Fouling communities are tractable ecological systems that form on manmade structures added to the marine environment. Their proximity to the water’s surface allows hypotheses about community assembly, disturbance, and invasion biology to be tested and these communities have become model systems in the marine realm. Much of this work has focused on sessile invertebrates but, on hard substrates in the marine environment, invertebrates and macroalgae, organisms from two different kingdoms, can compete for the same limiting resource, space. On rocky reefs, macroalgae and invertebrates are partitioned between horizontal and vertical walls respectively, but invertebrates are able to persist on horizontal surfaces if algae are excluded by shading, suggesting that algae presence may exclude invertebrates or impact invertebrate post-settlement mortality. I conducted four manipulative experiments on floating docks to test if algae also excluded invertebrates in fouling communities, including potential mechanisms for this by using mimicked (putative plastic) and modified (distal thalli removed) algae. In three out for four experiments, macroalgae did not exclude invertebrates but did alter invertebrate community composition, communities with algae tending to have more native species whereas communities without algae were dominated by invasive ones. In one experiment, macroalgae also appeared to facilitate invertebrate settlement in the early stages of community assembly, mediated by both algae structure and natural chemical cues. If macroalgae can confer invasion resistance i.e., its presence in a community can shift assemblages towards those composed of more native versus invasive species, then managing floating docks to enhance algae persistence can help prevent species invasions. Recommendations include constructing docks out of materials that allow sufficient light for photosynthesis, orienting docks towards the sun, and adding surfaces and structures that can enhance algae growth and persistence. Marinas should also be managed to reduce sediment and pollutants, factors that negatively impact algae survival. Furthermore, to advance the field of invasion biology, more studies should investigate facilitation, indirect effects, and interactions between organisms from different taxonomic groups.

I conducted four experiments in a New England fouling communities to investigate if macroalgae exclude sessile invertebrates on the shallow sunlit sides of floating docks, as well as potential mechanisms for this. The experiments were carried out at a depth of 1m on polycarbonate settlement plates suspended vertically off floating docks at Dorchester Yacht Club, Boston, Massachusetts, (42.305556°N, 71.046111°W). The fouling community was composed of sessile algae and invertebrates typical of this region, including red and green algae, sponges, bryozoans, polychaetes, molluscs, barnacles, and colonial and solitary ascidians.

At the end of the experiments, settlement plates were collected, all algae and arborescent invertebrates trimmed to enable primary space occupiers to be identified, and communities photographed using an Olympus Stylus Tough 8010 camera. Digital photographs were later enlarged on a screen and percent cover and species richness of space occupying sessile invertebrates quantified using 200 random points overlaid on the central 12x12 cm area of a settlement plate to avoid edge effects.