A recent article in “Nature Climate Change” combined a series of present-day experimental data with a wealth of data from the fossil record to better understand species stress. Key factors and combinations of factors that lead to species extinction were examined. Thus providing a broader foundation for understanding how present-day climate change may affect certain aquatic species.
One of the outcomes of this work was evidence that the combination of heat and oxygen deprivation creates conditions where many aquatic species come under lethal stress. While this is understandable, it also raises the questions of how heat and oxygen deprivation relate to each other in normal marine/aquatic processes.
Heat and metabolism
As ocean waters warm, the metabolic activity in many marine species increases. So, one of the immediate effects is to increase competition for food since the higher metabolic activity requires higher energy intake. However, at the bottom of the food chain, in the trophic level including algae and bacteria, the increased metabolic activity takes a more sinister path. If sufficient nutrients are available, then algae growth can explode with several types of negative results
High growth rates in the algae can locally deplete the oxygen in the water, thus killing off other animals. Also, these blooms often produce toxins that are harmful to aquatic life. Humans may also be affected if they consume seafood from the toxic waters.
Heat and stratification
In other cases, excessive warming can superheat the near-surface water making it less dense than the underlying, deeper waters. This stratification discourages mixing and isolates the deep water from surface oxygen exchange. Without mixing, these deeper waters slowly become oxygen-depleted, thus stressing life below the surface.
Intense stratification, as in the Black Sea, results in completely anoxic deep waters. It is important to note that not all stratification is due to heat. Other mechanisms also drive isolation of deep waters to create deep anoxia. This phenomenon is more common in closed bodies of water like lakes than in open marine conditions.
Heat and thermohaline circulation
Large scale marine anoxia can occur when polar areas warm, thus decreasing the temperature differential between the equator and the poles.
During the Cretaceous, the lack of a strong temperature gradient between the equator and poles created an earth where seasonal changes were less pronounced. Additionally, the earth’s capacity for transporting heat from the equator to the poles declined significantly. Robust thermohaline circulation in the earth’s oceans then disappeared, so currents like the North Atlantic Gulf Stream weakened or ceased to exist.
Weakened ocean currents and reduced atmospheric circulation had a knock-on effect on ocean chemistry. Oceans today enjoy a high rate of “turnover” where water from the surface sinks and deep ocean water upwells to the surface. This process keeps the deep ocean waters oxygenated.
Disruption of vigorous ocean circulation during the late Cretaceous led to the development of anoxic conditions in the deep oceans. This period of anoxia appears in the geological record as black shales. Deposits of these shales exist around the globe. During the deposition of these shales, oxygen-dependent life forms in the deep oceans disappeared.
Heat and oxygen deprivation are intimately related
Heat affects the ocean’s food chain (Source: ArcheanWeb) – https://archeanweb.com/2020/01/30/heat-affects-the-oceans-food-chain/ Also:
Changing currents cause North Atlantic ecosystem drift (Source: ArcheanWeb) – https://archeanweb.com/2020/05/07/changing-currents-cause-north-atlantic-ecosystem-drift/ Also:
Why do harmful algal blooms occur? (NOAA) – https://oceanservice.noaa.gov/facts/why_habs.html Also:
Global warming and extinction risk (University of Erlangen-Nuremberg; Science Daily) – https://www.sciencedaily.com/releases/2020/02/200210133210.htm Also:
Feature Image: New Orleans, Lake Pontchartrain with highlighted algae bloom, Louisiana (By Sentinel Hub) (Modified_) – https://commons.wikimedia.org/wiki/File:New_Orleans,_Lake_Pontchartrain_with_highlighted_algae_bloom,_Louisiana_-_January_6th,_2020.jpg – This file is licensed under the Creative Commons Attribution 2.0 Generic license. – https://creativecommons.org/licenses/by/2.0/deed.en