The role of post-settlement processes in determining fouling assemblages on artificial reefs

Author: Shea Cameron

Cameron, Shea, 2018 The role of post-settlement processes in determining fouling assemblages on artificial reefs, Flinders University, College of Science and Engineering

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Eco-engineering of marine habitats is becoming more relevant as anthropogenic additions to hard substrate through ocean sprawl accelerate. Knowledge is needed on the material and structure useful for artificial hard substrate habitats, and on methods for effective monitoring. This project compared assemblages on artificial and natural reefs and then investigated post-settlement processes that could have contributed to the differences between the reefs. This study also compared and contrasted new methods for comparing and monitoring reefs. The outcomes from this project will help to inform coastal managers and other researchers as well as provide the foundation for practical interventions to increase biodiversity.

Fouling communities on established artificial reefs were examined and compared to determine substrate specific differences after decades of succession. Whereas most comparative studies only use a single artificial and natural reef, this project compared seven different aged reefs made from three different artificial materials and one natural substrate type. The community analyses derived from two methods, image analysis and physical sampling, were also compared. Reefs had dissimilar fouling communities even if they were the same underlying substrate type. The only purpose built reef, made from tyre modules was one of the least diverse sites and was probably affected by regular disturbances caused by its elasticity in the materials used.

Sampling fouling communities using image analysis showed greater similarity across substrates and reef types, because of lower level taxa identifications than obtained from scrapings. Image analysis can underestimate assemblage diversity and miss cryptic invasive species, but was the most accurate method to efficiently sample colonial macroinvertebrates. The relationship between biotic complexity (macro algae and macroinvertebrate cover) and non-colonial invertebrates was also examined in this study using functional habitat groups. This study highlighted how the choice of methodology in visual surveys can influence the assessment of fouling communities.

Four common artificial substrates (concrete, rubber, steel, and wood) were deployed at four different sites along the coast of the Gulf of St. Vincent in South Australia every season and retrieved after three months. There were no seasonal differences in the recruitment onto the plates, but some site and substrate specific differences were recorded. This chapter indicated that there was continuous and wide spread larval recruitment in this southern temperate gulf and substrate specific recruitment trends over the sampled period.

Competition for space on hard substrates is well documented, but large macroinvertebrates and macroalgae also provide complex 3-D structures that could positively affect recruitment and perhaps functional diversity. As artificial reefs usually have communities that are different to those found on natural counterparts, similarity on a function level was examined instead. Functional diversity was explored using Biological Trait Analysis (BTA). To determine if sessile macroinvertebrates facilitate recruitment and increase species and functional diversity, three structurally complex macroinvertebrates were translocated onto settlement plates and colonisation compared after six months. The diversity of taxa was highest on the settlement plates that had the most complex shaped facilitator, a bryozoan. Functional diversity was not significantly different between any of the treatments, however, more mobile species had colonised plates with facilitators of greater structural complexity. This experiment emphasised the importance of biogenic habitat heterogeneity and positive relationships on artificial reefs. The results demonstrate that abiotic and biotic substrate differences affect the development of fouling assemblages on artificial reefs and that species and functional diversity can be increased by the presence of structurally complex organisms.

The overall findings of this study suggest that managing any hard substrate reef is a complex task. The idea of managing or creating ecosystems that have been lost to habitat degradation, may not be realistic or feasible. Approaching the task from the viewpoint of achieving minimal harm to the overall system and some functional similarity may be the best approach for marine resource managers.

Keywords: Artificial Reefs, Artificial Substrates, Biofouling, Fouling, Post-Settlement Processes, Facilitation, Biological Trait Analysis, Shipwrecks, Tyre Reefs, Jetty Pylons, Temperate Reefs, Marine Management

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Sabine Dittmann