Zooplankton Biomass in the Southern Gulf of St. Lawrence: Spatial Patterns and the Influence of Freshwater Runoff

Abstract

The role of advection of freshwater input in Zooplankton distribution was evaluated from 16 surveys carried out in the southern Gulf of St. Lawrence during 1967–71. Biomass was structured according to two main patterns: large-scale gradients (> 150 km) over the entire area and mesoscafe (≈25 km) patches. Patterns exhibited seasonal and interannual variability without any sign of recurrence through time, unlike the spatial structure of surface water temperature and salinity which was typical of large-scale dispersion gradients resulting from the seasonal input of freshwater into the area. In most cases, overall distribution patterns of biomass and hydrographic variables were not significantly correlated, indicating that biomass patterns in the southern Gulf were not related to water mass characteristics and advection of freshwater runoff over the entire area. Although mesoscale patchiness was unrelated to hydrographic processes, patch size (≈25 km) was similar to estimates obtained for the northern Gulf and for other coastal shelf areas. Large-scale gradients in biomass may correspond to species changes in relation to the salinity gradient and/or topography and bathymetric features. I conclude that biomass alone is not an adequate predictor for monitoring changes and variability of Zooplankton in response to runoff variability.