Estuaries in New Zealand

ANDREW SWALES
Estuaries are the interface between the land and the sea. They’re meeting places of water, animals and plants. So it’s a place where seawater is diluted by riverwater coming into it. Geologists view estuaries as sediment traps, so over about 12,000 years since estuaries were formed, they’ve continuously filled in with sediment coming from the land and sea, and they change over time.

And biologists look on estuaries as places where particular types of animals and plants, which are highly tolerant to large changes in environmental conditions like salinity, are able to live part or all of their lives.

Our estuaries have changed a lot since people arrived in Aotearoa some 700 years ago. We’ve had various waves of people arrive, we’ve had major deforestation of catchments. We’re getting a lot more sediment coming into the estuaries than before people arrived. So this has accelerated the infilling of estuaries and the formation of new habitats. We’ve seen shifts from quite sandy environments to more muddy environments generally.

And so we see changes in the animals that live in estuaries, typically from animals that are quite sensitive to turbidity or muddy sediments or being smothered by sediments to animals that are less sensitive to that. For example, some shellfish are sensitive to suspended sediments, and in many estuaries, we might have lost them.

So we’ve seen large-scale changes in New Zealand’s estuaries, and you couldn’t describe any New Zealand estuary as being pristine – they’re just at different stages along a continuum of environmental effect. And those estuaries which are more remote from human population centres are typically in better nick. Around major cities, a lot of estuaries have accumulated contaminants like heavy metals due to run-off, stormwater from urban areas etc. And in rural areas, we still see lots of sediment coming into estuaries depending on the size and the nature of their catchments.

There’s lots of room for improvement, and one of the things about this toolkit is that many estuaries aren’t actively monitored. There are many – there are hundreds of estuaries in New Zealand, and there’s only so much resource to go around in terms of monitoring these, so that there is the potential here for hapū and community groups to actually be the first groups to collect meaningful data on estuaries, so there’s a real opportunity here to add to the collective knowledge of our estuaries.

Acknowledgements:
Andrew Swales, Weno Iti, and Raiha Tuahine, NIWA