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 I4C 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 r 35% of sinlung 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 s tudes 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 carbonnitrogen coupling in the mesopelagial need to be considered in the oceanic biogeochemical models.