DIEL VARIABILITY IN NITROGENOUS NUTRIENT-UPTAKE BY PHYTOPLANKTON IN THE CHESAPEAKE BAY PLUME

Diel patterns in the uptake of nitrogenous nutrients were observed in the coastal plume of the Chesapeake Bay system, but the specific patterns varied with season. During the winter months, rates of NH4+ and urea uptake were significantly higher at night than during the day, and rates of NO3- uptake were higher during the day. During the summer, rates of NH4+ and urea uptake were significantly higher at night only during half the studies conducted; during the remaining studies, there was either no significant difference or rates of uptake of NH4+ were higher during the day. Rates of NO3- uptake during the summer months were also higher during the day than at night. Seasonal differences were also apparent in the time of day at which maximum observed uptake rates of each nitrogen nutrient occurred. During the winter-spring months, maximum observed rates of NO3- uptake occurred between first light and noon, whereas during the summer months, maximum observed uptake rates of NO3- occurred both morning and afternoon, and consistently 9-16 h after the maximum observed peak in the uptake of reduced nitrogen. We interpret these findings in terms of seasonal shifts in nitrogen nutritional status of the assemblages, as well as species-specific differences in the effect of a given stimulus (e.g. a nitrogen pulse at the mouth of the Bay) to entrain an uptake response, and we suggest that the extent of this variability must be understood before generalizations about the use of f-ratios as characteristics of specific populations or water masses can be drawn.