Dissolved oxygen (DO) concentration in the water
column is an environmental parameter that is crucial for
the successful development of many pelagic organisms. Hypoxia
tolerance and threshold values are species- and stagespecific
and can vary enormously. While some fish species
may suffer from oxygen values of less than 3 mL O2 L
−1
through impacted growth, development and behaviour, other
organisms such as euphausiids may survive DO levels as low
as 0.1 mL O2 L
−1
. A change in the average or the range of
DO may have significant impacts on the survival of certain
species and hence on the species composition in the ecosystem
with consequent changes in trophic pathways and productivity.
Evidence for the deleterious effects of oxygen depletion
on pelagic species is scarce, particularly in terms of the effect
of low oxygen on development, recruitment and patterns
of migration and distribution. While planktonic organisms
have to cope with variable DOs and exploit adaptive mechanisms,
nektonic species may avoid areas of unfavourable DO
and develop adapted migration strategies. Planktonic organisms
may only be able to escape vertically, above or beneath
the Oxygen Minimum Zone (OMZ). In shallow areas only
the surface layer can serve as a refuge, but in deep waters
many organisms have developed vertical migration strategies
to use, pass through and cope with the OMZ.