Silicic acid depletion and silicon limitation in the plume of the Mississippi River:evidence from kinetic studies in spring and summer

The surface distributions of dissolved silicic acid, chlorophyll and diatom abundance were measured in the plume of the Mississippi River and adjacent waters during spring (late Apnl and early May 1993) and summer (July 1992).In spring, the time of maximum river flow, there was an ~ntense diatom bloom with a mean diatom abundance of 1.5 X 10' cells 1-', more than an order of magnitude higher than in summer Mixing curves of silicic acid concentration ([Si(OH),]) versus salinity indicate that biological uptake within the river plume removed >99% of the Si(OH), supplied by the river In spring and 80 to 95% in summer.In spring [Si(OH),] was occasionally depleted to <0.2 pM-among the lowest values ever reported from the ocean-with extensive depletion to <0.5 pM over the shelf.In summer [Si(OH),] was less severely depleted; the lowest measured was 0.93 pM and all others were 22.4 pM. 3 0 ~i kinetic experiments were performed during both spring and summer to measure the degree to which the rate of Si uptake by the natural diatom asse~nblages was limited in sjtu by substrate availability.In spring the dependence of the specific uptake rate (V) on extracellular [Si(OH),] conformed much more closely to the Michaelis-Menten saturation function than has been observed in past studies.Strong dependence of V on [Si(OH),] was observed throughout the most Si(OH),-depleted (