Quick Dips

Rebuilding a Boulder Reef

Megan Graeme

In an innovative attempt to combine development and protection of the environment, the Port of Tauranga has relocated an boulder reef from the main entrance channel to Tauranga Harbour.

The reef, known as Tanea Shelf, was made up of loose stacked boulders, providing the habitat of a diverse fauna in an area of predominantly sandy beaches. Boulders from 0.2-2.0 metres in diameter were dredged from the harbour entrance onto a barge and shifted into a nearby secondary channel.

For 18 months I studied the outcome of this project to see if the original reef community re-established, and compared the rates of establishment on boulders with surviving encrusting life and boulders that were bare, having previously been buried.

Even though not enough time elapsed for a mature stable community to develop on the relocated boulders, the survey found indications that the new community would eventually replicate and replace that lost from Tanea Shelf.

By the end of the study in January, kelp and red algae cover was increasing but sponges and ascidians (notable members of mature, stable communities, often known as sea squirts) were not common. This could be because more time was needed for colonisation by these animal species, or more attractive settlement conditions were required. High suspended and settled sediment levels have probably hindered natural colonisation rates and sequences.

At the western end of the relocated reef, where comparison of "bare" and "pre-colonised" boulders was undertaken, many of the boulders were relocated with a patchy cover of green-lipped mussels. The mussels quickly grew in size, excluding all other would-be colonisers.

This relocated mussel population will eventually regress through predation or senescence, and juveniles are not present on the new reef to replace it. Once the mussels regress, it is predicted that the boulders will follow the sequence found on the bare boulders, being colonised by early settlers such as small red algae and short-lived seasonal species such as barnacles. These species will then be superseded by brown algae and ascidians, and finally sponges.

The pre-existing mussel cover does not seem to have advantaged community development, but has actually set back the successional sequences. Had the study boulders not been dominated by relocated mussels, but rather sponge and ascidian as found at the east end of the reef, a different outcome would be expected.

The effect of sponges would be more subtle than the aggressively competitive mussels. Sponges would probably influence community development by actively repelling some settling species and gradually competing for space by overgrowing less competitive species. In this scenario, unless other competitive equivalents such as ascidians established themselves on the boulders, it would be possible for the pre-established sponges to dominate the new Pilot Bay reef community.

Megan Graeme carries out research from the University of Otago.