Urea as a source of nitrogen to giant kelp (Macrocystis pyrifera)

Nitrate concentrations routinely fall below levels required to sustain growth of giant kelp (Macrocystis pyrifera) during summer and autumn in the Santa Barbara Channel, yet growth continues. We found urea to be consistently present at concentrations of 0.48-1.82 mu M, accounting for greater than 20% of the dissolved fixed nitrogen pool during summer (14% overall). Field experiments indicate direct uptake of urea by giant kelp at a rate of 0.19 mu mol N g dw(-1) h(-1), comparable to rates for ammonium (0.18 mu lmol N g dw(-1) h(-1)) but lower than for nitrate (0.39 mu mol N g dw(-1) h(-1)). Co-occurring phytoplankton took up nitrate, urea, and ammonium, 2-, 15-, and 39-fold faster than giant kelp; however, the nitrogen uptake advantage of phytoplankton varies by substrate and season. Together, our results suggest that urea is readily used by giant kelp and may help to sustain growth throughout the year.