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Several causes for sea lettuce blooms

Monday, 23 December 2013 10:00 a.m.

The sea lettuce plaguing the Tauranga Harbour for the past few weeks is probably at its peak right now, and is about average for the time of year, Bay of Plenty Regional Council says.

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

“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,” he said.

“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.

“These efforts include removing Tauranga and Omokoroa sewage in the mid-90s, closure of piggeries around the harbour, reduction in nutrients flowing from agrinutrient 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.”

“For most other nutrient inputs, the Regional Council has programmes in place to ensure that systematic reductions happen, mostly by regulations such as stormwater management, sewage overflow reduction programmes, catchment protection works, and improvements at the Port of Tauranga,” he said.

The number of farms and stock in the catchment had also decreased, and almost all had shifted to land-based treatment systems.

“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,” he said.

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