“Dead zones are hypoxic (low-oxygen) areas in the world's oceans and large lakes, caused by "excessive nutrient pollution from human activities coupled with other factors that deplete the oxygen required to support most marine life in bottom and near-bottom water.” [US National Ocean and Atmospheric Administration]
Every Northern Hemisphere Spring this dead zone occurs in the Gulf of Mexico and increases in size over time.
It occurs because every year the Mississippi River collects roughly ten thousand pounds of fertilizer and raw sewage pollution from 31 states and some of Canada. When spring and summer rains come down, they wash the excessive nutrients in fertilizers and sewage downstream and out into the Gulf.
It is only one of more than 400 hypoxic areas world-wide which were mapped in 2008.
Proceedings of the National Academy of Sciences of the United States of America (PNAS), Andrew H. Altieri et al, 2017, Tropical dead zones and mass mortalities on coral reefs:
Oxygen-starved coastal waters are rapidly increasing in prevalence worldwide. However, little is known about the impacts of these “dead zones” in tropical ecosystems or their potential threat to coral reefs. We document the deleterious effects of such an anoxic event on coral habitat and biodiversity, and show that the risk of dead-zone events to reefs worldwide likely has been seriously underestimated. Awareness of, and research on, reef hypoxia is needed to address the threat posed by dead zones to coral reefs.
Degradation of coastal water quality in the form of low dissolved oxygen levels (hypoxia) can harm biodiversity, ecosystem function, and human wellbeing. Extreme hypoxic conditions along the coast, leading to what are often referred to as “dead zones,” are known primarily from temperate regions. However, little is known about the potential threat of hypoxia in the tropics, even though the known risk factors, including eutrophication and elevated temperatures, are common. Here we document an unprecedented hypoxic event on the Caribbean coast of Panama and assess the risk of dead zones to coral reefs worldwide. The event caused coral bleaching and massive mortality of corals and other reef-associated organisms, but observed shifts in community structure combined with laboratory experiments revealed that not all coral species are equally sensitive to hypoxia. Analyses of global databases showed that coral reefs are associated with more than half of the known tropical dead zones worldwide, with >10% of all coral reefs at elevated risk for hypoxia based on local and global risk factors. Hypoxic events in the tropics and associated mortality events have likely been underreported, perhaps by an order of magnitude, because of the lack of local scientific capacity for their detection. Monitoring and management plans for coral reef resilience should incorporate the growing threat of coastal hypoxia and include support for increased detection and research capacity.
Anyone still in favour of allowing an expansion of coal mining in the Galilee Basin, Queensland?
Anyone still comfortable with the amount of agricultural/industrial run-off into the Great Barrier Reef, marine protected areas and Australian coastal waters, which is allowed under state and federal policies?
It’s not just our rivers and aquifers which are suffering from political inaction and vested interest greed.
BACKGROUND
The Australian Government’s OzCoasts website states:
A reduction in dissolved oxygen concentrations is amongst the most important effects of eutrophication on aquatic organisms [4]. Hypoxia can cause direct mortality, reduced growth rates and altered behaviour and distributions of fish [4] and other organisms. In addition, bottom-water hypoxia can interact with elevated water temperatures at the surface to produce a "temperature-oxygen squeeze" effect, which can greatly reduce the amount of summer habitat available for some species [12]. Eggs and larvae of fish (and crustaceans) may be particularly susceptible to this effect because these life history stages are less able to avoid unfavourable conditions, and because they live in near shore areas, such as estuaries, where too-high water temperatures and too-low oxygen conditions often occur [5]. Changes in fish assemblages and crustaceans in response to hypoxia and & anoxia can render these organisms more susceptible to fishing pressure, and can increase the abundance of non-targeted species in by-catch [4].
Dissolved oxygen status also influences the uptake or release of nutrients from sediment. When oxygen is depleted, the nitrification pathway is blocked, and efficiencies may be lowered. As a consequence, more nutrients (e.g. nitrogen and phosphorous) are released from the sediment in bio-available forms [7]. These nutrients help to sustain algal blooms, and therefore continue the supply organic matter to the sediments [7]. With organic matter (energy) diverted from invertebrate consumption to microbial decomposition, the natural pattern of energy flow is altered, and pelagic and opportunistic species are favoured [8]. Indeed, an increased ratio of planktivore:demersal fish biomass is an important effect of eutrophication [11]. Low bottom water oxygen concentrations are also conducive to the build-up of toxic compounds such as hydrogen sulfide and ammonia gas, which can also be harmful to benthic organisms and fish. Even short-lived anoxic events can cause the mass mortality of fish and benthic organisms [10].
Overall, anoxic and hypoxic events can cause large reductions in the abundance, diversity and harvest of fish in affected waters [4], and can contribute to an overall loss of bio-diversity[9]. However, the extent to which bottom water anoxia causes declines in overall fish production depends on a balanced between the negative and positive and effects of eutrophication in the full spectrum of habitats within the system [4]……
Major research institutions, universities and government (local and State) agencies gather oxygen data for specific research studies. Some information on anoxic and hypoxic events in Australian coastal waterways was compiled during the National Land & Water Resources Audit. In most cases, no data was available. However, localised or short-lived periods of hypoxia were reported in the Derwent and Huon estuaries (TAS) and in the Tuggerah Lakes (NSW). Prolonged and extensive anoxia is experienced in the Gippsland Lakes.
Note:
Anoxia is an extreme form of hypoxia.
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