Optimising macroalgal diversity and ecosystem function on coastal infrastructure - an eco-engineering approach

Artificial structures are becoming ubiquitous features of coastal landscapes. The habitat these provide differs from natural shorelines in several ways, and consequently there has been significant interest in eco-engineering, which combines ecological and engineering knowledge to design artificial structures to support more natural biological assemblages. A number of designs have been developed, but most have been tested in only one context. Research on how biota on artificial structures compares to natural shores has focussed primarily at the level of diversity and community structure, and we do not yet have a clear understanding of how ecological processes may differ between these two habitats. The biological responses of key habitat-formers on artificial structures are also not yet well understood. In terms of eco-engineering, stakeholders perceive a lack of evidence that such interventions contribute to biodiversity and ecosystem function on artificial structures, and thus there is a need to strengthen the existing evidence base. In Chapter 2, I cleared plots on seawalls and natural shores at two separate timepoints and monitored these over one year to determine the influence of timing of disturbance on recruitment and succession. Additionally, I cleared plots on one seawall at a single timepoint, installed exclusion cages and monitored these over 18 months in order to determine the influence of grazing pressure on colonisation. I found that timing of disturbance was significant in determining biofilm structure on both types of habitat after 3 months, and in determining species richness on both shore types after 6 months. Timing of disturbance also influenced the direction and trajectory of the subsequent successional sequence. Grazer exclusion increased biofilm concentrations on artificial structures, and influenced community composition in comparison to plots without exclusion. In Chapter 3, I characterised and compared populations of a key habitat-forming species on natural and artificial substrata in terms of abundance, growth rate, survival and aspects of reproduction. Data were collected from artificial structures and natural shores along the coasts of Ireland and Wales. Abundance and growth rate did not differ between artificial and natural shores. While there was considerable site-level variation, populations on natural shores produced a higher number of mature receptacles overall during the peak reproductive period in April, and had lower rates of dislodgement compared to artificial structures. In Chapter 4 I investigated the ecology and function of seaweed assemblages on artificial bolt-on rockpools on seawalls established in different environmental contexts along the Irish coastline. A total of 32 artificial rockpools were retrofitted on eight seawalls, and were monitored regularly for a period of two years. Data were collected in terms of productivity, community respiration, and community composition. Seaweed colonisation consisted of early dominance of ephemeral species followed by the appearance and establishment of perennial habitat-formers. Overall species richness differed both among sites and among structural sections of the artificial rockpools. By 24 months, all sites supported populations of large habitat-forming seaweeds. Productivity and community respiration differed among sites, but not among environmental contexts at any timepoint. This thesis serves to highlight the impacts of artificial structures on coastal biodiversity and ecosystem function, particularly in terms of ecosystem processes and the ecology of habitat-forming species. Knowledge of how these characteristics differ on artificial structures compared to natural shores, along with an understanding of how ecological processes are altered on these structures, will aid