Uptake of nitrogenous nutrients by phytoplankton in a barrier Island estuary: Great South Bay, New York

The uptake of urea, nitrate and ammonium by phytoplankton was measured using 15N isotopes over a one-year period in Great South Bay, a shallow coastal lagoon. The bay is a unique environment for the study of nutrient uptake since ambient concentrations of NO 3 − NH 4 + and urea remain relatively high through the year, and phytoplankton are probably never nutrient limited. Urea nitrogen averaged 52% of the total assimilated, while ammonium represented 33% and nitrate 13%. High rates of ammonium uptake occurred only at low urea concentrations (ca< 1-μg-atom urea l −1). Over the sampling period urea was present in relatively high concentrations, averaging 5·35 μg-atom N l −1, while means for ammonium and nitrate averaged 1·94 and 0·65 μg-atom N l −1, respectively. Total N uptake measured with 15N averaged about 3·3 times the calculated (from elemental ratios and 14C productivity measurements) N needs of the phytoplankton population. Highest nitrogen uptake occurred in the summer and coincided with the primary production maximum.