Ulva additions alter soil biogeochemistry and negatively impact Spartina alterniflora growth

Decaying mats of Ulva can be washed into salt marshes by the tides as large wrack deposits, especially in eutrophic estuaries, where they can negatively impact marsh vegetation. Using field and laboratory experiments, we examined the effects of decomposing Ulva on Spartina alterniflora growth, soil biogeochemistry and nitrogen dynamics. High levels of Ulva exposure resulted in reductions in above- and belowground biomass, while lower levels of Ulva exposure resulted in reductions in only belowground biomass. Porewater ammonium in soil that contained decomposing Ulva quickly attained potentially toxic levels. In addition, amending soil with Ulva led to elevated porewater concentrations of sulfide and trithiane, an organosulfur compound and potential biocide. Use of a ¹⁵N tracer documented plant uptake of Ulva-derived nitrogen, but higher nitrogen availability did not stimulate growth. Our findings support the hypothesis that decaying Ulva mats may create hotspots of adverse physiochemical conditions in salt marshes. However, because our Ulva additions were higher than typically found in coastal marshes, additional field and laboratory studies are needed to establish more firmly whether similarly adverse responses are observed under natural conditions.