Estuaries
The Tasman region is home to the second and third largest tidal lagoon estuaries in the South Island (Waimea and Whanganui Inlets). There are also several large tidal river mouth estuaries with extensive and productive intertidal deltas (Motueka, Takaka, Ruataniwha) and also many smaller tidal river estuaries whose entrances intermittently block or constrict and cause water quality problems (e.g. Grants Road near Puremahia River in Golden Bay) as well as several relatively unmodified tidal river and tidal lagoon estuaries (all located in the West Tasman region). The estuaries with the most valuable habitats are extensive tidal flats, saltmarsh and seagrass. These are predominantly the larger tidal lagoon estuaries and tidal river deltas.
Estuaries in Tasman are valued for recreation, including kayaking, water-skiing, swimming, walking, horse-riding, bird-watching, as well as harvesting of cockles and fish. Several estuaries in Tasman are considered nationally important for coastal birds – particularly Farewell Spit, Motueka River Delta and Sandspit and Waimea estuary.
The main pressures on estuaries in Tasman are sedimentation, vehicles damaging habitats, habitat loss through sea level rise, and ecological change through sea temperature and acidity change.
Tasman’s estuaries have suffered historic saltmarsh loss through reclamation - 30% loss across the district since 1900. Moutere and Ruataniwha estuaries have suffered the largest loss at 47% and 40% respectively. On average about two-thirds of the natural vegetated buffer margin around estuaries has been developed into pasture, residential and forestry land use.
Tasman District Council regularly monitors six estuaries from Waimea in the east to Whanganui in the west. Fine-scale assessments are undertaken at all of these except the Riuwaka-Motueka Delta estuaries. The purpose of this monitoring is to understand their condition and characterise and track changes in their ecological health. The Whanganui Inlet is the least modified monitored estuary with only 17% of the margin in pasture.
Half of all estuaries in Tasman and Golden Bays are excessively muddy (greater than 10% of the estuary area filled with soft muds). The Waimea estuary is most affected at 47% soft mud (i.e. with >50% silt or clay content), with Waitapu and Motupipi at approximately 25%. Despite moderate-to-high nutrient loads to a few of Tasman’s estuaries (Waimea and Motupipi in particular) they have low to moderate nutrient enrichment (which manifests in nuisance algal growth and low oxygen concentrations). This is due to the relatively high level of flushing.
