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Hypoxia

Feedback mechanisms

The hypoxic regime is maintained by persistent stratification, changes in nitrogen cycle, phosphorous release, hydrogen sulfide release, and a change in foodweb structure related to the benthic fauna. Macrobenthos cannot survive under hypoxic conditions; DO levels below 0.5 ml per liter are typically associated with mass mortality of benthic animals. It creates a series of reinforcing feedbacks described below:

Normoxia

  • Zooplakton feedback (Local, well established): abundant zooplankton controls micro algae populations, reducing their ability to consume DO and increase organic matter on the water column. Hence, with high DO on the water, conditions are optimal for further zooplankton development.
  • Macrophyte feedback (Local, well established): Macrophyte are a type of macro algae. They perform the same function, however, they capture nutrients and carbon in their large bodies and soil, contrary to micro algae. Thus, they control nutrients in the water column and reduce their availability for micro algae. Low levels of micro algae allow clear water conditions that in turn are optimal conditions for further macrophyte development.
  • Nutrients feedback (Local, well established): When nutrients input are low and micro algae population are controlled, DO is high. The later is a chemical conditions to allow nitrogen to be removed in its gas form N2.

Hypoxia

  • Zooplakton feedback (Local, well established): Under hypoxia regime, the zooplakton feedback works the other way around. As micro-algae abundance increase, DO decrease reducing in turn optimal conditions for zooplankton survival. Lower zooplankton levels further reinforce micro-algae abundance.
  • Macrophyte feedback (Local, well established): Similarly, the macrophyte feedback also works the other way around. Low macrophyte density increase nutrients in water by releasing the nutrients captured on their bodies and the soil. With more nutrients available, micro-algae populations increase causing more turbidity which in turn reduces the ability of macrophyte to survive.
  • Organic matter feedback (Local, well established): Low levels of DO increase the mortality rate of different organism. As they die organic matter content on the water increase, increasing the demand of oxygen required to decaying process.
  • Nutrients feedback (Local, well established): With more nutrients available in the water column and micro-algae metabolizing it, DO levels drop. Hypoxia together with the loss of the benthic fauna alters sedimentary habitats through the disruption of nitrification and denitrification processes. Instead of nitrogen being removed as N2, ammonia and ammonium together with phosphorous are released from the sediments. This further stimulates the growth of algae, the deposition of organic matter, the growth of microbes, and the depletion of oxygen

Anoxia

  • Hydrogen sulfide feedback(scale, uncertainty): Once DO levels drops below 0.5ml per liter hydrogen sulfide is released as a product of the decaying process carried out by microbes. As a consequence, acidity level (pH) increase and life is further limited to few adapted microbial organisms. 

Drivers

Stratification and flushing are important external drivers determined mainly by climate variation. Increased variability due to climate change could therefore have an important impact on the extent and severity of hypoxia.

Shift from Normoxia to Hypoxia and Anoxia

Important shocks (eg droughts, floods) that contribute to the regime shift include:

  • Floods (local, speculative): Heavy rains and floods have a double impact on hypoxia. On one hand, floods increase the amount of nutrients washed from the watershed, and consequently, turbidity in water. On the other hand, floods can increase flushing with in turns increase DO in water.

The main external direct drivers that contribute to the shift include:

  • Nutrients input (regional, well established): The main anthropogenic driver of hypoxia is the delivery of large quantities of nutrients from agricultural systems, namely nitrogen and phosphorous, leading to eutrophication(Howarth 2008). Eutrophication is associated with algal blooms in the upper water layers, which leads to increased deposition of organic matter in the deeper water layers. This promotes the growth of microbes that decompose the organic matter, and their respiration in turn consumes oxygen.
  • Water column stratification (local, well established): Physical processes that stratify the water column make the oxygenation of water even more difficult and exacerbate the hypoxic conditions. Lack of mixing reinforce depletion of oxygen in deep layers of water.
  • Upwellings (regional, well established): In addition to agricultural sources, changes in frequency or intensity of upwellings, therefore in nutrient inputs, might synergistically destabilize hypoxia-prone marine areas.

The main external indirect drivers that contribute to the shift:

  • Population growth (global, speculative): Population growth leads to higher demand of food and densification of coastal settlements as well as upstream settlements.
  • Food demand (local-regional, speculative): Food demand in turn stimulate more agricultural practices, especially the increase of efficiency; in other words more productivity in less area.
  • Agriculture (regional, well established): Agriculture often requires the use of fertilizers. When soils are eroded or washed, fertilizers run downstream to waterbodies.
  • Urban growth (global, well established): Urban growth increase the production of sewage which is rich in nutrients. It also increase the water runoff on the landscape. 
  • Deforestation (regional, well established): Deforestation increase landscape fragmentation and facilitates landscape conversion to agriculture. Both reduce the capacity of the landscape to retain water in the soil, accelerating erosive processes and runoff of nutrients.
  • Rainfall variability (regional, speculative): Rainfall variability is expected to change with climate change in some areas of the world. Although it is not clear where or to what extent, it is definitely likely to influence the frequency of flood events, flushing and the exacerbate erosive processes.

Slow internal system changes that contribute to the regime shift include:

  • Dissolved oxygen (local, well established): Levels of DO is what determine the state of the system. Low levels leads to mass mortality, excess of decaying matter that in turn increase DO consumption.

Shift from hypoxia or anoxia to normoxia.

Important shocks (eg droughts, floods) that contribute to the regime shift include:

  • Floods (local, speculative): On the other hand, floods can increase flushing with in turns increase DO in water.

The main external direct drivers that contribute to the shift include:

  • Flushing (regional, well established): Flushing counteract the effect of water stratification and increase the levels of DO in the water column. 

Key thresholds

Shift from Normoxia to Hypoxia: Dissolved oxygen below 5mL per liter

Shift from Hypoxia to Anoxia: DO below 0.5 mL per liter