Red tide. Blue glow. Black death

All that glitters is not gold. Turns out that all that glows has a dark side too.

The plankton party that has had everyone in awe by turning the wildside shoreline into a spectacular light show has got a sting in the tail. There were reports of a large number of dead fished washed up on Sards beach yesterday.

Local scientists had warned that this could be one of the side-effects of the recent red tide. The party can go bad for two reasons, both oxygen-related. 

Firstly the plankton are on such a mad jol with their nocturnal glow antics that they suck all the available dissolved oxygen out of the water at night cos of some heavy breathing (cellular respiration in science-speak). Pretty much the same way you'd get breathless busting out all your moves on the dance floor.

Secondly, all epic jols have to end. The mass death of the critters once the party is over results in a whole big pile of bodies on the ocean floor. Along come hordes of bacteria - who are the grim reapers of the sea and pitch up wherever things are dying. These ocean street cleaners have to decompose all the dead stuff. But guess what, decomposing death is hard work!

© Marizanne Ferreira

© Marizanne Ferreira

All this effort means the bacteria suck up most of the oxygen in the surrounding water. Although not a great thing, it isn't a train smash as long as the bottom waters are mixing well with the oxygen-rich surface waters - which means the oxygen content down below gets renewed. The hiccup comes if due to localised sea conditions the water becomes stratified. This is basically like a Water Apartheid.  The water on the top don't mix with the water on the bottom. It often occurs in bays and coves, such as Sards, that are cut off from the large-scale circulation patterns that promote mixing. Probably find the lack of swell on that side has amplified the weak local currents even further.

Because the oxygen-depleted bottom waters don't get refreshed, it causes hypoxia. This makes it really hard for fish and other marine creatures to breathe. Pretty much a case of "Move or die, fish". Sometimes the oxygen depletion happens so quickly that it cuts off escape routes and then results in the death of marine life through suffocation. This is called a black tide, and seems to have been the case at Sardinia Bay - with upwards of 300 fish washed up onto the beach on Saturday afternoon.

It's pretty ironic considering the bacteria are there to clean up the dead to start with. You're maybe not doing your job so lekker if you cause death whilst you're cleaning death up. Just saying. 

Nature can be weird sometimes.

And there's still possibly another twist in the tale. Low oxygen conditions can allow certain bacteria to convert sulphates in the water to hydrogen sulphide gas. Remember the stuff that smelt like rotten eggs in science class? This stuffs so hectic it can even corrode metal objects and cause respiratory problems if you live close by.

So if the water is ponging like your fat uncle's fart, probably don't swim with your Rolex or stand there for hours gulping in lungfuls of the stinky air. Let's hope these sulphate-converting bacteria don't show up, cos besides the pong it could result in even greater fish deaths.

Scientists from Bayworld have collected samples of the dead fish from Sards and will be investigating further.

© Nick Cadle Photography