Monitoring Southland's Estuaries

July 2014

Environment Southland with Fish & Game is monitoring estuaries to better understand them

Research in Southland estuaries has found they are “the canary in the mine” for water quality. In Southland there’s been a big focus in the past on the Waituna Wetland, but other estuaries in the region are providing a huge amount of information from a major monitoring programme.

Estuaries are important for two main reasons: water filtration and habitat protection.

There are 4 main ways that estuaries contribute to the human way of life.

  • Provisioning – goods produced or provided by ecosystems
  • Regulating – benefits obtained from regulation of ecosystem processes
  • Cultural benefits – non material benefits from ecosystem
  • Supporting – services necessary for production of other ecosystem services.

They buffer both the land and sea by filtering what comes off the land. As estuaries are damaged, they lose this filtering ability and this has implications on both recreational and commercial species such as the oyster fishery in Fouveaux Strait.

Estuaries also soak up a lot of water during times of flood and during storm surges from the sea. When that ability is lost through reclamation or urbanisation, species loss or sedimentation, their ability to buffer is lost.

Nick Ward, coastal environmental scientist from Environment Southland says we can’t afford to under-estimate these effects.

New River Estuary at Invercargill and Jacob’s River Estuary at Riverton are two of four main catchments in Southland.

Monitoring has been occurring at the New River Estuary since 2001. Nick describes the monitoring as taking a holistic approach to ecosystem health by focusing in fine detail on a few specific sites in each of the two estuaries.

The scientists have been looking at how muddy the estuaries are, what biology is in the sediment, where the oxygen level is and the concentration of nutrients and toxicants in the sediment. They are also looking at habitat changes and wide scale biological changes including seagrass changes over time. They have mapped this information.

Seagrass is a great habitat and mapping it gives an indication of whether it is increasing or decreasing. In 2001 there were 94ha of seagrass in the New River estuary, which was 3.1% of the area. In 2012 only 53ha were mapped, which was 1.7% of the area.

That’s a dramatic decline in something you want to hold onto, Nick says.

At the same time, the macroalgal cover in the estuary increased from less than 1% by area in 2001 to more than 13% in 2012. There are three main species of this nuisance seaweed that have proliferated at the same time. These species are native, but opportunistic. They are sea lettuce Enteromorpha lactuca, Enteromorpha intestinalis, and Gracilaria chilensis (used to make agar jelly for use in petri dishes).

The Jacob’s River estuary at Riverton has seen a similar pattern to New River estuary. In 2003 seagrass covered 17.4ha that was 4.2% of the estuary. By 2008 it covered only 16.7ha and by 2013 was down to 10.3ha or 2.5% of the estuary. By contrast, macroalgal cover went from less than 5% in 2003 to 35% in 2008 and then 30% in 2013.

The two main issues in the estuary are nutrients and sediments, and both of these inputs have accelerated over time, swamping the seagrass, which predominantly gets stressed by the turbidity and lack of clarity in the water.

When the macroalgae bloom and then rot down, the bacteria suck the oxygen out of the system, making the estuary sediment very low in oxygen, a bit like putrid compost.

Estuaries act like the kidneys and liver of the land, filtering what comes off the land before it gets into the sea. As we over-extend them, a bit like binge drinking, they break down and don’t work effectively. As estuaries degrade, they move into a state where they are putrid, smelling, with fish stocks possibly collapsing and harmful algal blooms. Nick says of all the estuaries in Southland, he thinks the New River is in the worst condition.

The monitoring is trying to get a handle on this continuum from the previous pristine state where swimming, fishing and shellfish collection could all occur, to where they are now.

Zane Moss, operations manager for Fish & Game Southland says estuaries are sending a clear message about what is happening on our land.

The New River Estuary is a highly diverse place with a wide range of bird species including wading birds that arrive from Siberia. This is predominantly why it is designated a RAMSAR site (Wetland of International Importance).