The removal of indigenous forest cover from most of the land draining into the Hauraki Gulf has resulted in around five times more sediment entering the marine area. After heavy rain it is common to see plumes of sediment billowing out from river mouths where they meet the sea.
Current major causes of sediment are earthworks, forestry harvesting and grazing of steep erodible land. Erosion of river banks is another significant source.
This increase in sediment has been accompanied by the expansion of mangroves in many estuaries around the Hauraki Gulf and most markedly in the Firth of Thames.
High levels of sedimentation can fundamentally change the marine environment. Sediment reduces water clarity and can result in the loss of species reliant on photosynthesis such as seagrass and seaweeds.
Murky water can make it harder for juvenile fish such as snapper to find prey and reduce their ability to survive. The sediment can also cause their gills to deform.
Particles in the water can make it difficult for filter feeders such as cockles, pipi and scallops to feed efficiently and it reduces the survival chances of larvae and juvenile shellfish.
Overall, high levels of sedimentation reduce the abundance and diversity of species and the ability of the marine system to support productive fisheries.
“I’ve seen water going past the Whitianga wharf yellow with clay from what appeared to be forestry runoff.”
“Every time we have a Coromandel downpour, if you stand up on Shakespeare Cliff, you see an enormous brown plume coming down the Purangi and Whitianga rivers and flowing right out into the bay.”