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Stable isotopes reveal that foraging strategy dictates salt marsh residents’ trophic response to black mangrove (Avicennia germinans) range expansion

Katherine B. Loesser*, Christina E. Powell, Brandeus Davis, Melissa M. Baustian, Michael J. Polito

*Corresponding author:

ABSTRACT: Climate warming has facilitated the expansion of black mangrove (Avicennia germinans, hereafter “Avicennia”) into southeastern Louisiana’s smooth cordgrass (Spartina alterniflora, hereafter “Spartina”) salt marshes. As macrophytes contribute to soil organic matter (SOM) and primary production, this transition could alter the basal energy pathways supporting salt marsh food webs. We used bulk-tissue and compound-specific stable isotope analyses (SIA) to determine if changes in dominant macrophytes alter basal energy pathways for two salt marsh residents: grass shrimp (Palaemonetes spp.) and marsh periwinkle snails (Littoraria irrorata). Specifically, we used Bayesian stable isotope mixing models to quantify the relative contribution of basal energy sources to SOM and resident food webs across a SpartinaAvicennia gradient in southeastern Louisiana. We found that sources of SOM changed in Avicennia-dominated habitat and that foraging strategy dictated salt marsh residents’ trophic responses to Avicennia expansion. Marsh periwinkle snail basal energy sources shifted from Spartina to algae (phytoplankton and epiphytic macroalgae) reliance, while grass shrimp basal energy sources remained reliant on algal production, regardless of macrophyte dominance. Compound-specific SIA improved basal energy source distinctions and provided more constrained estimates of their contributions to resident food webs than bulk-tissue SIA. The importance of algal energy across the landscape warrants future investigations into the ability of Avicennia to support the diversity and abundance of algal energy sources present in Louisiana salt marshes. Understanding coastal wetland food web dynamics could help with planning and evaluating the most effective coastal restoration techniques (e.g., prioritizing salt marsh or mangrove habitat) in southeastern Louisiana.