What is an estuary?


Blueskin Bay estuary

Estuaries are located at the land and the sea interface and are the transition zone between terrestrial, freshwater and marine environments. Estuaries are sheltered, semi enclosed bodies of water going by many names such as harbours, bays, lagoons, wetlands and fiords. They are often well mixed and are complex ecosystems, which are valued for their biodiversity, ecosystem services, cultural and recreational values. 


Why are estuaries important?


Tautuku estuary

Estuaries are unique environments that provide diverse habitats and feeding areas for many different species, fish, birds, mammals, plant communities and macrofauna (critters that live within the sediment such as cockles and worms). Many species are specially adapted to the unique estuarine environment, equally they are important feeding grounds for threatened species such as godwit and as nursery habitats.


Due to the complex nature of estuaries as the connection between freshwater and saltwater, they provide a range of ecosystem services such as nutrient cycling, nutrient storage, carbon storage and other highly visible services like food. The plants and animals within these systems influence the processes and functions that affect these ecosystem services.

Alongside ecological benefits of estuaries, estuaries are nice places to spend time, whether swimming, fishing, bird watching or gathering food.

Good estuarine health is needed to provide these services and diversity of habitats and species, with degraded estuaries less able to sustain diverse ecology and therefore, there are reduced ecosystem services and human values. 

What are we doing to monitor and manage our estuaries?


Te Hakapupu

At Otago Regional council we run a state of the environment estuary monitoring programme which includes fine scale and broad scale monitoring practices utilising the methods in the national estuary monitoring protocol. Fine scale monitoring is long term monitoring that monitors mud content, heavy metals, sedimentation and macrofauna at selected sites within estuaries to assess trends in estuarine health over time. Broad scale monitoring maps the habitats within the estuary, salt marsh, seagrass, areal extent of mud etc to determine estuary wide health, this monitoring occurs approx. every 5 years. The information combined provides a spatial and temporal overview of estuarine health over time. Equally ORC undertakes targeted projects such as sediment source tracking (Pleasant River estuary), nutrient modelling and other projects looking at certain stressors affecting estuaries across Otago or individual estuaries that are under greater stress than others.


The information from both the state of the environment monitoring and the targeted projects helps to inform both regulatory and non-regulatory management of estuaries in Otago. The regulatory component is through providing information for policy to implement plans and information for consents and compliance as required. Equally a big opportunity to improve estuarine health across Otago is through non regulatory management working with community groups, landowners and other stake holders to provide information and advice on changes that can be undertaken, will lead to improving estuarine health from the mountains to the sea.

Mapped estuaries

Blueskin Bay

The Blueskin Bay is a large, shallow, tidal estuary located approximately 25km north of Dunedin on New Zealand’s east coast. Overall, this is a healthy estuary, with high-value seagrass and cockle beds. This is due to a combination of small freshwater inflows, high tidal flushing, and the catchment sediment and nutrient loads that the estuary is currently able to absorb.

Catlins Estuary

The Catlins Estuary is a large tidal lagoon located at Pounawea. It has a single tidal entrance that is always open, a large lower estuary basin, an upper estuary basin named Catlins Lake, and two dominant freshwater inflows from the Catlins and Owaka Rivers. The catchment is dominated by pasture (61%) and indigenous forest (20%).

Kaikorai Estuary

Kaikorai Estuary is an extensively modified, moderate-sized tidal lagoon located at Dunedin. The catchment is dominated by pasture (48%) and urban areas (21%). 

Pleasant River Estuary

Pleasant River Estuary is a medium sized tidal lagoon, and a shallow intertidally dominated estuary (SIDE), located west of Palmerston and approximately 50km north of Dunedin. 

Shag Estuary

Shag Estuary is a relatively modified, moderate sized tidal lagoon near Palmerston, approximately 50km north of Dunedin. It has a single, narrow tidal opening that is occasionally restricted, a central river channel, a large basin in the lower estuary, and two smaller basins in the upper estuary. The catchment is dominated by pasture (71%).

Tautuku Estuary

Tautuku Estuary is a medium-sized tidal lagoon, and a shallow intertidally dominated estuary (SIDE), located approximately 140km south of Dunedin. 

Tokomairiro Estuary

Tokomairiro Estuary is a modified, moderate-large sized tidal river located downstream of Milton. The tidal mouth though often constricted, is nearly always open, while the upper estuary is at times poorly flushed, stratified/layered and susceptible to phytoplankton blooms. The estuary substrate is dominated by muddy sediments but supports large areas of saltmarsh, subtidal macrophyte growth, and small beds of nuisance macroalgae. The 398kmcatchment is dominated by high producing exotic pasture (54%) and plantation forestry (35%).

Waikouaiti Estuary

Waikouaiti Estuary is a relatively modified, moderate-large sized tidal lagoon located at Karitane. It has a single narrow tidal opening that is occasionally restricted, a central river channel, and two main arms. The catchment is dominated by pasture (74%).