Diel variability in nitrogenous nutrient uptake by phytoplankton in the Chesapeake Bay plume

Diel patterns in the uptake of nitrogenous nutrients were observedin the coastal plume of the Chesapeake Bay system, but the specificpatterns varied with season. During the winter months, ratesof NH₄⁺ and urea uptake were significantly higher at night thanduring the day, and rates of NO₃^– uptake were higher duringthe day. During the summer, rates of NH₄⁺ and urea uptake weresignificantly higher at night only during half the studies conducted;during the remaining studies, there was either no significantdifference or rates of uptake of NH₄⁺ were higher during theday. Rates of NO₃^– uptake during the summer months werealso higher during the day than at night. Seasonal differenceswere also apparent in the time of day at which maximum observeduptake rates of each nitrogen nutrient occurred. During thewinter-spring months, maximum observed rates of NO₃^– uptakeoccurred between first light and noon, whereas during the summermonths, maximum observed uptake rates of NO₃^– occurredboth morning and afternoon, and consistently 9–16 h afterthe maximum observed peak in the uptake of reduced nitrogen.We interpret these findings in terms of seasonal shifts in nitrogennutritional status of the assemblages, as well as species-specificdifferences in the effect of a given stimulus (e.g. a nitrogenpulse at the mouth of the Bay) to entrain an uptake response,and we suggest that the extent of this variability must be understoodbefore generalizations about the use of f-ratios as characteristicsof specific populations or water masses can be drawn.