Dead lands: how lifeless zones appear on our planet and why they grow

Algal bloom development

Water bloom is the result of excess nutrients in the water.

substances, especially phosphorus. Excess carbon or nitrogen is not the cause of water bloom.

Introduction to the hydrocenosis of phosphates in highconcentrations causes the growth and reproduction of algae and higher plants. The latter are displaced under these conditions and die off. Dead organic matter becomes food for bacteria that decompose it. The more food you eat, the more bacteria develop and the less dissolved oxygen they use in the water.

When the dissolved oxygen is reduced, many fish and aquatic insects begin to die. Flowering can lead to the formation of zones of death.

  • Bright green bloom is the resultreproduction of blue-green algae, which are actually bacteria (cyanobacteria). Blooms can also be caused by non-plankton macroalgae. It can be recognized by the large blades of algae that are washed ashore and make the water dark. Black waters were first described in the Florida Bay in January 2002.
  • Some types of algae produce neurotoxins; in high concentrations, these poisons can cause serious consequences for wildlife.
  • Algae blooms sometimes occur in drinking water. In such cases, the toxins can be removed using standard water treatment procedures.
  • Flowering can be observed in freshwaterin aquariums when the fish are overfeed and the excess nutrients are not absorbed by the plants. It is usually not harmful to fish and this can be remedied by changing the water and then reducing the amount of food dispensed.
  • Water bodies can also bloom under thick ice cover.

Where toxic blooms appear

  • Australia

Two Winters 2010 and 2011 on the beaches of Australiabegan to throw out dead fish en masse. This was due to several factors: the record strong phase of the Southern Oscillation - La Niña, during which the surface water layer in the equatorial Pacific Ocean heats up. In addition, the Leuvin Current increased off the southern coast of Australia, carrying warm water from the Indian Ocean, and the blocking anticyclone caused an anomalous inflow of heat from the atmosphere into the ocean.

Also during this period there was an annual maximumtemperatures in the Southern Hemisphere. As a result, by February 2011, the ocean along the coast of Western Australia was three, and on some days even five degrees warmer than usual. Marine ecosystems have been severely affected.

  • Pacific Ocean

In the northwest Pacifica huge patch of warm water formed: it blocked all the nutrient-rich cold waters. Because of this, the population of phytoplankton decreased, and then coho and chinook salmon, about a million seabirds died in Alaska. In addition, for the first time in history, scientists have observed a massive bleaching of Hawaiian coral reefs.

  • North America

A nitrogen surge has turned floating sargassum brown algae into the world's largest harmful bloom, according to researchers at the Florida Atlantic University.

The authors concluded that increased availabilitynitrogen from natural and anthropogenic sources, including wastewater, supports blooms, which in turn lead to harmful blooms and disastrous consequences for coastal ecosystems, economies and human health. Globally, harmful algal blooms are associated with increased nutrient pollution.

The results show that the percentage of algal tissue increased by 35% and simultaneously decreased the amount of phosphorus by 42% in sargassum tissue from the 1980s to the 2010s.

Removal of sargassum from the beaches of Texas during the early andless severe flooding was estimated at $ 2.9 million per year, while Miami-Dade County in Florida alone estimated the latest removal costs at $ 45 million per year. Cleaning up the entire Caribbean in 2018 cost $ 120 million, excluding declining tourism revenues. The release of sargassum also affects marine life and also causes breathing difficulties due to decomposition and increases the number of faecal bacteria.

Have dead spots appeared before

New study by American and Japanese scientistsshowed that this happened regularly in the recent geological past. In the cores obtained while drilling the bottom of the Bering Sea, over the past 1.2 million years, 27 dead zones have been counted without traces of the vital activity of organisms.

The main reason is hypoxia or a sharp depletion of oxygen in water. Moreover, a clear relationship was established between dead zones and climate warming.

Such harsh hypoxic phenomena are common in the geological record. They almost always occur during warm interglacial periods like the one we see today.

Ana Cristina Ravelo, professor and co-author of the study

Is the toxic bloom increasing?

Yes.Dead zones have expanded significantly in recent decades, including in bodies of water on land, according to a study in the journal Nature. Deoxygenation of freshwater lakes is now several times faster than sea water. To find out, scientists analyzed data over the past 80 years on the oxygen content in 393 lakes in Europe and the United States.

It turned out that since 1980 the oxygen level inof the surveyed lakes decreased by 5.5% at the surface and by 18.6% at depth. This was due to an increase in water temperature and general warming in the temperate zone.

The amount of oxygen that can holdwater decreases with increasing temperature. Since surface water warms an average of 0.38 degrees Celsius over a decade, oxygen concentration declines by 0.11 milligrams per liter over the same period.

The problem is that many lakes have averagestrips, the water temperature reached values ​​favorable for the mass reproduction of cyanobacteria - blue-green algae that release toxins. This greatly affects the ecosystem and the quality of drinking water.

What factors affect the spread of dead zones

Scientists identify several factors that can affect the increase in the number and area of ​​dead zones:

  • Blocking anticyclones in the atmosphere

They stay in place for a long time, and the temperatureis steadily increasing. This was the case in the winter of 2013-2014 in the South Atlantic. And in 2019, sea heat waves in the southwestern part of the Atlantic Ocean were formed due to a blocking anticyclone that arose thousands of kilometers over the Indian Ocean and then moved towards South America.

  • Sea currents and surface temperature changes

So, in 2015-2016, a heat wave in the Tasman Seabetween Australia and New Zealand caused the strengthening of the East Australian Current, which carries warm waters from the equator. And the most powerful heat waves in the Pacific Ocean in 2011 and 2014-2015 coincided with the warm phase of the Southern Oscillation.

  • Global warming

In a recent study published in the journalScience, scientists at the University of Bern have calculated that the likelihood of heat waves has increased 20 times over the past decades. The authors associate this with human activity.

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