An ecological survey of Tauranga Harbour by Massey University researchers and local iwi is under way in a bid to better understand and identify areas of concern about its ecosystem.
The survey was recommended as part of a report prepared by the Manaaki Taha Moana (MTM) research consortium whose members include Massey University, Waka Taiao, Te Manaaki Awanui of Tauranga Moana and IT company, WakaDigital.
While the report identified overall stable nutrient levels in the harbour and water quality suitable for recreation, it noted concerns with a 34 per cent decline in sea grass since the middle of last century and the expansion of mangroves. It also stated that some species of fish and shellfish have declined while nutrient levels in some rivers entering the harbour are unusually elevated.
Marine ecologist Joanne Ellis, from the Cawthron Institute in Nelson, is leading the survey, with involvement of the MTM team, Bay of Plenty Regional Council, University of Waikato, and local volunteers to help collect samples from up to 75 locations throughout the harbour, including intertidal sandflats, shellfish beds and seagrass areas.
Members of MTM, which has received funding for the project from the Ministry of Science and Innovation until 2015, will use the survey data to develop models that can be used to monitor improvements or decline in the health of the harbour over time or in resource management applications such as predicting future species distribution under varying scenarios.
Professor Murray Patterson, from Massey's School of People, Environment and Planning, who leads MTM, says the research consortium is also developing a coastal cultural health index with Maori. This will enable the assessment of the harbour's health through criteria such as measuring the abundance of shellfish in the water catchment.
An important aspect of Manaaki Taha Moana is that it incorporates local Maori knowledge as well as "scientific data".
A copy of the report can be downloaded from the Manaaki Taha Moana website http://www.mtm.ac.nz
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 run-off from recently harvested forest.
Water quality for recreation: Despite frequent bacterial contamination in rivers and streams within the catchment, according to the Bay of Plenty Regional Council 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 per cent sea grass decline over 40 years, with losses of up to 90 per cent 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 per cent over the last 60 years. On-going 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 regional council is, however, targeting the source of the problem by working with landholders in local catchments to reduce sediment run-off.
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 on-going risk of invasive species from port activities. It was concluded, however, that it is difficult to assess the overall state of biodiversity in the harbour due to the lack of data and on-going monitoring.