Combined effects of substratum enhancement and seeding techniques on assemblages in artificial harbor habitats

Coastal urbanization is resulting in shorelines being increasingly built-over and protected by artificial structures which are generally colonized by species poor assemblages dominated by opportunistic organisms. “Eco” approaches integrating ecological research into the design of these structures, such as enhancing substrate complexity and seeding of surfaces with target organisms, provide the chance to improve the structure and function of target-designed novel ecosystems. I tested the effects of two design techniques, specifically tiles mimicking increasing substrate complexity and seeding of habitat forming oysters, on the development of benthic intertidal assemblages at two jetty sites of Ravenna’s port. I used orthogonal combinations of different complexity tiles (flat, 2.5 cm high ridges tiles and 5 cm high ridges) and two seeding conditions (unseeded and seeded with oyster recruits), and measured the developing assemblages over the first 6 months. I also analysed the effect of substrate complexity on seeded oyster survivorship and growth. A great difference between the assemblages at the two sites was detected due mostly to higher presence of algae at the southern site and bare substrate at the northern site. Lower abundance of the opportunistic algae Ulva spp. and Cladophora sp. were registered on more complex tiles. The same algae were more abundant on unseeded tiles. The seeded oysters reduced the presence of opportunistic species by both space pre-emption and by creating biogenic microhabitats. No effects of substrate complexity was observed on oyster survivorship and growth. These results suggest that both increasing substrate complexity and seeding could help reducing the presence of opportunistic species on port structure in the short term. Longer-term observations will be needed to explore the efficacy of these interventions over longer times and their practical applications on port seawalls and other types of artificial structures.