Phytoplankton biomass and nutrient dynamics in a tidally induced upwelling. The role of the NO₃:SiO₄ ratio

Data from 2 cruises in July 1979 and 1980 in the head region of the Laurentian Channel, in the St. Lawrence estuary, were used to study variations of nutrient and phytoplankton biomass following upwelling relaxation.During both cruises, the newly upwelled water mass was characterized by high salinity (> 29%), low temperature (<4 'C), low phytoplankton biomass (< l pg I-') and high nutrient levels [NOs = 13 ~I M , PO4 = 1 4 btM, Si04 = 18 yM, NH4 = 0.6 pM) As the water mass warmed up, phytoplankton biomass increased exponentially up to -17 ~t g 1-' and a concurrent decrease in the concentration of all nutrients was observed.The decrease of N o 3 : PO4 and NO3 : SIO, ratios during this period indicates a gradual nitrate impoverishment of the mixed layer.At the warmest stations, in July 1980, nitrate and ammonium reached undetectable concentrations while silicic acid concentrations fell below 2 FM.High values of phytoplankton POC: PON ratios measured in July 1980 also suggest that algae were nitrogen limited during this cruise.Data from this study and other world-wide upwelling areas support the hypothesis that the initial concentration of nitrate and silicic acid in the newly upwelled waters is the factor which mainly determines which nutrient will become exhausted (limiting) first.The nearly simultaneous exhaustion of nitrate and silicic acid in the St. Lawrence estuary is attributed to the 3-layer circulation pattern of its water masses.Silicic acid is trapped in the bottom layer (deeper than 100 m) which is hardly mixed with surface waters.In the intermed~ate layer (between 20 and loom), the origin of the upwelled waters, these 2 nutrients are generally found in equal proportion.