Te Awarua-o-Porirua Harbour has been extensively modified over the years, particularly the Onepoto arm where almost all of the historical shoreline and salt marsh has been reclaimed for roading and development. Most of the arm is now lined with steep straight rock walls flanked by transport corridors. The Pāuatahanui arm is slightly less modified, with areas of salt marsh remaining in the north and east on the seaward side of the road.
Catchment land use in the Onepoto arm is dominated by residential and commercial development. In the steeper Pāuatahanui arm grazing is the dominant land use, although residential development is significant in some areas.
Sedimentation is a major problem in the estuary, particularly in the Pāuatahanui arm, where potential sources include land disturbance associated with residential and road development.
Elevated nutrient inputs are also causing moderate degradation of estuary health by decreasing sediment oxygen levels and encouraging algal growth.
Results: Intertidal monitoring sites
Intertidal sediment quality in Te Awarua-o-Porirua Harbour is generally ‘good’, with low levels of nutrients and metal contaminants.
Monitoring has identified a gradual decline in species at all sites except at the site near Porirua City. This is consistent with sedimentation monitoring that reveals a long-term harbour-wide increase in the extent of mud-dominated sediments.
Results: Subtidal monitoring sites
Sediments at all subtidal sampling sites are muddy, except for the site located off Camborne, which is sandy. The muddy sediment concentrates contaminants from rivers, stormwater, and road runoff in the deeper muddy areas in the Onepoto arm and at the southern end of the Pāuatahanui arm.
Overall, sediment in the Pāuatahanui arm was found to have lower levels of contaminants than the Onepoto arm. This is likely due its distance from the Porirua City Centre and the concentration of urban and industrial sources of pollution. Ecologically, the Onepoto subtidal area scored lower than the Pāuatahanui subtidal area, with fewer species of sediment dwelling invertebrates found closer to the city.
Species in the Onepoto arm generally limited to those that can tolerate mud and disturbances. Despite this lack of diversity, there are double the number of animals in the Onepoto arm compared to the Pāuatahanui Arm.
