Macroalgae are an important part of coastal communities, providing food, refuge and habitat to numerous organisms. Macroalgal blooms, however, negatively impact coastal ecosystems as well as commercial and recreational activities. To date, most research on the causes of macroalgal blooms has centered on the role of bottom-up (physical) forces including light, temperature and nutrients. Far less research has examined the function of top-down (biological) forces in macroalgal bloom formation and persistence. This study examined the impact of grazing by the omnivorous mud snail, Ilyanassa obsoleta, on three species of bloom-forming macroalgae: Ulva lactuca, U. intestinalis and Gracilaria tikvahiae, in Narragansett Bay, RI. Paired choice feeding assays were conducted to assess if I. obsoleta had a preference for a particular type (attached versus drift) or species of macroalgae. Each assay consisted of 20 replicates, in which three juvenile I. obsoleta were placed in a container with two different macroalgal blades. Autogenic algal changes in biomass were controlled for using 20 non-herbivore replicates. Algal wet mass was measured at the start and end of each assay; assays ran until 1/3 of the algal tissue was consumed, or to 14 days.

Results/Conclusions

In most feeding assays, I. obsoleta showed no consumption preference for different types or species of algae. Similar results were obtained for feeding assays using the herbivorous shrimp Palaemonetes pugio. However, in all feeding assays, algae in the experimental treatment grew more than the algae in the non-herbivore, control treatment. As a result, we designed subsequent assays using the same experimental set up as before, except that we sequestered the snails in a mesh cage to prevent grazing. In these experiments, no significant difference in algal growth was detected between control and experiment treatments. These results indicate that the growth observed in the feeding assays was likely due to overcompensation by the algae via herbivory, not due to causes such as nitrogen enrichment from herbivore excretions; data on %nitrogen tissue content support this conclusion. Thus, common herbivores in this estuarine system may be promoting the growth of macroalgal bloom species, instead of controlling them.