In 2017, an 8,000 square mile large "dead zone," (an area with low-oxygen levels in the ocean) developed along the coasts of Louisiana, Texas and Florida. This anoxic event was in part caused by human activity along the Mississippi River, flowing into the Gulf of Mexico. Wastewater from cities contains a lot of nitrates and phosphates, acting as a fertilizer in the water. Sediments washed by rain from the agricultural fields into the river also provide nutrients and elements like iron that are important for plant growth. Fast-growing algae, living in the surface layer of the ocean that receives sunlight, benefit from this surplus of nutrients and minerals. As the algae grow and multiply, they consume all the free oxygen in the water.
Decreasing oxygen levels are followed by a mass mortality, at first of larger animals followed by smaller creatures and finally also the algae. The dying animals and plants provide a sudden surplus of nutrients for bacteria. As the bacteria multiply and feed on the decaying organic matter, they consume the last traces of oxygen and the dead zones exponentially grow in size.
Geological evidence suggests that such oceanic anoxic events (OAE) happened 95 million years ago during the Cretaceous. A series of thin strata of dark mud interbedded in white limestone and found worldwide are the remains of such ancient algal blooms. As animals, plants and microorganisms dying on the surface of the oceans didn't decompose, the organic material accumulated on the bottom of the sea. When life returned the white limestone (formed by the calcareous shells of microorganisms) was deposited.
The exact cause of the past OAE is still debated, but it's likely that volcanic eruptions provided nutrients as volcanic ash dispersed into Earth’s atmosphere, and that the warmer climate of the Cretaceous also played a role. Warm water can hold less dissolved oxygen than cold water. Warmer oceans are therefore more vulnerable to oxygen depletion during an algal bloom as cold oceans. A warmer climate also favors plant growth, making algal blooms more likely. Research has shown that in the past 50 years and as the climate warms such modern dead zones multiplied, especially along the coasts of the continents, which are most exposed to human activity and wastewater.
In our modern times, anoxic events are not only disastrous for marine ecosystems, but also for the fishing industry. Fish or shrimp farms along the coast can't simply relocate in areas not affected by low-oxygen events. Also, the small-scale traditional fishing industry, especially in developing nations, can't follow the fish swarms away from the coast to the open ocean.
