Multiple roles of a dominant seaweed in driving environmental conditions and community structure

Samuel A. Mahanes, Ecology & Evolutionary Biology, University of California, Irvine, Irvine, CA, Matthew E. S. Bracken, Ecology & Evolutionary Biology, University of California Irvine, Irvine, CA and Cascade J. B. Sorte, Ecology & Evolutionary Biology, University of California, Irvine, CA

Background/Question/Methods
Climate warming and ocean acidification are threatening marine biodiversity. The ecological effects of these changes may be mediated by dominant species, which may alter local environmental conditions and structure ecosystems. To investigate these potential roles, we studied Neorhodomela oregona, which is the most abundant seaweed species in tide pools near Sitka, Alaska. We tested two hypotheses regarding the effects of dominant species. First, we hypothesized that dominant species can affect environmental conditions. We specifically predicted that Neorhodomela would raise pH during the day, cause pools to acidify more quickly at night, and reduce water temperature. Second, we hypothesized that dominant species are important drivers of community structure, and we predicted that Neorhodomela would increase species richness, reduce the abundance of other producers, and increase the abundance of periwinkle snails (the most abundant mobile invertebrate at the site). We manually removed all Neorhodomela biomass from five tide pools and left five as controls. We then measured the rate of pH change over time during the day and at night, recorded temperature with data loggers, and assessed community change with bi-weekly surveys. In a complementary experiment, we added Neorhodomela biomass to five mesocosms, with three control mesocosms, and measured the rate of pH change over time.
Results/Conclusions
In the removal and mesocosm experiments, Neorhodomela abundance was associated with more rapid acidification during the night, likely due to respiration, but did not affect the rate of pH change during the day. Neorhodomela removal did not affect water temperature, nor did it alter total species richness. However, Neorhodomela removal resulted in an increase in producer abundance and a decrease in the abundance of periwinkle snails, effects that diminished as Neorhodomela grew back. Our results suggest that Neorhodomela may be competitively limiting other producers, potentially via shading, while providing habitat for some invertebrates, such as periwinkle snails. These results illustrate the multiple roles that a dominant species can play in modifying environmental conditions and community structure, with potential to leverage ecosystem responses to global change.