Abstract Fouling communities are tractable ecological systems that commonly develop on artificial structures in the marine environment, where hypotheses about community assembly, disturbance, and species invasions can be tested. These systems are strongly space limited, and competition for primary substrate is a key process that shapes the structure and composition of sessile assemblages. While research has largely focused on sessile invertebrates, which dominate on artificial surfaces and are frequently non-native in origin, far less is known about how macroalgae influence invertebrate assemblages in these systems. Using a series of manipulative experiments on floating docks in a New England marina, I examined whether macroalgae affect invertebrate establishment, settlement, and community composition, and whether these effects differ between native and non-native taxa. In longer-term experiments, macroalgae did not reduce invertebrates but were associated with shifts in community composition, with higher relative abundances of native taxa, particularly sessile molluscs, in communities with algae present. In contrast, communities without algae were characterized by higher relative abundances of non-native colonial ascidians. In a short-term settlement experiment, macroalgae facilitated invertebrates, with both structural features and potential chemical cues appearing to contribute to higher recruitment and diversity. These results suggest that macroalgae can influence community assembly by altering the balance of invertebrate assemblages towards those containing native species and by facilitating settlement, rather than by directly excluding invertebrate species. Because outcomes varied with life stage, algal identity and experimental timescale, the role of macroalgae in modifying invasion outcomes is likely to be context dependent. Understanding interactions between autotrophs and invertebrates in fouling communities may help elucidate indirect pathways that influence invasion patterns on artificial marine structures.
Martine Wagstaff (Mon,) studied this question.