Soutenance mémoire
ECOSYSTEM FUNCTIONING AND SERVICES
Ecosystem functioning describes the integrated sum of the processes performed by the biota encountered in a specified ecosystem. Stachowicz et al. (2007) de fi ne it as « aggregate or emergent aspects of ecosystems (e.g., production, nutrient cycling), carrying no inherent judgment of value » . In this document, ecosystem functioning will include mechanisms that influence ecosystem functions, while ecosystem functions are quantifyable products (such as the amount of released nutrients). When ecosystem functions are associated with a value to human society , we speak of ecosystem services (Naeem et al., 2009). For ex ample , forest growth can be seen as wood production and carries an economical value. From an integrated economical perspective, the knowledge about marine ecosystem functioning will be important for developing a guideline for the sustainable use of marine resource s, which is necessary to ensure the coexistence of humankind and the ocean’s biota as its food and pleasure source (Wolanski 2006). Economical values are sometimes difficult to assign to biogeochemical cycles, but the complex web of interactions and energetic and trophic links makes biologie al production or decomposition part of the nu trient cycles (Naeem et al., 2012). The theory of global stability assumes that an ecosystem will always need to return to its equilibrium stable state after a perturbation. However, according to Gray and Elliot (2009), marine systems are more likely to follow the theory of the neighbourhood stability, where the system may exist and switch to an alternative steady state after perturbation. Following the ide a of alternative steady states, perturbation such as climate change can cause an ecosystem to fall into a stable state with, e.g., changed diversity patterns – that may coincide with changes in ecosystem functions. It is therefore likely that climate change will provoke changes in ecosystem functions and services (Cardinale et al., 2012).
Biodiversity and ecosystem functioning
In the face of decreasing biodiversity (Chapin III et al., 2000; Hooper et al., 2012), the interest in describing the role of biodiversity for ecosystem functioning has significantly increased during the last decades (Hulot et al., 2000; Loreau et al., 2002; Solan et al., 2009; Naeem et al., 2012). Several models for the biodiversity – ecosystem function relationship have been proposed and discussed particularly in terrestrial ecology literature (Hooper et al., 2005). It has generally been accepted by the expert community that biodiversity does affect ecosystem functions, but there is still an ongoing debate on the underlying mechanisms and direction of the biodiversity – ecosystem function relationship (e.g. Loreau et al., 2002; Stachowicz et al., 2007; Naeem et al., 2009). Part of the debate involves, how biodiversity is measured. Traditionally, the number of species, taxonomie richness, has been used to explain plant biomass production, but soon the concept of functional traits evolved, assuming that it is not the mere number of species, but rather their complementarity in mediating different functions or occupying different niches which are important for the total ecosystem functioning (Naeem et al., 2012). If species are complementary in their traits, ecosystem functions should increase linearly with species richness. If species are redundant in their functions, the relation between species richness and ecosystem functions can be asymptotic or show a rivet-like distribution (Lore au et al., 2002). Such redundancy cou Id serve as an insurance against ecosystem functioning changes, if environmental change reduces biodiversity (Yachi and Loreau, 1999). Finally , sorne species can provide a very particular, idiosyncratic role for the ecosystem’s functions not encountered in other species, thus that we speak of an identity effect of the species (Lore au et al., 2002). Biodiversity can also be measured as phylogenetic, genetic, landscape or other kinds of diversity (Naeem et al., 2012). However, hereafter 1 will only treat diversity in terms of taxonomie and functional group richness and community composition, which are within the scope of my thesis.
Resource availability and ecosystem functioning
Few studies within the diversity-ecosystem function debate have inc1uded the external factor resource availability to explain relationships. In fact, most studies are placed in experimental or controlled environmental settings, such that external variability cannot confound the diversity-ecosystem function relationship. However, in an experimental setup,Fridley (2002) found that resource availability dominates the relation between plant species richness and their production, and Cardinale et al. (2009) found important effects of resource availability on primary production patterns of phytoplankton species. Contrary to experimental studies, observational studies are strongly influenced by the variability on the natural setting (Maestre et al., 2012). Non-intertidal benthic communities are supposedly limited by food supply from the water column (Jahnke, 2004; Klages et al.,2004). Hence, benthic activity may be greatly influenced by vertical flux patterns (Klages et al., 2004; Renaud et al., 2007a; Gradinger et al., 2010). Consequently, benthic remineralisation not only depends on the diversity of a benthic community, but also on the presence and quality of organic matter in the benthic environment (Sun et al., 2009). Resource availability may also act indirectly on ecosystem functions (Fig. 1). Many studies have demonstrated that on a regional scale, diversity or abundance of communities increase with increasing quantity of food supply (Rex et al., 2006; Hoste et al., 2007;Witman et al., 2008). Thus, more resources would mean higher diversity, which increases ecosystem functions. Clearly, such interactions must be taken into account when looking for factors that can best predict areas of high ecosystem functions.
The influence of temporal and spatial variability in natural systems
Ecosystem functioning varies on the spatial (Glud, 2008; Schmid et al. , 2009) and temporal scales (Yachi and Loreau, 1999; Farias et al., 2004; Frid, 2011 ). While experimental setups control for such confounding factors (or are specifically looking for it), spatio-temporal variation introduces an additional source of variation when studying the influence of diversity and/or resource availability on ecosystem functions in the natural environment. For example, rernineralisation in benthic environments is higher in summer than in winter (Renaud et al., 2007a), thus measures at different sites are comparable only in the same season. On the spatial scale, benthic rernineralisation of nutrients is influenced by zonation of the redox-front in the sediments. In oligotrophic, weU-oxygenated benthic environments, the oxic layer of the sediments can be several centimetres into the sediment. Thus, incoming organic matter is foremost aerobicaUy remineralised and the sediments act as a source of most nutrients (Hensen et al., 2006). In less oxygenated sediments, the redox front can be c10ser to the sediment-water interface, and degradation of organic matter may require nitrate or even nitrite as reaction partner, thus that sediments in less oxygenated regions can act as a sink for nitrate (Hulth et al., 2005). The habitat precondition can affect how diversity of bioturbating macrofauna wiU be related to nutrient fluxes (Laverock et al., 2011).The influence of temporal and spatial sc ales on ecosystem functions can also interact in their influence on variation. Benthic remineralisation of shallow water communities was different in one site of a lagoon than another, but only in one of two studied years (Thouzeau et al., 2007). This shows how seasonal differences can intervene if we want to generalize ecosystem processes from field study results. Depending on the spatial and time scale observed, a measured change might therefore simply represent a stochastic change (the system will faU back to the equilibrium state), or a progressive change (the system will shift to another steady-state). To distinguish stochastic from progressive change, it is crucial to complete our knowledge on ecosystem processes with long-term studies inc1uding seasonal aspects as weIl as multiple spatial scales (Klages et aL, 2004; Piepenburg, 2005; Stachowicz et al., 2007).
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Table des matières
REMERCIEMENTS
AVANT-PROPOS
RESUME
ABSTRACT
LISTE DES TABLEAUX
LISTE DES FIGURES
INTRODUCTION GÉNÉRALE
1. ECOSYSTEM FUNCTIONING AND SERVICES
Benthic ecosystem functioning
2. FACTORS RELEVANT FOR ECOSYSTEM FUNCTIONING
Biodiversity and ecosystem functioning
Resource availability and ecosystem functioning
The influence of temporal and spatial variability in natural systems
3. BENTHIC ECOSYSTEMS IN THE ARCTIC
Pelagic-benthic coupling and characteristics of the environ ment
Known patterns of benthic ecosystem functioning and its relation to resource availability and diversity in the Arctic
Spatial and temporal variability of ecosystem functioning in the Arctic
Known patterns of benthic ecosystem functions and food availability on the Canadian Arctic shelves
Known patterns of diversity in the Canadian Arctic
Importance of finding hotspots of benthic ecosystem functioning in the face of a changing Arctic
4. GENERAL OBJECTIVE
5. SAMPLING DESIGN
CHAPITRE 1 SPRING-TO-SUMMER CHANGES AND REGIONAL V ARIABILITY OF BENTHIC PROCESSES IN THE WESTERN CANADIAN ARCTIC
RÉSUMÉ DU PREMIER ARTICLE
SPRING-TO-SUMMER CHANGES AND REGIONAL V ARIABILITY OF BENTHIC PROCESSES IN THE WESTERN CANADIAN ARCTIC
Introduction
Materials and methods
Results
Discussion
Conclusions
ACKNOWLEDGMENTS
CHAPITRE 2 MULTIVARIATE BENTHIC ECOSYSTEM FUNCTIONING IN THE ARCTIC – BENTHIC FLUXES EXPLAINED BY ENVIRONMENTAL PARAMETERS IN THE SOUTHEASTERN BEAUFORT SEA
RÉSUMÉ DU DEUXIÈME ARTICLE
MULTIVARIATE BENTHIC ECOSYSTEM FUNCTIONING IN THE ARCTIC – BENTHIC FLUXES EXPLAINED BY ENVIRONMENTAL PARAMETERS IN THE SOUTHEASTERN BEAUFORT SEA
Introduction
Material and methods
Results
Discussion
Conclusions
ACKNOWLEDGMENTS
CHAPITRE 3 HOTSPOTS IN THE COLD – A PERSPECTIVE FROM BENTHIC REMINERALISATION IN THE CANADIAN ARCTIC
RÉSUMÉ DU TROISIÈME ARTICLE
HOTSPOTS IN THE COLD – A PERSPECTIVE FROM BENTHIC REMINERALISA TION IN THE CANADIAN ARCTIC
Introduction
Origin of data and statistical analyses
Benthic remineralisation function in the Canadian Arctic in 2008-2009
Canadian benthic remineralisation hotspots and what they can tell us
Conclusion
ACKNOWLEDGEMENTS
CHAPITRE 4 ARE HOTSPOTS AL WAYS HOTSPOTS? TEMPORAL VARIABILITY AND ITS ROLE FOR THE RELATIONSHIP BETWEEN DIVERSITY AND ECOSYSTEM FUNCTIONING IN ARCTIC BENTHIC ENVIRONMENTS
RÉSUMÉ DU QUATRIÈME ARTICLE
ARE HOTSPOTS ALWAYS HOTSPOTS? TEMPORAL VARIABILITY AND ITS ROLE FOR THE RELATIONSHIP BETWEEN DIVERSITY AND ECOSYSTEM FUNCTIONS IN ARCTIC BENTHIC ENVIRONMENTS
Introduction
Methods
Results
Discussion
Conclusion
ACKNOWLEDGMENTS
CONCLUSION
First comprehensive analysis of spatio-temporal variation in benthic remineralisation across the Canadian Arctic shelves
First explicit integrative study on the influence of abiotic environ mental parameters, resource availability and diversity on ecosystem function in Arctic benthic systems
Where are benthic remineralisation hotspots?
Future directions
RÉFÉRENCES BIBLIOGRAPHIQUES
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