Effet de la position verticale des larves competentes dans la colonne d'eau sur l'emplacement de la deposition initiale en zone intertidale: Le cas de la mye commune (Mya arenaria) et de la moule bleue (Mytilus edulis)

Most marine benthic invertebrates have in their life cycle a larval planktonic phase and an adult benthic phase. Pre-settlement (e.g. larval transport, physiological conditions in the water column, larval survival rate) and post-settlement processes (e.g. predation, competition, desiccation) then become important determinants of the distribution and abundance of benthic communities. Studies carried out on benthic communities were mostly on post-settlement processes until the 1980's, period during which ecologists started to put more emphasis on pre-settlement processes and larval supply.;The study was carried out in 2007 and 2008 in two tidally contraste(' regions. The vertical distribution of larvae in the water column and the distribution pattern of early settlers on the sediment were described for both the softshell clam (Mya arenaria) and the blue mussel ( Mytilus edulis), two common species found along the eastern coast of Canada. The sampling was carried out at the beginning, middle and the end of the seulement period of the softshell clam, period during which mussel larvae are also abundant. The vertical distribution of larvae in the water column was determined with water samples pumped at three different water depths (surface, middle of the water column, and near the sea floor). The distribution of early settlers was determined at three different levels in the intertidal zone (high, mid, and low intertidal levels). The early settlers were sampled with two types of collector (PVC tubes and " SOS tuffies ") as well as directly from the sediments' top layer (control). The sampling was carried out at Kouchibouguac National Park in the Northumberland Strait and near Saint Andrews in the Bay of Fundy. The presence of semi-diurnal tides in the Saint Andrews site allowed us to verify the effect of the day/night cycle on the distribution of planktonic larvae and early settlers.;The distribution of planktonic larvae and early settlers of the two bivalve species does not vary with the day/night cycle or the site (tidal regime). The results show that the sampling day influences the distribution of planktonic larvae and early settlers. No general pattern in the distribution of planktonic larvae and early settlers is however apparent. Within all possible relationships, few significant relationships were observed between the number of early settlers and the number of larvae found in the water column during the previous high tide, regardless of the studied species, the collector type and the study year. Our results suggest that day-to-day and local hydrodynamic processes affect the distribution of planktonic larvae and early settlers. The wind direction appeared to have an impact on the distribution of larvae in the water column and initial deposition at the Kouchibouguac site. Soft-bottom habitats are unstable environments due to the action of currents on the sediments. This could explain the absence of a relationship between both distributions as early settlers may be easily and rapidly transported once deposited on the sediment.;The number of competent larvae available for settlement is highly variable. Many factors influence the vertical distribution of larvae in the water coluinn. Studies showed that there is a relationship between the concentration of competent larvae in the water column and the number of early settlers deposited on the substrate. Most of those studies have been conducted on sessile species living on hard-bottom habitats. Soft-bottom habitats (e.g. sand, mud) have a different dynamic compared to hard-bottom habitats. Soft-bottom habitats are unstable and individuals are exposed to erosion. The objective of this study is to describe the vertical distribution of competent larvae in the water column and early settlers on the sediment and to try to establish a relationship between both distributions for two bivalve species.