Yves Gratton

Effect of a freshwater pulse on mesoscale circulation and phytoplankton distribution in the lower St. Lawrence Estuary

As part of a multidisciplinary program to study the physical-biological interactions regulating carbon flows in the lower St. Lawrence Estuary (LSLE), three cruises were conducted in June-July 1990 during a neap-spring tidal cycle when biological production was expected to be maximal. Nutrient (nitrates and silicates), phytoplankton biomass (chlorophyll), oxygen, temperature, salinity, and current fields were used to elucidate the effect of a freshwater pulse produced by the discharge of the St. Lawrence and Saguenay rivers on the current fields and the biological variability and productivity of the LSLE. A simple Rossby adjustment model is presented to explain the temporal (3-5 days) and spatial (40-50 km) scales of motion in our study region (impact of the freshwater pulse on the circulation). Prior to the passage of the pulse during the neap tide, the circulation was dominated by a downstream outflow and phytoplankton blooms were limited to areas of weak baroclinic currents downstream and along the south shore. The arrival of the pulse during the tidal transition led to the intensification of a transverse current that most likely reduced flushing and allowed phytoplankton biomass to develop further upstream and toward the north shore. During the spring tide, lower salinity waters and the bloom spread along the north shore as the transverse current weakened. Based on these observations, a new conceptual model of mesoscale physical-biological interactions in the LSLE is presented that emphasizes the importance of transverse motions in regulating mesoscale patterns in phytoplankton blooms.

Orientation of the infaunal bivalve Mya arenaria L. in relation to local current direction on a tidal flat

Abstract The deep-burrowing and sedentary bivalve Mya arenaria L. has a preferential orientation on a tidal flat. Circular distribution of individuals is axial, the mean bearing of an organisms sagittal plane is perpendicular to the principal component of current direction, and variability in orientation is related to the variability of current direction. This mean orientation can be explained through optimization of energy acquisition during an entire tidal cycle. This position prevents recirculation during flood and ebb because exhalant and inhalant apertures are always in a plane normal to the tidal bidirectional current. No effects of individual orientation on shell growth, on somatic or reproductive tissue weights, and on gonadosomatic index could be demonstrated for individuals 6–19 yr old. This could result from a strong selective pressure of current direction on youngest individuals.

Mesoscale physical—biological variability during a summer phytoplankton bloom in the lower St Lawrence Estuary

Abstract Wide estuaries can produce and sustain eddies that can then have a substantial impact on the fluxes of nutrients and on the production and export of carbon. As part of a multidisciplinary programme to evaluate the biological impact of cold-core eddies in the lower St Lawrence Estuary, six cruises were conducted between mid-June and mid-July 1989, during the period of the summer phytoplankton bloom. A description of the temperature (T), salinity (S), phytoplankton biomass (Chl), nutrient (NO 3 − ), and primary production data for the surface layer (top 30 m) gathered during these cruises is presented along with a correlation analysis and a spatial classification of T-S and Chl-NO 3 − vertical profiles. The results show considerable variability and complexity in the physical-biological structure over this 1-month period. One-dimensional measures of physical structure, e.g. vertical stratification, could not account for the biological variability. Two types of coherent mesoscale physical-biological structures, unrelated to the semi-diurnal tide, were found: one with features oriented mostly along-shore and another displaying a cross-shore thermohaline front. The initiation of the summer phytoplankton bloom was associated with the development of the cross-shore front. The three-dimensional mesoscale physical-biological structure fluctuated every few days, faster than the biweekly spring-neap tidal cycle, and changes in biological production were not related to spring-neap transitions as expected. The physical processes responsible for these variations in distribution and production will be the subject of further study.

Properties of unstable waves in the lower St Lawrence Estuary

A two part hay bale carrier of simplified economical construction is formed by a wheel unit and a handle member. The wheel unit comprises a pair of wheels journaled by respective ends of an axle which rigidly supports normally upright tines intermediate its ends which are inverted and manually inserted into an end corner surface of a rectangular bale. The bale and wheel unit is then righted. The handle member comprises a pair of parallel tines joined at one end in U-shaped fashion by a bight portion which projects outward of the opposite end of the bale after the hand member tines are longitudinally inserted into the bale end opposite the wheel unit enabling an operator to substantially balance the mass of the hay bale on the wheel unit and move the bale in two-wheel dolly fashion across the surface of the earth.

Wind-Induced Eddy Motion in the Lower St. Lawrence Estuary

The lower St. Lawrence Estuary is a wide estuary, in the sense of being several internal Rossby radii in breadth. Thus, the Coriolis effects are significant and complex circulation patterns are to be expected. We analyse the response of this body to a strong up-channel pulse of wind, using data from an array of current meters and sea level gauges. Interesting features of the flow are found, such as coastal jets accelerating into the wind as the wind speed relaxes (apparently due to the establishment of an along-channel pressure gradient), and a pair of synoptic-scale eddies developing in the current pattern.