Thumbnail Image

Monitoring of subtidal reef biota off Motuotau Island in relation to dredge spoil dumping by the Port of Tauranga Ltd 2014-2018

Background • Dredge spoils generated from the development and maintenance of the Port of Tauranga is disposed of on the inner shelf off Mt Maunganui. • Major channel deepening and widening programmes were undertaken in 1992 and 2015/16. Maintenance dredging is undertaken more regularly. • Since 1990, monitoring has been conducted to determine if the dumping of dredge spoils was affecting subtidal reef communities around Motuotau Island, as island off Mt Maunganui beach and inshore of the dredge spoils disposal area. • Three sites at Motuotau have been monitored since 1990. A site to the east of Motuotau has been monitored as a control site and two sites to the north-east and north-west of Motuotau have been monitored as impact sites. • Monitoring has consisted of photographic monitoring of permanent transects close to the reef-sand boundary to record (a) changes in the elevation of the seabed relative to the rocky reef, and (b) changes in the biological community over time. • Monitoring has taken place in 1990, 1991, 1993, 1995, 1998, 2001, 2006, 2009 and 2012. • One of the difficulties of the monitoring programs was that the marker pins, which are cemented onto the reef, would often be dislodged between surveys and would need to be reinstated prior to each new survey taking place. This made it difficult to be sure that the same section of reef was monitored on each occasion and that the measurement of changes in seabed height was accurate. • In 2013, a decision was made to reinstate marker pins for the permanent transects. In January 2014, holes were drilled into the reef and stainless steel rods were secured in these holes using epoxy cement. It is unlikely that these marker pins will ever be dislodged from the reef. • At the same time, the monitoring methodology was reviewed and a number of changes recommended. Most significantly, it was decided that the reef being monitored as a control site did not really constitute a control against the effects of dredge spoil dumping and subsequently all three sites would be considered as impact sites. • Monitoring took place in January and May 2014, November 2015 and April 2018 and is reported on here. Findings • Marker pins were reinstated in January 2014 and four and a half years later remain solidly in place. • In general terms, the reefs around Motuotau appear to be healthy and in a state that is comparable to other reefs of similar depth, aspect and exposure along the Bay of Plenty coastline. • The 2014, 2015 and 2018 surveys did not record any major changes in the biological communities of the reefs around Motuotau. • Measurements of the distance between the bottom marker pin at each site and the reefsand boundary indicate that the reefs are not being inundated by dredge spoils. Seabed heights have fluctuated by as much as 30 cm since 2014 and this is not thought to be outside the scale of fluctuations that would naturally occur given the aspect and energy environment of the surveyed reefs. • Photographic surveys continue to prove useful in capturing information about the diversity of subtidal reef communities. Although there is some evidence of minor changes in these communities over time this is to be expected. There is also evidence of stability in these communities with a number of sponges on these transects having been present for several decades.
Technical Report
Type of thesis
ERI report
Ross, P. M. (2018). Monitoring of subtidal reef biota off Motuotau Island in relation to dredge spoil dumping by the Port of Tauranga Ltd 2014-2018 (ERI report). Hamilton, New Zealand: Environmental Research Institute, University of Waikato.
Environmental Research Institute, University of Waikato
© 2018 the authors.