Phytoplankton dynamics in a coastal jet frontal region

Abstract

We studied the physiological status and turbulent transport of phytoplankton across the quasi-permanent 150 km long estuarine front separating the Gaspe current from the Anticosti gyre (NW Gulf of St. Lawrence, Canada). In early June, chl a concentrations reached 35 pg 1-' in the current, but remained < 1 pg 1-' in the gyre. At that time of maximum river runoff and horizontal shear stress, the phytoplankton biomass developing in the current was rapidly transported across the front by turbulent mixing. The strong correlation between chl a and salinity (r = -0.96) lndcated that the rate of crossfrontal dilution far exceeded the rate of phytoplankton production. The N : C and C .ch l a ratios increased and decreased, respectively, with cross-frontal mixlng as nutrient-deficient cells from the Gaspe current experienced higher nutrient concentrations and a deepening of the mixed layer in the front. As freshwater runoff and frontal mixing decreased in summer, phytoplankton biomass accumulated in the density front and at the base of the jet current, where low advection and sufficient light and nutrient conditions prevailed. Sporadic upwelling along the front contributed an estimated 5 and 9 O/O of the total seaward flux of nitrate and silicate, respectively. Thus, nutrients from the drainage basin of the St. Lawrence, rather than the frontal upwelling of deep water, would b e responsible for the bulk of the inorganic nutrient flux from the NW Gulf of St. Lawrence to the southern Gulf and the Scotian Shelf.