Nature of environmental variability experienced by benthic and pelagic animals in the St. Lawrence Estuary, Canada

We compared the short-term variability in salinity, temperature and concentration of suspended particulate matter experienced by benthic and pelagic animals in the middle St. Lawrence Estuary on both horizontal and vertical axes. In addition, we examined the effect of 3 patterns of vertical distribution (no vertical migrations, die1 and tidal vertical migrations) on the environmental variability experienced by pelagic animals. Results demonstrated that the nature of environmental variability is different for the 2 types of animal. For benthic animals, environmental variability is experienced on the horizontal axis, whereas the vertical axis is more important for pelagic animals. The magnitude and frequency of abiotic fluctuations experienced by benthic animals depend on the environmental factors that control the longitudinal position of the estuarine gradient, whereas for pelagic animals they mostly depend on the animals' behaviour on the vertical axis. Consequently. favourable habitats for benthic and pelagic animals in the estuary are different. The most upstream section of the estuary is the most variable for benthic animals because the tides of greatest range generate the highest abiotic fluctuations on the horizontal axis. In contrast, the same section is the most abiotically stable for pelagic animals because tides of highest energy generate the strongest vertical mixing. The highest abiotic fluctuations experienced by these animals are in the most vertically stratified waters downstream in the estuary. As a result, minimum numbers of species of benthic and pelagic animals are reached at different locations in the estuary corresponding to the areas which are the most abiotically variable for each type of animal. We propose that well-mixed parts of estuaries should be considered as abiotically stable habitats for the pelagic fauna, whereas vertically stratified areas should be considered as abiotically variable except in the case where vertical rnigratlons are not necessary for the retention of animals.