Urea uptake by Sargasso Sea phytoplankton: saturated and in situ uptake rates

Abstract

Uptake rates, determined with saturating additions (10 μg-at. N l −1) of 15N-labeled ammonium, nitrate, and urea were measured at two stations in the western Sargasso Sea and at one station in continental slope water off Cape Hatteras in August, 1985. Daily rates of nitrogen uptake were determined during 24 h incubations in water samples collected within the euphotic zone. 15N-Urea uptake was roughly constant during 24 h incubations in the Sargasso Sea, but uptake rates were faster during the day and slower at night in continental slope water. In slope water, NH 4 + uptake rates were 2–3 times faster than urea uptake rates and 3–3.5 times faster than NO 3 − uptake rates. Urea uptake rates were 5 times faster than NH 4 + uptake rates in surface water of one Sargasso Sea station, but below 30 m, uptake of NH 4 + was more rapid than urea. At the other Sargasso Sea station, NH 4 + uptake rates were faster than urea uptake rates. 14C-Urea and 15N-urea uptake rates were not equivalent: the median 14C-urea rate/ 15N-urea rate ratio was 1.5. In situ urea turnover times, determined using trace additions of 14C-urea, were ca. 13 h in the surface mixed layer and ca. 2 days in water samples collected from the base of the euphotic zone in the Sargasso Sea. The ratio of saturated urea uptake rate/ in situ urea uptake rate was near unity in four of the six samples. These results show that in situ urea uptake rates are near the maximum potential uptake rates, and that phytoplankton in these regions have a high affinity for dissolved urea.