The Interaction between an omnivorous mud snail and bloom-forming macroalgae is context-dependent in shallow estuaries

Eutrophication due to anthropogenic nutrient enrichment is the greatest factor leading to estuarine degradation. Even after external nutrient loading has been reduced, internal nutrient recycling has the potential to keep the system in a eutrophic state. In field studies, an association between the Eastern Mud Snail, Ilyanassa obsoleta (Say), and the opportunistic green macroalgae Ulva sp., has been observed and attributed to the detrital food source associated with the algal mat. In this study, we sought to determine the spatial and temporal context-dependence of this relationship, the reciprocal benefits of the association, and the potential feedbacks to macroalgal bloom formation in shallow coastal systems. In West Falmouth Harbor (WFH), MA, we conformed the association in the more eutrophic Inner Harbor (IH) during the reproductive period when the macroalgae likely provides valuable oviposition substrate for I. obsoleta, but saw no trend in the less-impacted South Harbor (SH) or later in the summer when macroalgal biomass was lower and snails were not reproductive. In a microcosm study using sediments from two sites in WFH, I. obsoleta increased NH₄⁺ flux to the water column likely due to direct excretion of NH₄⁺ and dissolved organic nitrogen and grazing of benthic microalgae that cap the sediment surface and prevent the release of NH₄⁺ to the water column. Gross primary production and net ecosystem metabolism were both decreased in the presence of snails, but only for the relatively sandy, low organic matter site (SH). Ulva sp. grew better when fertilized with snail excreta that with other individual inorganic and organic nitrogen sources. We observed differences in algal growth in the lab in the presence of sediment, and in the field across site and season, but could not confirm enhanced growth of macroalgae in the presence of snails in spite of the clear effect of snails on the release of nutrients from the benthos to the water column. The interaction between I. obsoleta and Ulva sp. is context dependent, with a stronger relationship in muddy eutrophic environments and early in the summer. Overall, our study uncovered new information regarding the complex relationship between I. obsoleta and Ulva sp. that is useful in understanding how internal nitrogen cycling may be controlled by biotic feedbacks and act to maintain macroalgal blooms in shallow estuaries