Urea as a nitrogen source for the phytoplankton in the Oslofjord

Ambient concentrations of urea in the inner Oslofjord, Norway, showed a pronounced yearly cycle in 1980, with values in the range 0.1 to 10.0 μg-at N l^-1; this cycle resemble that of ammonia although urea concentrations were usually lower. The uptake of urea by phytoplankton was investigated using ¹⁵N. Urea was usually a less important N source than NH ₄ ⁺ , and accounted for 0 to 53% (mean 19%) of summed NH ₄ ⁺ +NO ₃ ^- + urea uptake rates from April to October. Absolute as well as relative (specific) uptake rates of urea were higher in the summer (June–August) than at other times. Uptake of urea was inhibited by NH ₄ ⁺ concentrations higher than 1 to 2 μg-at N l^-1. The summed NH ₄ ⁺ +NO ₃ ^- + urea uptake rate was exponentially related to temperature.