Bay of Plenty Regional Council says the sea lettuce blooms in Tauranga Harbour in recent days is probably at its peak.
Regional Council general manager natural resources Warwick Murray said the peak was probably about average for the time of year and it was clear the blooms were not just caused by human-made nutrients flowing into the harbour.
"Sea lettuce is a naturally occurring algae, and the variations in abundance from year to year and place to place are strongly influenced by uncontrollable factors such as wind, tides and coastal currents which affect water temperature and nutrient levels.
"Monitoring of sea lettuce blooms shows they occur at times when nutrients derived from land or discharges such as sewage and stormwater are lowest, indicating other sources such as deep oceanic water play an important role.
"Human sources of nutrients can be an influencing factor however and they are controllable, so it is for this reason that the regional, district and city councils have put a lot of effort over recent years to reduce both point sources and diffuse nutrient flows into the harbour. This has met with considerable success with substantial reductions achieved over the past 20 years,"Mr Murray said.
Regional Council efforts have greatly improved the water quality in the harbour, he said.