Soutenance mémoire
Spatial variation in recruitment
Over the l OO -km extent of our stud y area, we observed substantial spatial variation in recruitment. Such variation is not surprising as the recruitment of invertebrates with planktotrophi c life cyc les is notori ously variable in marine systems (Harris et al. 1998). However, our experimenta1 design permitted us to examine this variatio n over three spatial scales: between sites (10 km – « regional »); within sites (100 m – » local »), and between collectors (10 m – « neighborhood »). Although substantial vari ation was observed at aIl three scales, there was only signiftcant variation at the smallest sca le, i.e., at the neighborhood leve l, a very unexpected result that suggests that the distribution of larvae is more homogeneous at the larger sca les and that the ecological processes influencing larval settlement are very heterogeneous at small spatial scales. At this point, there is no obv ious environmental factor th at we can identify that might explain this pattern of variation, and it stands in contrast to earlier studies (McKindsey & Bourget 2000, Smith et al. 2009) conducted in this same system which showed heterogeneo us pattern s of recruitment dominating at a much large r scales (4-30 km). It is likevvise surprising that there was no correlation between recruitment in subtidal habitats and recruitment in intertidal habitats. Given the proximity of the two habitats at a given site (usuall y less than 1 km) relative to the 10-km separation of the sites, one would expect both habitats to have a similar exposure to potential recruits (i.e. , be in the same water mass). Perhaps the lack of any patterns was due to the general lack of signift cant variation at this spatial scale (see above), but this result suggests that one cannot sim ply assume that different habitats that are in close proximity will have similar conditions. In particular, there may be substantial bathymetric differences in water movement that determine the supply of larvae to a particular location (Archambault & Bourget 1999). For example, bottom topography might interact with coastal currents to alter the flow of water across the bottom. Other hypotheses have also been proposed to expl ain variability in recruitment between habitats including variable rates of the survival of larvae and postsettlement stages (Yoo & Ryu 1985), larval behavior (Dobretsov & Miron 2001), settlement density (Grant 1977), tidal conditions during settlement periods (Porri et al. 2007), and physical barri ers (Broitman et al. 2008). The lack of any correlation between the abundance of adults and recruits in intertidal environments is also surprising in certain senses. Given the wide dispersa l of musse l larvae during their planktonic period, no relationship between adults and recruits would be expected in terms of local reproduction (i.e. , local larval retention wou Id be unlikely). However, as a cue for settlement, the presence of adult musse ls is weil known to enhance recruitment (Menge 199 1), either directly (i.e., settl ement within ex isting ad ult beds) or indirectly by prov iding loca l sources for secondary recruits (post-settlement juveniles that disperse locall y via wave action). Thus, sites of hi gh adult abundance would a priori be expected to have hi gher recruitment. However, possible negative effects of the fi ltration of larva l stages by adults (Lehane & Davenport 2004) or annual vari ation in larval supp ly could counter any such trend, making it diffi cult to detect. Finally, predation ofrecruits can influence the abundance of later stages (Keo ugh & Downes 1982), and we know li ttle about the sources of mortality of early post-settlement stages of musse ls. Final ad ult distributio n may thus be the result of a combinati on of physica l processes and biological postsettlement mechanisms (Johnson & Ge ll er 2006).
Contrasting habitats
Blue musse ls are ecological known as intertidal mussels, likely due to bath the bias of benthi c studies bein g dispro portionally conducted in this environment and the reality that survival really is best in this environment. lt is thus rather ironie that in terms of its fundamenta l and realized niches, this mussel appears to be exc luded from the habitats where its intrinsic performance is highest, namely the subtidal environment. Our limited data on the differences in growth between subtidal and intertidal habitats are nonetheless convincing, and correspond with more extensive data fo r musse ls that are continuously submerged intentionall y (e.g., musse ls in aquaculture; Mallet & Carver 1989) or unintentionall y (Page & Hubbard 1987, Bourget et al. 2003) where growth rates are almost al ways faster compared to intertidal populati ons (exceptions occur when fo uli ng organisms are killed by emersion). It seems then that by inhabiting submerged habitats, musse ls decrease physiological stress and increase access to food, resulting in hi gher growth rates and greater reproducti ve effort. Under more natural systems, however, it appears that the benefits of occupyin g the optimal habitat are more than offset by the res ul ting decrease in survival from predation. Thus, as seen for many intertidal organisms, abundances are highest in the spatial refuges created by environmental stresses that excl ude predators.For these two contrasting habitats, we thus see two diffe rent eco logies, one in which the species is an ecosystem engineer, prov iding both physica l structure and secondary production to the local environment and the other in which it is a rare species, contributing little to the local assemblage. This knowledge is essenti al fo r a proper understanding of the ecosystem in many regards. Most obviously, we know that in spite of the enormous potential for secondary producti on in subtidal environments, little is actually occurring, at least by mussels. Less obviously, in terms of the metapopulation dynami cs of musse ls in the SLME (Sm ith et al. 2009), our results show that f r estimating dynamics and connectivity, the acti on is happening principall y in the intertid al environments and that a knowledge of the relative abundance of intertidal populations is sufficient for estimating the sources of propagules. Thu s, by knowing more about the entire role of this species in the multiple environments, we can be better able to understand its dynami cs and its influence on the rest of the community
CONCLUSIONS
L’objectif principal de cette thèse est de déterminer la répartition bathymétrique et spatiale des populations de moules dans une large zone de l’EMSL et d’évaluer la contribution potentielle des « populations» infralittorales à la démographie de l’ensemble de la métapopulation . Pour comprendre le fonctionnement d ‘ une métapopulation, il est essentiel de connaître la répartition réelle de toutes les sous-populations qui pourraient contribuer à sa dynamique. Pour les espèces de la zone côtière, il faudrait donc vérifier la présence, ou l’ absence, de populations notoires dans les étages infralittoraux et médiolittoraux (Lawrie & McQuaid 2001). Dans le cas de la moule bleue, il s’agit d’ une espèce dominante et bien connue dans un des habitats (l’étage médiolittoral), mais cette espèce pourrait également vivre et être fréquemment observée dans un autre type d’ habitat (l’étage infralittoral). Dans un contexte de métapopulation, il en va de la validité des modèles de bien évaluer l’importance des populations infralittorales. Étant donné le cycle vital complexe de plusieurs invertébrés benthiques, il faudrait également connaître la dynamique des stades adulte et larvaire. Pour ce faire, j’ai évalué l’abondance des adultes (c ‘est-à-dire la source des larves) ainsi que le recrutement (c’est-àdire la dissémination des larves) dans ces deux habitats contrastants. Pour répondre aux deux questions fondamentales sur le suivi des larves « où vont les larves? » et « d ‘où proviennent les nouvelles recrues? », il est recommandé d’intégrer des approches spatiales et temporelles (Levin 2006). En effet, on ne peut pas se fier à une étude qui ne comprend qu’un seul ou même quelques sites. Nous avons donc documenté l’abondance des moules dans les zones médiolittorales et infralittorales de dix sites distribués le long d ‘ une côte homogène de 100 km. Le résultat le plu év ident est qu ‘ il n’existe pas de populations importantes en milieu in fralittoral dans ce secteur de l’EMSL. Par extrapolati on, il est donc peu probable que des populations infralittorales soient présentes à l’échelle de l’ensemble de l’écosystème du Saint-Laurent. Ce la veut dire que les modèles de métapopulations de moul es qui se basent sur l’évaluation des populations méd iolittora les (Smith et al. 2009, Le Corre et al. 2013) ont supposé correctement que l’on peut ignorer la contribution démographiq ue des moules in fraiittorales. Étant donné que d’abondantes populations de moules bl eues peuv ent parfois être observées en mili eu infra littoral « naturel » (c’est-à-dire des populations importantes au fond) et souvent observées dans des conditions « artificiell es » (c’est-à-dire l’élevage en aquaculture), il faut chercher des expli cations écologiques aux résultats constatés dans l’EMSL. D’ abord, mes données démontrent clairement qu ‘ il ne s’agit pas d’ un manque de recrutement, car les collecteurs installés sur la zone d’étude ont montré un fort recrutement dans la zone infralittorale. Bien que le taux de recrutement observé ne représente que la moitié de ce lui observé dans les habitats médiolittoraux, les taux de recrutement des deuxmilieux ne sont pas significativement différents et les populations de mou les ne semblent pas limitées par le recrutement. Cependant, il faut aussi tenir compte du fait que les données des collecteurs ont relatives, on ne peut donc pas supposer que le taux de recrutement sur des substrats naturels est du même ordre de grandeur. Les collecteurs étant sélectionnés pour leur attractivité par rapport aux larves, les taux de recrutement sont donc sans doute surestimés.
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Table des matières
REMERCIEMENTS
AVANT-PROPOS
RÉSUMÉ
ABSTRACT.
TABLE DES MATIÈRES
LISTE DES TABLEAUX
LISTE DES FIGURES
LISTE DES ABRÉVIATIONS, DES SIGLES ET DES ACRONYMES
INTRODUCTION GENERALE
CHAPITRE 1. DYNAMIQUE DE POPULATION DANS DES HABITATS CONTRASTANTS: DISTRIBUTION ET FACTEURS LIMITANTS POUR LA MOULE MYTfLUSS PP. AUX MEDIOLITTORAL ET INFRALITTORAL DANS L’ESTUA IRE DU SAINT- LAURENT
RÉSUMÉ
lntroduction
Methods
Results
Discussion
CHAPITRE 2. CONCLUS IONS ET PERSPECTIVES
RÉFÉRENCES BIBLIOGRAPHIQUES .
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