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

Abstract

Abstract. 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 O 2 L −1 through impacted growth, development and behaviour, other organisms such as euphausiids may survive DO levels as low as 0.1 mL O 2 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.Correspondence to: W. Ekau (wekau@zmt-bremen.de) 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.

Effects of hypoxia in the gills of the Manila clam Ruditapes philippinarum using NMR-based metabolomics

“…Most studies on the effects of hypoxia on marine life have focused on metabolic processes associated with energy consumption (Ekau et al, 2010;Childress & Seibel, 1998;Seibel, 2011). A thorough understanding of the hypoxia-induced whole-body stress and the mechanisms underlying the adaption of the marine organisms to the hypoxic environment remains to be achieved.…”

Expansion of oxygen minimum zones may reduce available habitat for tropical pelagic fishes

“…Habitat compression also impacts the preferred prey of billfishes and tunas (primarily small scombrids, cluepids and carangids), which share similar high-oxygen-demand physiology 20,25,26 . Hence, these pelagic predators and their preferred prey tend to be compressed together in the oxygenated narrow surface mixed layer habitat above the thermocline 8,9 .…”

“…Direct oxygen tolerance measurements for adult billfishes are not available, although one juvenile sailfish (Istiophorus platypterus) study indicated high oxygen consumption and typical metabolic rates associated with tropical tunas 19 . These high-oxygen-demand species also share obligate ram ventilation respiration, large gill surface and intolerance to low ambient dissolved oxygen 15,17,20,21 . Here, we consider the plausible hypothesis that these species have oxygen limitations that impact vertical habitat use.…”

Investigating hypoxia in aquatic environments: diverse approaches to addressing a complex phenomenon

“…Rapidly changing oxygen concentrations and the occurrence of hypoxia impose stress on aquatic organisms, and the physiological responses and survival strategies of the biota are very complex (Vaquer-Sunyer and Duarte, 2008;Zhang et al, 2010;Levin, 2003;Ekau et al, 2010). When oxygen concentrations decrease, pelagic organisms may escape vertically, develop migration strategies to avoid hypoxic areas, or develop adaptation mechanisms .…”