Development and long term dynamics of a fouling assemblage of sessile marine invertebrates

The construction of a new pier in upper Spencer Gulf, South Australia, was used as an opportunity to test theories about recruitment and dynamics of subtidal assemblages of sessile invertebrates. The fouling fauna was monitored for ca six years after initial immersion of piles using photographs of fixed positions and direct observation by divers. Assemblages were traced through time using non-metric multidimensional scaling (MDS). Faunal composition differed at sites along the pier throughout the study, but the composition at all sites tended to change in a similar way over time, and seemed to be changing more slowly near the end of the study. Abundances of key taxa fluctuated markedly from site to site along the pier, but for some taxa a trend was discernible over and above the variability. Predictions based on experiments on piers in more sheltered waters in an adjacent gulf were not fulfilled; although over 50% of pile surface area was covered by encrusting or mound-forming colonial animals such as sponges and colonial ascidians, solitary organisms such as bivalves and solitary ascidians which were expected to be overgrown persisted in great abundance throughout the study. Significant differences amongst sites after ca six years, both in assemblages and in abundances of key taxa, did not match environmental variables such as degree of shading, depth of seabed, disturbance due to wave action, or release of treated ballast water, although there were signs of an effect of high current speeds associated with a local gyre.