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Phytoplankton dynamics and the distribution of fish larvae and their nutritional resources across an estuarine plume front Levasseur, Maurice


In the marine environment, export production leading to the traditional food chain is a relatively rare event taking place primarily in hydrographic features such as frontal areas. When export production persists, massive reproduction of herbivores is expected to occur. Since copepod eggs and nauplii are the main prey of a majority of fish postlarvae, the spawning of dominant fish species is expected to be associated with fronts. The aims of this study were to determine the influence of an estuarine plume front upon the phytoplankton dynamics (distribution and physiological status) and to assess the role of the cross-frontal circulation upon the distribution of fish larvae and their prey. The frontal area under study is located in the northwestern part of the Gulf of St. Lawrence, at the interface between a coastal jet (Gaspe Current) flowing along the Gaspe Peninsula and the Anticosti Gyre. In early June, maximum phytoplankton concentrations (up to 35 μg chl a L⁻¹) were found in the Gaspe Current. In the salinity gradient, a significant correlation was found between salinity and phytoplankton concentrations (and seston in general), indicating that physical processes (vertical and horizontal mixing) were more important in controlling the seston distribution than biological processes. The dominance of physical processes is probably due to the high current velocities and shear stress in the Gaspe Current in early June. Later during the season, the cross-frontal mixing was less vigorous due to the lower freshwater runoff, and the front acted as a retention zone for estuarine plankton. Maximum diatom concentrations (up to 50 μg chl a L⁻¹) were measured in the front per se. Measurements of nitrogen and silicate concentrations (ambient and intracellular) and uptake rates suggested that silicate generally limited diatom growth across the front. In June, estuarine larvae (capelin, Mallotus villosus and sand lance, Ammodytes hexapterus) were concentrated in the diatom-rich Gaspe Current and front where immature copepod stages were abundant. The five-fold increase in immature copepod concentrations between the gyre and the current/front resulted probably from a food-induced increase in copepod reproduction. Thus it appears that the dispersion strategy of the estuarine species in relation with local hydrography favours the exploitation of the resource-rich Gaspe current and front by the first-feeding postlarvae. The extrusion of redfish (Sebastes spp.) larvae appears to be synchronized with the copepod reproduction that followed the gyre April/May bloom. Later, redfish larvae were also found in abundance in the resource-rich front.

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