Snail grazing facilitates growth of two morphologically similar bloom-forming Ulva species through different mechanisms

Summary

1. Herbivores can facilitate macrophyte growth through stimulation of overcompensation pathways, removal of neighbouring or fouling competitors or increasing nutrient availability via nitrogenous waste inputs.
2. We examined the facilitative relationship between the mud snail Ilyanassa obsoleta and two morphologically similar bloom-forming Ulva species, Ulva compressa and Ulva rigida. Through mesocosm experiments, we determined the occurrence and underlying mechanisms responsible for facilitating Ulva growth. In situ snail enclosure experiments determined the occurrence of this facilitative pathway under natural conditions.
3. We found that both U. compressa and U. rigida are facilitated by I. obsoleta, but despite their morphological similarity, the primary facilitative mechanism differed for each species. Ulva compressa experienced increased growth via snail nitrogenous waste inputs, while U. rigida was facilitated by the removal of fouling microalgae. These results suggest that physiological differences exist between these two species, potentially sustaining their coexistence within blooms.
4. Ground tissue assays showed I. obsoleta preference for microalgal foulers in whole-tissue assays was driven by Ulva blade morphology rather than chemical or nutritive properties. This indicates that the underlying mechanism for I. obsoleta's facilitation via grazing is an inability of the snail's radula to effectively puncture Ulva blades, confining their grazing activities to fouling organisms on the blade surface.
5. In contrast to our mesocosm results, in situ experiments failed to find a positive impact of mud snail presence on Ulva blade growth. However, Ulva blades within these experiments were located at a highly eutrophic site and isolated from potential competitors. Consequently, this may have created an environment where I. obsoleta facilitation was not required for maximum Ulva growth.
6. Synthesis. We demonstrated two mechanisms by which top-down forces can facilitate macroalgal growth: herbivore nitrogenous waste inputs and removal of microalgal fouling organisms. This facilitation may occur within the large mats of macroalgae that form during bloom events, exacerbating bloom conditions.