Re-colonisation of a relocated boulder reef in Tauranga Harbour, New Zealand

Abstract

Tanea Shelf at the base of Mount Maunganui (Bay of Plenty, New Zealand), is a subtidal boulder reef that supports a diverse marine community. To allow for expansion of the Port of Tauranga, 3.2 ha of this reef was dredged up and relocated less than 0.5 km away in Pilot Bay. Colonisation of the relocated boulders was assessed every three months from May 1993 to May 1994 and with a final survey in January 1995. Random photo-quadrats were taken on marked and random boulders, and a random dot method was used to estimate percent cover and species diversity.

Monitoring of the relocated reef and the original Tanea Shelf documented the initial colonisation of the Pilot Bay boulders and demonstrated the increasing species diversity of the newly formed community with time. By the end of the study the boulders at the west end of Pilot Bay reef had gained a species diversity similar to that found on remaining Tanea Shelf boulders. However, community composition at the end of this study varied between sites, with greater sponge and Ecklonia radiata cover recorded on Tanea Shelf.

Epibiota was also compared between those boulders that had a cover of encrusting organisms that survived the relocation and those relocated boulders that were initially bare. After 20 months of monitoring, bare boulder surfaces had been colonised sequentially by barnacles, small turfing red algae, and large brown algae. Relocated sponges decreased in percent cover over time, either outcompeted by mussels or smothered by sediment. Relocated ascidians however maintained their space over time but total area covered was small (~5%). Mussels Perna canaliculus were also introduced to the community on some of the west end boulders. Initially the pre-colonised boulders had a higher diversity than the bare boulders. However, the mussels grew to dominate the boulders on which they were present, limiting the epibiotic diversity. It is suggested that the relocated mussels did not hasten community development. Once the mussels die and are not replaced, as indicated by the lack of recruiting spat, it is expected that species succession will revert to the pattern observed on the initially bare boulders, with no net gain provided by the mussel phase.

Monitoring of the eastern end of the relocated Pilot Bay reef started in April 1992. Community composition of the boulders at the east end differed from that at the west end as mussels were absent from the eastern end of the reef. Suspended sediment loads over the reef were monitored using sediment traps. The eastern end of Pilot Bay reef was more heavily affected by sediment deposition. This sediment probably affected species' settlement and growth. Short-term colonisation plates reflected, among other factors, the effects of settled sediment.

Development of the encrusting community on the relocated boulders is advancing and the reef now (January 1995) supports a variety of motile species. However, further monitoring once the relocated community has developed to a stable situation is necessary to determine whether the relocated reef has indeed replaced the habitat lost on Tanea Shelf. Conclusions about the possible benefits of using pre-colonised boulders to enhance community development were confounded by the effects of sediment and the unexpected inclusion of mussels in the surveyed community and their subsequent role in succession.