Foundation species identity and trophic complexity affect experimental seagrass communities

The abundance and species composition of marine foundation species is changing due to range expansion or contraction, with potentially important ecosystem-level consequences. In Chesapeake Bay, USA, warming is likely to favor the more heat and stress-tolerant Ruppia maritima (widgeongrass) over Zostera marina (eelgrass). Because of the key role of seagrasses in providing habitat and trophic support, it is important to determine whether the more stress-tolerant seagrass provides similar ecological functions to the species it may replace. We addressed how trophic control differs between communities associated with the 2 seagrass species in a mesocosm experiment. Grazing of epiphytic algae can benefit seagrasses over competing algae, and crustacean mesograzers are an important link for higher trophic levels. We manipulated seagrass density, species identity, and presence of grazers and predators, and examined the resulting communities of recruiting algae and invertebrates. Overall, predation was higher in Ruppia than in Zostera, although mesograzer species individually differed in their susceptibility to predation and response to seagrass species. The presence of grazers and predators had a greater overall effect on multivariate metrics of fouling community development than did seagrass species identity. Initial densities of seagrass and grazer species had interactive effects on some recruiting microalgae and tunicates. Differences in grazer composition and predation between seagrass species could have consequences for higher trophic levels that rely on fauna in seagrass beds. However, given the considerable effects of manipulated seagrass and mesograzer density on trophic interactions and the fouling community, it may be most important to consider the overall density and distribution of seagrass present, rather than seagrass species identity. Our results highlight the importance of testing redundancy in ecological functions among habitat-forming species