As our region continues to evolve, it’s important that we understand the pressures our environment faces and the changes that are happening.  

A growing population, changing land use and a booming economy have all had an effect on Tauranga Harbour, and there are a few issues we are working with our communities, iwi and hapū as well as local industries to resolve.

Sea grass

New Zealand's only native species of sea grass, Zostera capricorni, is a flowering marine plant which colonises soft sediment in estuaries and some coastal rocky platforms. The plant is very hardy and able to tolerate both low and high salinity (salinity refers to salt content of water) waters, and being exposed to the air at low tide. Sea grass beds are declining worldwide, including New Zealand, an issue that is hard to fix as they do not re-establish easily.

Sea grass beds are declining worldwide, including in New Zealand, and are hard to fix as it does not re-establish easily. The aerial photo below shows where sea grass was previously abundant in Tauranga Harbour and is now gone. It is estimated that within the whole harbour, 34% of the sea lettuce was lost between 1959 and 1996.  In some areas of the harbour up to 69% was lost over this 37-year period.  Areas near the harbour entrance with little land runoff or influence from other catchments have shown the smallest decline in seagrass abundance.

This image shows Northern Tauranga Harbour and the presence of seagrass in 1959 (red) and 1996 (blue).

This image shows Northern Tauranga Harbour and the presence of seagrass in 1959 (red) and 1996 (blue).

The decline is linked to higher suspended sediment rates in the water which blocks out the available light the sea grass needs to survive.  Higher nutrient rates in the water column are also related to the decline as higher nutrient levels result in the growth of algae in the water column which also block the amount of available light. 

Other reasons the sea grass beds have been in decline are:

  • Reclamation of the sea bed.
  • Dredging which removes plants and also increases suspended sediment in the water column.
  • Physical disturbance from vehicles, boats, structural works and people.
  • Exotic species including the black swan which grazes the sea grass beds and removes patches up to 1m in diameter.