Biogeosciences Discuss., 6, 5073–5144, 2009 Biogeosciences www.biogeosciences-discuss.net/6/5073/2009/ Discussions BGD © Author(s) 2009. This work is distributed under 6, 5073–5144, 2009 the Creative Commons Attribution 3.0 License. Biogeosciences Discussions is the access reviewed discussion forum of Biogeosciences Impacts of hypoxia on the structure and processes in the Impacts of hypoxia on the structure and pelagic community processes in the pelagic community W. Ekau et al. (zooplankton, macro-invertebrates and Title Page fish) Abstract Introduction Conclusions References W. Ekau1, H. Auel2, H.-O. Portner¨ 3, and D. Gilbert4 Tables Figures 1Fisheries Biology, Leibniz Center for Tropical Marine Ecology, Fahrenheitstr. 6, 28359 Bremen, Germany 2 Marine Zoology (FB 2), University of Bremen, P.O. Box 330 440, 28334 Bremen, Germany J I 3Integrative Ecophysiology, Alfred Wegener Institute, Am Handelshafen 12, 27570 Bremerhaven, Germany J I 4 Maurice-Lamontagne Institute, Fisheries and Oceans Canada, Mont-Joli, Quebec´ G5H3Z4, Back Close Canada Full Screen / Esc Received: 30 April 2009 – Accepted: 4 May 2009 – Published: 19 May 2009 Correspondence to: W. Ekau ([email protected]) Printer-friendly Version Published by Copernicus Publications on behalf of the European Geosciences Union. Interactive Discussion 5073 Abstract BGD Dissolved oxygen (DO) concentration in the water column is an environmental param- eter that is crucial for the successful development of many pelagic organisms. Hypoxia 6, 5073–5144, 2009 tolerance and threshold values are species- and stage-specific and can vary enor- −1 5 mously. While some fish species may suffer from oxygen values of less than 3 ml L Impacts of hypoxia and show impact on growth, development and behaviour, other organisms such as eu- on the structure and −1 phausiids may survive DO levels as low as 0.1 ml L . A change in the average or the processes in the minimum or maximum DO in an area may have significant impacts on the survival of pelagic community certain species and hence on the species composition in the ecosystem with conse- 10 quent changes in trophic pathways and productivity. W. Ekau et al. Evidence of the deleterious effects of oxygen depletion on species of the pelagic realm is scarce, particularly in terms of the effect of low oxygen on development, re- cruitment and patterns of migration and distribution. While planktonic organisms have Title Page to cope with different DOs and find adaptive mechanisms, nektonic species may avoid Abstract Introduction 15 areas of inconvenient DO and develop adapted migrational strategies. Planktonic or- ganisms may only be able to escape vertically, above or beneath the Oxygen Minimum Conclusions References Zone (OMZ). In shallow areas only the surface layer can serve as a refuge, in deep Tables Figures waters many organisms have developed vertical migration strategies to use, pass and cope with the OMZ. J I 20 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. J I Back Close 1 Introduction Full Screen / Esc Oxygen has come into the focus of science only recently as an environmental factor impacting physiology, behaviour and life cycles of marine organisms. While significant Printer-friendly Version 25 research has been done on oxygen consumption or transport, etc. on cellular or in- dividual basis the impact of low oxygen conditions, i.e. hypoxia on the physiological Interactive Discussion 5074 and behavioural capacity and life cycles is much less investigated (5200 hits in ASFA). About 35 percent of this work has been done on fish (1800 hits). The majority of the BGD studies has been conducted in estuaries or semi-enclosed seas such as Baltic or Black 6, 5073–5144, 2009 Seas. The investigation of hypoxia in the pelagic ecosystems is a rather new topic. 5 The pelagic community includes representatives from nearly all zoological phyla. Most marine animals spend at least part of their lifetime in the pelagic environ- Impacts of hypoxia ment, either as planktonic or nektonic organisms. Neglecting the primary produc- on the structure and ers (e.g. diatoms, dino-flagellates, green algae) at this stage we concentrate here processes in the on the consumers (e.g. copepods, jelly fish, euphausiids, decapods) and top preda- pelagic community 10 tors (e.g. squid, fish, mammals), and within these groups we focus on the dominant species. W. Ekau et al. −1 −1 The impact of low oxygen or hypoxia (<1.42 ml O2 L ; 60 µM; 2 mg O2 L ) can be twofold: on a macro-scale in the environment hypoxia constrains the vertical and hor- Title Page izontal distribution of species and forces them to depart from their normal behaviour; 15 and on a micro-scale it causes physiological changes and shifts and thus alters life- Abstract Introduction cycle performance, growth capacity and reproductive success and the vulnerability to diseases. Conclusions References Hypoxia tolerance and threshold values are species- and stage-specific and can vary Tables Figures enormously (Miller et al., 2002). Fishes seem to be more vulnerable to low oxygen 20 levels than crustaceans and molluscs (Vaquer-Sunyer and Duarte, 2008). Even within J I species, different development stages may suffer differently from hypoxia. While some −1 fish larvae may suffer at oxygen values of less than 3 ml O2 L and show impact on J I growth, development and behaviour, other organisms such as euphausiids may survive −1 Back Close to 0.1 ml O2 L . While sensitive species avoid even medium range oxygen levels, 25 others such as gobies have no problem to spend hours sitting on the oxygen-free mud Full Screen / Esc at the sea floor! A change in the average and the minimum and maximum DO in an area may thus have significant impact on the survival of certain species and the Printer-friendly Version species composition of an ecosystem with consequent changes in trophic relationships and productivity. Interactive Discussion 5075 After elucidating the role of oxygen and the impact of hypoxia at the physiological level, we review in this paper the effects of hypoxic conditions on some of the domi- BGD nant organism groups in the pelagic environment and discuss possible consequences 6, 5073–5144, 2009 for the structure of pelagic ecosystems and predator-prey-relationships in the pelagic 5 ecosystem. Lluch-Cota et al. (2007) in their review of the Gulf of California still mention that “. no discrete sampling of the pelagic community of the water column as related Impacts of hypoxia to oxygen level. ” has been performed. Only recent congresses and symposia have on the structure and taken up this topic. processes in the Depending on the scientific discipline, oxygen is presented in various units, mainly pelagic community 10 in DO concentration or saturation. Geochemical studies mostly use µmol per liter as unit for measured values, while mg or mL per liter are widely spread in physical and W. Ekau et al. environmental studies. Physiological studies relate to partial pressure measured in hPa (mbar) or kPa, some are still in mm Hg. However, the oxygen demand of individuals Title Page is temperature-dependant and saturation is a unit including these two parameters. In 15 many cases, temperature is not explicitly given by the authors and thus comparison Abstract Introduction between results is sometimes difficult. We have tried to harmonise the units in the review to a certain extent. Table 1 may help to transform and compare between units. Conclusions References Tables Figures 2 Physiological background of hypoxia tolerance in marine organisms J I Ecophysiological studies aim to understand the physiological basis of zoogeographical J I 20 patterns within the system of currents and in a latitudinal cline, as much as these distribution patterns are determined by the temperature regime and its interactions Back Close with other abiotic factors, like water velocities, oxygen levels etc. The responsible physiological mechanisms involve those limiting and adjusting cold and heat tolerance Full Screen / Esc and these research topics have, in general, gained interest in the context of global 25 warming and associated shifts in the geographical distribution of marine ectothermic Printer-friendly Version animals. Interactive Discussion Traditional concepts in animal physiology have considered the effects of individual 5076 factors like oxygen, temperature, carbon dioxide or salinity and have addressed the specific responses in organisms from various habitats. For analyses of the complex BGD interactions and effects of various environmental factors at the ecosystem level these 6, 5073–5144, 2009 traditional concepts need to be integrated in order to achieve more realistic estimates 5 of sensitivity to environmental change. Progress in recent years has identified how such integration may occur and can be implemented (e.g. Portner¨ and Farrell, 2008). Impacts of hypoxia The traditional principles and their integration operate similarly across animal phyla and on the structure and will accordingly be briefly summarized from a general point of view. Those principles processes in the are emphasized which provide a link between physiology and ecology and support an pelagic community 10 understanding of ecosystem level processes. In the context of organismic requirements for ambient oxygen and of the associ- W. Ekau et al. ated levels of hypoxia sensitivity the concepts of oxyconformity, oxyregulation, and the critical PO2 have been revisited based on an integration of systemic and cellular pro- Title Page cesses in declining ambient oxygen tensions (Portner¨ and Grieshaber, 1993). It has 15 been recognized early that animals may show different patterns of oxygen consumption Abstract Introduction in response to changes in ambient PO2. Some keep their oxygen consumption more Conclusions References or less constant in a wide range of PO2 and are called oxyregulators. Others reduce their oxygen uptake with decreasing oxygen tensions and have, consequently, been Tables Figures termed oxyconformers.