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.
For more information about Seagrass in Tauranga Harbour read our
Tauranga Harbour FactSheets
Sea grass beds have high ecological value and contribute to the
harbour ecosystem in terms of:
- Stabilising the sea bed and preventing erosion
- Provide food and shelter for estuarine animals including
juvenile fish and water fowl
- Increase productivity
- Increased habitat complexity and hence species diversity
A declining population
Sea grass beds are declining worldwide, including in New
Zealand, which is hard to fix as sea grasses do not re-establish
easily. The Tauranga Harbour is a national example of the
rapid decline in sea grass bed populations. The aerial photo
below shows areas where sea grass was previously abundant and is
now gone with an estimated loss within the whole harbour of 34 %
between 1959 and 1996. Some areas of the harbour lost up to
69% over the 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.
Figure 1 - Northern Tauranga Harbour showing 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
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
- Exotic species including the black swan which grazes the sea
grass beds and removes patches up to 1m in diameter (Figure 2)
Figure 2 - Damage to sea grass beds from swan grazing.
Black swans are managed by Fish and Game to limit population
increase and thus the damage to sea grass beds.
Bay of Plenty Regional Council is funding a University of
Waikato PhD student to study the pressures on sea grass growth in
Tauranga Harbour which includes suspended sediment, swan grazing
and nutrient loading. Additionally, changes in the
distribution of sea grass beds are being monitored in order to
identify links to other processes. The best method of
remediation is the recent improvements in land management such as
retiring riparian areas, which reduces land runoff of sediments and
nutrients and will hopefully result in no further decline in the
sea grass beds.
Want to know more?
The NIWA website has some interesting facts and articles and
good pictures of sea grass:
The role of sediment in keeping seagrass beds healthy
Connecting habitats in estuaries
Comparing seagrass meadows across New Zealand
Does seagrass contribute to marine biodiversity?
The Department of Conservation website also has some informative
technical reports and information:
Management and conservation of seagrass in New Zealand: an
For more information
Contact Bay of Plenty Regional Council to find out more.
Contact details below.