UREA DECOMPOSITION BY BACTERIA IN THE SOUTHERN CALIFORNIA BIGHT AND ITS IMPLICATIONS FOR THE MESOPELAGIC NITROGEN-CYCLE

We studied urea decomposition in depth profiles at stations in the Southern California Bight. Urea decomposition was measured by adding C-14 urea as a tracer. In situ urea concentration was measured by the urease method; we, therefore, studied the decomposition of only the urease-sensitive fraction of the urea pool. In the mesopelagic zone the decomposition of urea, which was mainly due to the bacterial size-fraction, was equivalent to 78 +/- 35% of sinking particulate nitrogen (N) flux and 41 +/- 20% (n = 4) of the new production and generally exceeded N demand for heterotrophic bacterial production. Urea decomposition was intense in the 100 to 200 m depth interval where earlier studies found high nitrification rates. Our results suggest that a substantial fraction of sinking N flux may follow the pathway: sinking-N --> urea --> ammonium --> nitrate rather than sinking-N --> ammonium --> nitrate. The significance of N flux via urea and its implications for bacterial production and carbon-nitrogen coupling in the mesopelagial need to be considered in the oceanic biogeochemical models.