which suggests the low-density exposure is not altering the salt marsh carbon flux dynamics.