Silicate and nitrate uptake in the Monterey Bay upwelling system

Rates of silicate (Si(OH)4) and nitrate (NO3) uptake by natural phytoplankton assemblages were measured simultaneously in an upwelling center in Monterey Bay, California, using the tracers 68GeGe(OH)4 and 15NNO3. The high precision of the 68GeGe(OH)4 method and low non-biological adsorption of 68GeGe(OH)4 showed the method to be useful for field studies. The 68GeGe(OH)4 results for Si(OH)4 compared well with published results using 30SiSi(OH)4 obtained for the Peru upwelling center providing a basis for confidence in the method as a way to estimate Si(OH)4 uptake. Data from four cruises, each of 1-week duration, collected in 1992 and 1993, are reported. Spatial patterns of uptake were found to be essentially identical for both nutrients with maxima close to the coast during weak upwelling and displaced off the coast during strong upwelling conditions. Nutrient addition experiments showed Si(OH)4 concentration effects over a large range and much above the accepted values for the half saturation constant for Si(OH)4 uptake. The results of this study indicate control of new production by Si(OH)4 in the Monterey Bay upwelling ecosystem.