The report found some good news – overall, stable nutrient levels in the harbour and water quality suitable for recreation – but also concerns with other trends. Sea grass area has declined 34 per cent since the middle of last century while mangroves have expanded; some species of fish and shellfish have declined; and nutrient levels in some rivers entering the harbour are elevated.
Professor Murray Patterson, from the School of People and Environment and Planning leads the research consortium called Manaaki Taha Moana, which contributed to the report as part of a harbour research project along with the Cawthron Institute, Manaaki Awanui Trust and a Tauranga based IT company Waka Digital.
He says the reasons for the harbour’s unhealthy state were unclear. “Although there has been a justified concern over the grounding of the Rena and its associated ecological impact on the coast, the longer term decline of coastal systems in Tauranga Harbour is just as concerning – happening more insidiously and under the radar.”
Lead author Jim Sinner from the Cawthron Institute says the survey, which starts next week, is necessary to address the shortfall in knowledge about the ecology of the harbour environment.
“More research is required to determine the cause of the problems before we can begin to restore the health of the harbour.”
His colleague, marine ecologist Dr Joanne Ellis, will lead the survey with support from iwi and hapu, the Bay of Plenty Regional Council and Professor Chris Battershill of the University of Waikato.
The survey is the latest stage of the six-year-long harbour research project that has received funding from the Ministry of Science and Innovation till 2015.
A copy of the report can be downloaded from the Manaaki Taha Moana website http://www.mtm.ac.nz
Further details of the report’s main findings:
The Health of Te Awanui Tauranga Harbour report, which chronicles the current ecological health of the harbour based on the scientific information already available, found some good news and some cause for concern – particularly:
- Nutrients: Reported levels of nitrogen and phosphorus showed little change within Tauranga Harbour between the early 1990s and 2005. Most major point source discharges of nitrogen and phosphorous were removed from the harbour in the early to mid 1990s. In many rivers and streams entering the harbour, nutrient levels have declined due to improved rural practices. However, many of these rivers still have elevated nutrient levels, and some show increasing trends associated with agriculture and runoff from recently harvested forest.
- Water quality for recreation: Despite frequent bacterial contamination in rivers and streams within the catchment, according to BOPRC monitoring reports the microbiological water quality standards for recreation are rarely exceeded in Tauranga Harbour, although shellfish contamination can occur, which is of concern to iwi and hapu.
- Shellfish: Macroinvertebrate species richness, an indicator of ecosystem health, remained stable during 1990-2000. Information on shellfish abundance is limited and mixed; pipi from Otumoetai declined between 2001 and 2010 whereas numbers of cockles are reported to have risen.
- Seagrass Decline: 34% sea grass decline over 40 years, with losses of up to 90% in subtidal areas (areas not exposed by the tide). Seagrass communities contain so much biodiversity that they have been called the ‘tropical rainforests of marine environments’. They provide animals with food and shelter, offer a safe home for juvenile fish, stabilise the sediment with their roots and remove nutrients from the water.
- Mangrove Expansion: Mangroves have increased by 160% over the last 60 years. Ongoing sedimentation from land has been identified as the primary cause of mangrove spread. Sediment inputs raise the level of the seabed, allowing mangroves to colonise areas that were once frequently inundated by the tide. The Bay of Plenty Regional Council is, however, targeting the source of the problem by working with land holders in local catchments to reduce sediment runoff.
- Sea lettuce blooms: In recent years the harbour has been plagued with well documented sea lettuce blooms. These blooms are caused by high levels of nutrients in the harbour but it is unclear, based on the scientific evidence, where these nutrients are coming from – possibly from land use activities, although there is some evidence to suggest a link with El Nino weather patterns pushing deep nutrient-rich water to the surface.
- Biodiversity. The report assesses the biodiversity of the harbour, noting the decline of some fish and shellfish species, effects of toxic phytoplankton, and the ongoing risk of invasive species from Port activities. It was concluded, however, that it is very difficult to assess the overall state of biodiversity in the harbour due to the lack of data and ongoing monitoring.