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 fact sheets
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 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 (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 introduction
For more information
Contact Bay of Plenty Regional Council to find out more. Contact details below.