Hypoxic Events

DO depletion in coastal and estuarine waters is a growing global concern. Bottom water DO concentrations are near zero under anoxic conditions, whereas they are < 2.0mg/l under hypoxic conditions.

The incidence of anoxic and hypoxic events is a suggested water quality indicator in the ANZECC/ ARMCANZ water quality guidelines....
and was adopted as an indicator of ecosystem integrity during the National Land and Water Resources Audit.

WHAT CAUSES HYPOXIC EVENTS?

Anoxic and hypoxic events are caused by the decomposition of organic matter by oxygen utilising bacteria. In mny cases, anoxia and hypoxia result from eutrophication (e.g. enhanced sedimentation of POM to bottom waters) and reflect the underlying problem of excessive nutrient loads.

Some P.S. of nutrients to coastal waterways are coastal discharges, including outfalls from industry, wastes from aquaculture operations and sewage discharged from yachts, boats and ships. The risk imposed by P.S of nutrient in coastal waterways is higher in areas with large population densities or with a significant tourism, and can be estimated by the number of P.S per unit unit area of coastline. N.P.S of nutrients from intensive agricultural in catchment and urban stormwater are often larger and more difficult to control.

Rainfall following the dry season in tropical regions can also mobilise organic rich detritus (e.g rotting weeds, grasses, cane trash and stormwater trash) into coastal waters and these can have a very high biological oxygen demand.

WATERWAYS SUSCEPTIBLE TO ANOXIC AND HYPOXIC EVENTS

Severe DO depletion in the coastal zone is usually associated with density stratification in wave dominated coastal systems (e.g. deltas, estuaries, and strainplains and coastal lagoons) characterised by low levels of tidal mixing, and subject to elevated nutrient loads.

Density stratification occurs when freshwater flows seaward over denser and more saline marine water, and us therefore intensified after periods of heavy rainfall.

Tenperature stratification can also occur - most commonly in systems such as coastal lagoons and strandplains - and results from solar warming of surface waters.

In both cases, stratification restricts vertical mixing and therefore the replenishment of bottom water with oxygen derived from the atmosphere. Severe oxygen depletion is not likely to be a significant issue in tide dominated coastal systems (e.g. deltas, estuaries and tidal creeks), because tidal mixing reduces the potential for stratification.

SIGNIFICANCE OF ANOXIC AND HYPOXIC EVENTS

A reduction in DO concentrations is amongst the most important effects of eutrophication on aquatic organisms. Hypoxia can cause direct mortality, reduced growth rates and altered behaviour and distributions of fish 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. Eggs and larvae of fish 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 temperature and too low oxygen conditions often occur. 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.

DO status also influences the uptake or release of nutrients from sediment. When oxygen is depleted, the nitrification pathway is blocked, and denitrification efficiencies may be lowered. As a consequence, more nutrients (e.g. nitrogen and phosphorus) are released from the sediment in bio-available forms. These nutrients help to sustain algal blooms, and therefore continue the supply organic matter to the sediments. 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. Indeed, an increased ratio of planktivore: demersal fish biomass is an important effect of eutrophication. 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.

Overall, anoxic and hypoxic events can cause large reductions in abundance, diversity and harvest of fish in affected waters, and can contribute to an overall loss of bio-diversity. 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.

CONSIDERATIONS FOR MEASUREMENT AND INTERPRETATION

Anoxic and hypoxic events are often recognised by the deleterious effects they have on aquatic ecosystems (e.g. fish kills). As such, by the time such events are detected, substantial damage may have already occured to the ecosystem. Ideally, the spatial and temporal extent of anoxia, the response of benthic communities and impacts on fisheries stocks should be documented. Biological and chemical indices from dated sediment records are useful for documenting changes in the magnitude and frequency of oxygen stress over time, and as to whether such changes result from human or natural causes.

Major research institutions, universities and goverment agencies gather oxygen data for specific research studies.