Anoxic waters

Areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen
Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. Anoxic waters can be contrasted with hypoxic waters, which are low (but not lacking) in dissolved oxygen. This condition is generally found in areas that have restricted water exchange. In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier, as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen. Anoxic waters are a natural phenomenon, and have occurred throughout geological history. The Permian–Triassic extinction event, a mass extinction of species from the world's oceans, may have resulted from widespread anoxic conditions combined with ocean acidification driven by a massive release of carbon dioxide into Earth's atmosphere. Many lakes have a permanent or temporary anoxic layer created by respiration depleting oxygen at depth and thermal stratification preventing its resupply. Anoxic basins exist in the Baltic Sea, the Black Sea, the Cariaco Trench, various fjord valleys, and elsewhere. Eutrophication has likely increased the extent of anoxic zones in areas including the Baltic Sea, the Gulf of Mexico, and Hood Canal in Washington State.

Eutrophication, an influx of nutrients (phosphate/nitrate), often a byproduct of agricultural run off and sewage discharge, can result in large but short lived algae blooms. Upon a bloom's conclusion, the dead algae sink to the bottom and are broken down until all oxygen is expended. Such a case is the Gulf of Mexico where a seasonal dead zone occurs, which can be disturbed by weather patterns such as hurricanes and tropical convection. Sewage discharge, specifically that of nutrient concentrated "sludge", can be especially damaging to ecosystem diversity. Species sensitive to anoxic conditions are replaced by fewer hardier species, reducing the overall variability of the affected area.

The temperature of a body of water directly affects the amount of dissolved oxygen it can hold. Following Henry's law, as water becomes warmer, oxygen becomes less soluble in it. This property leads to daily anoxic cycles on small geographic scales and seasonal cycles of anoxia on larger scales. Thus, bodies of water are more vulnerable to anoxic conditions during the warmest period of the day and during summer months. This problem can be further exacerbated in the vicinity of industrial discharge where warm water used to cool machinery is less able to hold oxygen than the basin to which it is released. Daily cycles are also influenced by the activity of photosynthetic organisms. The lack of photosynthesis during nighttime hours in the absence of light can result in anoxic conditions intensifying throughout the night with a maximum shortly after sunrise.

Individual species’ reactions to eutrophication can vary widely. For example, some organisms, such as primary producers, can adapt very quickly and even thrive under anoxic conditions. However, most organisms are extremely susceptible to slight changes in aquatic oxygen levels. Simply put if a respirating organism is presented with little to no oxygen, its chances of survival will decrease. Therefore, eutrophication and anoxic conditions in water leads to a decrease in biodiversity. For example, the soft coral Xenia umbellata can resist some anoxic conditions for short periods of time, but after about 3 weeks, mean survival decreases to about 81% and about 40% of surviving species experience size reductions, lessening in coloration, and compromised pinnate structures (Simancas Giraldo et al., 2021). Another example of a susceptible organism is observed with The Sydney Cockle, Anadara trapezia. Enriched sediments have lethal and sublethal effects on this Cockle and, as stated in [Vadillo Gonzalez et al., 2021], “movement of cockles was reduced in enriched sediments compared to natural treatments.” These are just a few examples of the hundreds of thousands of aquatic species that exist, but these and other examples show important results. A study collecting over 850 published experiments "reporting oxygen thresholds and/or lethal times for a total of 206 species spanning the full taxonomic range of benthic metazoans." Individual species will have different adaptive responses to anoxic conditions based on their biological makeup and the condition of their habitat. While some are able to pump oxygen from higher water levels down into the sediment, other adaptations include specific hemoglobins for low oxygen environments, slow movement to reduce rate of metabolism, and symbiotic relationships with anaerobic bacteria. In all cases, the prevalence of excess nutrients results in low levels of biologic activity and a lower level of species diversity if the area is not normally anoxic.