Stay in the know about wildlife, water quality, and ecosystems on Sanibel and Captiva Islands and in Southwest Florida

Discolored Water More Common As Anoxic Events Increase

September 16, 2020

Sometimes SCCF’s Marine Lab gets a report of discolored water they think may be a phytoplankton bloom or an algae spill. 

Eutrophication occurs when bodies of water are overly enriched with nutrients and algal growth increases. In our estuaries, this process can cause ecosystem imbalances that become positive feedback loops that further destabilize the system. 

One of the most destabilizing processes is decomposition since it can reduce dissolved oxygen concentrations to levels that cause the suffocation of aquatic animals. Some of those, like some snails, are algal grazers. Red tide toxins can kill other benthic algae grazers like mullet. Once grazing rates are decreased, the macroalgae can grow unchecked to problem levels again.

Eventually, the algal accumulations may be displaced or experience a rapid salinity change, and it may all start to decompose.  When large amounts of the algae, or any organic material, like dead fish, decompose in shallow water, bacteria can use up the oxygen in the water faster than it can be replaced. Wave action can accelerate oxygen transfer. 

But, when the water is calm, the diffusion rate is very slow. Dissolved oxygen concentrations can drop to levels where only half of marine species can survive, or to levels where most all animals and many plants can’t survive.  If the water column is stratified, the oxygen supply to the lower layer is very slow.

When the oxygen is consumed in saltwater, anoxic sulfate-reducing bacteria start blooming and hydrogen sulfide is formed.

If little light available, as in deep water, this hydrogen sulfide can combine with dissolved iron to form a black precipitate (as pictured above). It can make the water look from gray to pitch black. When sufficient light is available, hydrogen sulfide can be oxidized by other types of sulfur bacteria that store elemental sulfur.  The sulfur accumulations look chalky white and will make the water look milky or pastel-colored, as pictured here in Matlacha Pass earlier this year. One type of photosynthetic sulfur bacteria has purple pigments and can make the water look pink. 

As eutrophication progresses, and water temperatures increase, anoxic events are becoming larger and more common worldwide. These events are correlated with runoff which increases with development and wetland loss. Lee County was once 60% wetlands, but since the wetland protections in the 1972 Clean Water Act have been ignored, we’re down to about 10% wetlands today.



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