Macroalgal bloom-forming species occur in coastal systems worldwide. However, due to overlapping morphologies in some taxa, accurate taxonomic assessment and classification of these species can be quite challenging. We investigated the molecular and morphological characteristics of 153 specimens of bloom-forming Ulva located in and around Narragansett Bay, RI, USA. We analyzed sequences of the nuclear internal transcribed spacer 1 region (ITS1) and the chloroplast-encoded rbcL; based on the ITS1 data, we grouped the specimens into nine operational taxonomic units (OTUs). Eight of these OTUs have been previously reported to exist, while one is novel. Of the eight OTUs, all shared sequence identity with previously published sequences or differed by less than 1.5% sequence divergence for two molecular markers. Previously, 10 species names were reported for Ulva in Rhode Island (one blade and nine tube-forming species) based upon morphological classification alone. Of our nine OTUs, three contained blade-forming specimens (U. lactuca, U. compressa, U. rigida), one OTU had a blade with a tubular stipe, and six contained unbranched and/or branched tubular morphologies (one of these six, U. compressa, had both a blade and a tube morphology). While the three blade-forming OTUs in Narragansett Bay can frequently be distinguished by careful observations of morphological characteristics, and spatial/temporal distribution, it is much more difficult to distinguish among the tube-forming specimens based upon morphology or distribution alone. Our data support the molecular species concept for Ulva, and indicate that molecular-based classifications of Ulva species are critical for proper species identification, and subsequent ecological assessment or mitigation of Ulva blooms.

Herbivory often has a negative effect on plants. However, there is a growing number of examples, primarily in terrestrial ecosystems, where herbivory causes an increase in plant size, growth rate, and/or reproductive output. In marine ecosystems, the positive effects of herbivores on macroalgae are not as well studied, although limited evidence exists for herbivore-induced increases in macroalgal growth rates via 2 mechanisms: nutrient addition via grazer defecation, and epiphyte removal. In this study, we examined the effects of grazing by the mud snail Ilyanassa obsoleta on Ulva lactuca, the dominant bloom-forming macroalga in many New England estuaries. We found that the presence of I. obsoleta had a significant positive, rather than negative, effect on U. lactuca. Through caging experiments, we established that this positive effect was not due to fecal nutrient inputs from the snail. Similarly, snail contact in the absence of grazing did not affect U. lactuca growth rates. In contrast, grazing by I. obsoleta significantly reduced the microalgal cover on heavily fouled U. lactuca blades, indicating that snail herbivory of microalgal films likely facilitates U. lactuca growth. Our study demonstrates the surprising, positive effect that an abundant herbivore can have on the growth of a macroalgal bloom species.

Summary1. 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.

Short CommunicationLiving particulate fluxes in throughfall and stemflow during a pollen event.

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