Regional Council freshwater ecologist Dan Fake taking a sample of the sea water around Napier. Photo: supplied by HBRC.

Dead sea cucumbers, kina, starfish and crayfish that washed up on the beach at Hardinge Road a week ago are likely to have died due to low dissolved oxygen levels.

On Thursday, the Regional Council received samples back from the lab, showing that an algal bloom that has been sitting offshore at least two months had collapsed. The bloom caused low dissolved oxygen in the bottom waters and resulted in stress and death for the marine species living on the seafloor.

“Where we had numbers of blooming algal cells over 33 million cells per litre of water, they are currently sitting at below 1000 cells per litre,” said HBRC science manager Anna Madarasz-Smith.

The phenomenon was also observed in a small area at Westshore and at Haumoana and Te Awanga beaches. Testing has taking place in these areas as well.

Earlier in the week a statement from the Regional Council had posited this as their “best working theory” on why the sea creatures had died, but had not yet ruled out other potential causes such as toxic spills.

Testing last Friday when the dead creatures were first discovered, showed dissolved oxygen was regularly below 30-40%, and the normal range is 90-110%, with high levels of salinity driven stratification (this prevents mixing and re-oxygenation of the water), she said.

“Blooms are made up of millions of tiny, microscopic algae, and if these persist, or when they start to die-off, they can use up a lot of the dissolved oxygen in the water, causing very low dissolved oxygen levels.
 
“We have measured dissolved oxygen saturation levels in the bottom waters of the area as low as 34%, when normal levels are between 90% and 110%. The animals washing up live on the bottom of the seafloor, so are vulnerable.”
 

Initially, a wide line of enquiry was taken to eliminate other potential causes such as freshwater drowning, sediment smothering and toxic spills, but Madarasz-Smith said they had not been convinced that freshwater drowning was the cause because testing showed the bottom waters were still relatively saline.
 
“While sediment smothering of these animals is plausible, the river discharges have been high in sediment for many months and it would be unusual for this to cause a sudden, acute and localised issue.”
 
“Spills can also cause low dissolved oxygen levels in water, however our compliance team have been investigating the local area with nothing unusual, and with more animals washing up around Haumoana then again a localised spill does not seem as likely.”

It was unknown how many creatures died as a result of the dissolved oxygen levels because people had come and  collected the dead sea creatures before the council was able to investigate. 

The council reiterated its warning to the public not to gather or consumer any of the dead sea life.

Public Interest Journalism funded through NZ On Air.

Share



Join the Conversation

3 Comments

  1. Can someone from the awesome baybuzz journalism team please investigate further as to the cause of the algal bloom itself? The best explanation from council is something along the lines of “naturally occurring phenomenon in favourable conditions”… this seems to appease the public and excuse any underlying contributors (rising sea temperatures? Effluent runoff? Sewerage disposal? Pollutants from industrial zones around Pandora pond?) so that we, the wider community can continue life status quo rather than addressing and modifying behaviours to reduce said “favourable conditions”.
    Over the past couple of months, the look of the algal bloom, on some days has been horrendous, the beaches have been noticeably less populated with swimmers (granted the weather has not been enticing) – do we want this to become the norm during summer months? Or can we address the route cause, generate discussion and action change to prevent future blooms… to optimistic perhaps?
    Many thanks for your time

  2. This is serious stuff. Loss of ecosystems. Is it going to happen every year?
    I like Sam’s comments about ‘appeasing the public’. I experience that feeling too about council replies to questions. When I asked what ‘low risk’ meant in regards the algae, the answer included ‘treat with caution’. What does that mean? I figured ‘low risk’ was more serious than ‘no risk’.
    I can’t help coming back to a fact in HB. The pulp/paper mill at Whirinaki pumps hot water into HB 24/7. How can that not affect water temperature? Instead of pouring a resource (heat) into the ocean, could it be used for another purpose? Powering local residents hot water??
    I did see the algae flowing into the Ahuriri Estuary on the tide. Quite noticeable red colour in its first flush of youth!
    I learned the annual ‘red tide’ in Florida has $$millions spent on it, to boost tourism – a kind of flocculent to absorb the algae, but then that sinks and causes new issues!!
    I’d like to know more about this algae too. HBRC say it’s not the same as the Florida one.

Leave a comment

Your email address will not be published. Required fields are marked *