Pelagic mesocosms

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May 18, 2010 - ... manipulation. Figure 6.6 Possible time courses for CO2 manipulation. .... the mesocosms; rates of key biological processes (e.g. auto- and heterotrophic activities, calcification). ... should be available as an appendix or online. The motivation .... Turbulence and random processes in fluid mechanics. 154 p.
Part 2: Experimental design of perturbation experiments

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Pelagic mesocosms

Ulf Riebesell1, Kitack Lee2 and Jens C. Nejstgaard3 Leibniz Institute of Marine Sciences (IFM-GEOMAR), Germany Pohang University of Science and Technology, South Korea 3 Uni Environment, Research AS, Norway

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6.1

Introduction

One of the greatest challenges in understanding and forecasting the consequences of ocean acidification is the scaling of biotic responses at the cellular and organism level to the community and ecosystem level, and their parameterisation in ecosystem and biogeochemical models of the global ocean. Here mesocosms, experimental enclosures designed to approximate natural conditions, and in which environmental factors can be manipulated, provide a powerful tool to link between small-scale single species laboratory experiments and observational and correlative approaches applied in field surveys. A mesocosm study has the advantage over standard laboratory tests in that it maintains a natural community under close to natural, self-sustaining conditions, taking into account relevant aspects from “the real world” such as indirect effects, biological compensation and recovery, and ecosystem resilience. The mesocosm approach is therefore often considered the experimental ecosystem closest to the real world, without losing the advantage of reliable reference conditions and replication. By integrating over multiple species sensitivities and indirect effects up or down the food web, the responses obtained from mesocosm studies can be used to parameterise ocean acidification sensitivities in ecosystem and biogeochemical models. As stated by Parsons (1982): “The main advantages … unique to enclosed ecosystems are: 1. The ability to study the population dynamics of two or more trophic levels for a protracted period of time. This includes both biological studies regarding species dynamics as well as chemical studies towards achieving a mass balance for the distribution of certain elements in the water column. 2. The ability to manipulate the environment of the water column either by natural means, such as physical upwelling, or by unnatural means, such as by the introduction of a pollutant.” Later mesocosm studies highlighted a third advantage that relates to the broad spectrum of processes captured in mesocosm enclosures, namely: 3. The ability of bringing together scientists from a variety of disciplines, ranging from for example molecular and cell biology, physiology, marine ecology and biogeochemistry to marine and atmospheric chemistry and physical oceanography. Combining a broad spectrum of disciplines in a single study offers the unique opportunity to study interactions of ecosystem dynamics and biogeochemical processes and track the consequences of ocean acidification sensitivities through the enclosed system (e.g. Heimdal et al., 1994; Riebesell et al., 2008a). Although the first mesocosm experiment was reported in 1939 (Petterson et al., 1939), it was not until the 1960s and 1970s that studies in larger sized enclosures grew popular (Parsons, 1981; Banse, 1982). Over the past four decades, mesocosm studies have been successfully used for a wide range of applications and have provided a wealth of information on trophic interactions and biogeochemical cycling of aquatic ecosystem in lakes (Sanders, 1985; Gardner et al., 2001), marine systems (Lalli, 1990; Oviatt, 1994) as well as in ecological risk assessment (Boyle & Fairchild, 1997). Effects of acidification on aquatic ecosystems were first studied in freshwater systems (Almer et al., 1974; Schindler et al., 1985; Schindler, 1988), where mesocosm studies on plankton, periphyton and metals gave results that were similar to those observed in whole-lake experiments (Schindler, 1980; Müller, 1980). Recently, a series of multinational mesocosm experiments were conducted to examine the effects of ocean acidification on Guide to best practices for ocean acidification research and data reporting Edited by U. Riebesell, V. J. Fabry, L. Hansson and J.-P. Gattuso. 2010, Luxembourg: Publications Office of the European Union.

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Part 2: Experimental design of perturbation experiments

marine pelagic ecosystems (Delille et al., 2005; Engel et al., 2005; Kim et al., 2006; Riebesell et al., 2008a) with mesocosms moored in sheltered bays and free-floating in open waters (Figure 6.1). Results from these experiments highlighted the sensitivity of key components of the pelagic ecosystem to ocean acidification and revealed associated biogeochemical feedback processes (Riebesell et al., 2008b). It needs to be acknowledged, however, that artefacts, like wall growth, and constraints of enclosures have to be considered when extrapolating mesocosm results to natural systems (Pilson & Nixon, 1980; Brockmann, 1990; Petersen et al., 1998). Enclosures of all kinds are inherently limited in their ability to include higher trophic levels, and to approximate water column structure and advective processes occurring in nature (Menzel & Steele, 1978; Carpenter, 1996). Enclosure effects may also influence food web dynamics to varying

Figure 6.1 Mesocosm studies in ocean acidification research - upper left: PeECE III study in the Espegrend Marine Biological Station, Bergen, Norway (Riebesell et al., 2008a); upper right: mesocosm facility at Jangmok on the southern coast of Korea (Kim et al., 2008); lower panel: free-floating mesocosms deployed in the Baltic Sea (Riebesell et al., unpubl.).

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Figure 6.2 Examples of two basic designs of mesocosm enclosures - left: MERL (Marine Ecosystem Research Laboratory) mesocosms at the University of Rhode Island, USA, with two different mixing schemes (left – plunger; right – rotating paddle, enabling a stratified water column); diameter: 1.83 m, height: 5.49 m, volume: 13.1 m3; from Donaghay & Klos (1985); right: flexible-wall in situ enclosures with floatation rings at the surface, used in CEPEX (Controlled Ecosystem Pollution Experiment) studies in Saanich Inlet, BC, Canada, in the late 1970s (Menzel D. W. & Case J., 1977. Concept and design: controlled ecosystem pollution experiment. Bulletin of Marine Science 27:1-7.).

degrees, creating trophic interactions that can differ with mesocosm dimension and which may deviate from those of the natural system intended to be mimicked (Kuiper et al., 1983; Stephenson et al., 1984; French & Watts, 1989). Despite these difficulties and the intense debate they have spurred over the past decades (e.g. Drenner & Mazumber, 1999), mesocosm enclosure studies still remain the most generally applicable means to experimentally manipulate and repeatedly sample multi-trophic planktonic communities (Griece et al., 1980) and thus provide an essential link between small-scale experiments on individual organisms and observational approaches in field surveys and natural high-CO2 environments. This link becomes indispensible when trying to investigate organism and population responses to ocean acidification at the ecosystem level and to parameterise them to be included in marine ecosystem and biogeochemical models. 6.2

Approaches and methodologies

Although the basic approach of mesocosm studies is straightforward and uniform, i.e. enclosing a body of water and studying the processes of interest in it over an extended period of time, the specifics of implementing a mesocosm experiment are often very different. This relates to aspects such as materials, design and location of the enclosures, for example land-based solid structures versus in situ flexible-wall enclosures (Figure 6.2), as well as the procedures used to fill, manipulate, mix and sample the mesocosms. Enclosure dimensions range from