According to Bay of Plenty Regional Council staff the sea lettuce plaguing the Tauranga Harbour for the past few weeks is probably at its peak, and is about average for the time of year.

Sea lettuce washed up on the beach. Photo: file.

Regional Council General Manager Natural Resources Warwick Murray says despite recent opinions about the cause, it's clear that the blooms were not just caused by human-made nutrients flowing into the harbour as some people believed.

'Sea lettuce is a naturally occurring alga - 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,” says Warwick.

'Monitoring of sea lettuce blooms shows they occur at times when nutrients derived from land or discharges such as sewage and storm-water are lowest - indicating other sources such as deep oceanic water play an important role.”

Warwick says human sources of nutrients are an influencing factor, but are controllable. 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. There has been considerable success with substantial reductions achieved over the past 20 years.

'These efforts included removing Tauranga and Omokoroa sewage in the mid-90s, closure of piggeries around the harbour, reduction in nutrients flowing from agri-nutrient and other industrial sites, reduction of septic tank seepage from communities with on-site effluent systems and removal of stock access to catchment streams and rivers.”

'The Regional Council has programmes in place to ensure that systematic reductions happen, mostly by regulations - such as storm-water management, sewage overflow reduction programmes, catchment protection works, and improvements at the Port of Tauranga,” says Warwick.

'This has greatly improved the water quality in the harbour and will continue to improve it in years to come. However we know that, while nutrients flowing off land into Tauranga Harbour are an important factor in sea lettuce growth, they are not the main drivers of the harbour-wide blooms we've been seeing.”