. 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 stage-specific 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. This paper elucidates the role of DO for different taxa in the pelagic realm and the consequences of low oxygen for foodweb structure and system productivity. We describe processes in two contrasting systems, the semi-enclosed Baltic Sea and the coastal upwelling system of the Benguela Current to demonstrate the consequences of increasing hypoxia on ecosystem functioning and services.

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Impacts of hypoxia on the structure and processes in pelagic communities (zooplankton, macro-invertebrates and fish)

Hypoxia is an ever increasing threat to aquatic systems. While fluctuating levels of dissolved oxygen (DO) can be a natural phenomenon, hypoxia caused by eutrophication and organic pollution is now...

The effects of hypoxia on fishes: from ecological relevance to physiological effects

Fish cover a large size range, from milligrams to tonnes, and many of them are regularly exposed to large variations in ambient oxygen levels. For more than half a century, there have been various, often divergent, claims regarding the effect of body size on hypoxia tolerance in fish. Here, we attempt to link old and new empirical data with the current understanding of the physiological mechanisms behind hypoxia tolerance. Three main conclusions are drawn: (1) body size per se has little or no impact on the ability to take up oxygen during hypoxic conditions, primarily because the respiratory surface area matches metabolic rate over a wide size range. If size-related differences are seen in the ability for oxygen uptake in a species, these are likely to reflect adaptation to different life-styles or habitat choice. (2) During severe hypoxia and anoxia, where fish have to rely on anaerobic ATP production (glycolysis) for survival, large individuals have a clear advantage over smaller ones, because small fish will run out of glycogen or reach lethal levels of anaerobic end-products (lactate and H(+)) much faster due to their higher mass-specific metabolic rate. (3) Those fish species that have evolved extreme adaptations to hypoxia, including haemoglobins with exceptionally high oxygen affinities and an alternative anaerobic end-product (ethanol), reveal that natural selection can be a much more powerful determinant of hypoxia tolerance than scaling of physiological functions.

Does size matter for hypoxia tolerance in fish

Anthropogenic modifications to aquatic ecosystems have altered connecting pathways within, and in some cases, between watersheds. Human structures, such as hydroelectric facilities, often impede fi...

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Non-physical barriers to deter fish movements

SUMMARY 1 Hypoxia occurs seasonally in many stratified coastal marine and freshwater ecosystems when bottom dissolved oxygen (DO) concentrations are depleted below 2–3 mg O2 L )1  2 We evaluated the effects of hypoxia on fish habitat quality in the central basin of Lake Erie from 1987 to 2005, using bioenergetic growth rate potential (GRP) as a proxy for habitat quality We compared the effect of hypoxia on habitat quality of (i) rainbow smelt, Osmerus mordax mordax Mitchill (young-of-year, YOY, and adult), a cold-water planktivore, (ii) emerald shiner, Notropis atherinoides Rafinesque (adult), a warm-water planktivore, (iii) yellow perch, Perca flavescens Mitchill (YOY and adult), a cool-water benthopelagic omnivore and (iv) round goby Neogobius melanostomus Pallas (adult) a eurythermal benthivore Annual thermal and DO profiles were generated from 1D thermal and DO hydrodynamics models developed for Lake Erie’s central basin 3 Hypoxia occurred annually, typically from mid-July to mid-October, which spatially and temporally overlaps with otherwise high benthic habitat quality Hypoxia reduced the habitat quality across fish species and life stages, but the magnitude of the reduction varied both among and within species because of the differences in tolerance to low DO levels and warm-water temperatures 4 Across years, trends in habitat quality mirrored trends in phosphorus concentration and water column oxygen demand in central Lake Erie The per cent reduction in habitat quality owing to hypoxia was greatest for adult rainbow smelt and round goby (mean: )35%), followed by adult emerald shiner (mean: )12%), YOY rainbow smelt (mean: )10%) and YOY and adult yellow perch (mean: )85%) 5 Our results highlight the importance of differential spatiotemporally interactive effects of DO and temperature on relative fish habitat quality and quantity These effects have the potential to influence the performance of individual fish species as well as population dynamics, trophic interactions and fish community structure

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Seasonal and interannual effects of hypoxia on fish habitat quality in central Lake Erie

When an oxygen gradient ranging from c. 10 to 95% air saturation was formed in a 5 m chamber, largemouth bass Micropterus salmoides avoided water in which dissolved oxygen values were <27% air saturation. There was a significant (P<0·05) correlation between fish mass and the level of dissolved aquatic oxygen that was selected. Small fish (23–500 g) utilized waters of lower oxygen levels than did the larger fish (1000–3000 g). The results of this study suggest that largemouth bass are able to sense and avoid hypoxic water, and select aquatic oxygen tensions that maintain their metabolic scope for growth and activity.

D.R. Wilhelm

Papers

The influence of fish size on the avoidance of hypoxia and oxygen selection by largemouth bass