Bay of Plenty Regional Council say nutrients such as farm run-off are not the main cause of a sea lettuce bloom in the Tauranga Harbour.
The bloom appeared several weeks ago and has spread through the harbour, but the council says the level of the algae is about average for this time of year.
It says there are programmes in place to control run-off into the harbour, which include stormwater and sewage overflow management, and there have been improvements at the Port of Tauranga.
The council says the algae's growth is strongly influenced by uncontrollable factors such as wind, tides, and coastal currents, and there are variations in its abundance year to year.