Sea-level rise and macroalgal blooms may combine to exacerbate decline in Spartina patens and Spartina alterniflora marshes

Abstract

Inundation stress from sea-level rise will alter the structure and function of coastal marshes, but will not act singularly on marsh flora. In eutrophic estuaries, where macroalgal blooms occur, sea-level rise will increase macroalgal deposition. Macroalgae may stimulate marsh grass productivity via nutrient addition, but may inhibit growth of marsh flora via shading or culm breakage. We tested the response of Spartina alterniflora and Spartina patens (representative of low and high marsh flora, respectively), to three levels of projected sea-level rise with the presence or absence of macroalgae. We observed declines in stem density in all treatments, but found that extreme inundation leads to more rapid and almost total decline. Responses of Spartina species diverged under moderate inundation, where S. alterniflora stems exhibited growth, and density declines were not as severe. Macroalgae had a negative impact on both grasses, resulting in exacerbated stem density decline. We observed a significant interaction of sea-level rise and macroalgae resulting in declines of stem density in S. alterniflora. Our findings demonstrate differences in marsh zone response to inundation and highlight negative impacts of macroalgae on marsh health, pointing to mechanisms by which sea-level rise alters marsh structure.