Warmer water isn’t always a good thing

Submitted to the Marlborough Express to be published with Climate Action Week, February 2024

Tom Powell & Bill McEwan

The Marlborough Sounds is getting warmer; that is the conclusion of 8 years monitoring by the District Council. Last summer was the warmest yet in the sounds, with peak water temperatures over 20⁰C.

Swimmers this summer will rejoice at the warmer water.  For marine life, it is a different story.  Water temperature has a major impact on sea life.  Just as on land, plants and animals adapt to the temperature range of their environment.  Change that range and the whole marine neighbourhood changes.

One important aspect is the amount of oxygen the water holds.  We all breathe oxygen and sea life do too, catching it in their gills.  Unfortunately, though, for sea creatures, the amount of dissolved oxygen that seawater holds goes down with increasing temperature.  So, sea life now have to “breathe” harder to get the oxygen they need.  It is similar what people experience tramping at high altitude. 

Fish can adjust to higher temperature by moving to deeper, cooler water or by leaving the sounds entirely.  Sea creatures like shellfish and sponges don’t have that option.  

Other aspects of the marine neighbourhood also change.  Kelp and sponges, which are a food source for other organisms, tend to thrive in a narrow temperature range.  Change the water temperature and many species of kelp and sponge disappear.   If you are a fish depending on these organisms for food, the supermarket shelves are now getting emptier; another reason to move out if you can.

Some foreign marine organisms thrive in warmer waters and these will become sounds sea life’s new neighbours. Like new neighbours everywhere, some will fit in and some won’t.  New predators and marine diseases will arrive.  Blooms of toxic algae will become more common.  The new neighbourhood will be in a state of flux as temperatures continue to increase. 

Add to this other stressors humankind has put on the marine environment in the sounds: gradually acidifying seawater due to rising CO2 emissions, siltation due to logging and landslides on human-disturbed land, sea life caught and killed by commercial and recreational fishers, destruction of tube worm reefs by historic dredging, and all manner of rubbish and chemicals dumped into the water.
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So, when you take that refreshing swim in the sounds this summer, spare a thought about the plight of the sea creatures living just below your feet.