Bacterial urea production and decomposition were studied in samples from coastal waters in the Southern California Bight (the Bight), USA, and an estuarine system of the Mankyung and Dongjin rivers (MD estuary) in Korea. Bacterial urea production ranged from undetectable to 139 nM d-', and the mean value of bacterial urea production (58 nM d", n = 6) was equivalent to 35-91 % of the estimated phytoplankton N demand in the Bight. The rates of bacterial production of urea were 2 orders of magnitude higher than the bacterial urea decomposition rates. Consequently, bacteria were consistently net producers of urea in the euphotic zone. The concentration-dependence of urea decomposition showed the presence of a high affinity but low capacity system (K,+S,,: 26 to 33 nM, V,,,: 3 to 11 nM d-l). The low K, values indicate that in typical seawater samples, which have >l00 nM urea, the bacterial ureolysis system is always near-saturated. The significance of bacteria as urea producers should be incorporated into models of nitrogen regeneration in surface waters.

Bacterial abundance, production, and environmental parameters were investigated to study the distribution of bacterial variables and interrelationships between bacteria and phytoplankton along a transect from the southwestern tip of the Korean Peninsula to the axis of the Yellow Sea in April 1991 The study area showed a tidally induced turbidity maximum in the middle region of the transect. The turbidity maximum had lower phytoplankton abundance, primary production, and bacterial production than the adjacent waters. Diatom blooms were observed in the inner bay and in waters outside of the turbidity maximum. Bacterial abundance and production showed relatively large variations along the transect from 3 to 32 X 10' I-' and from undetectable to 11.9 pg C 1" d-l, respectively. Bacterial and phytoplankton variables did not correlate significantly (p z 0.12). Further, depth-integrated bacterial production over the euphotic zone comprised less than 4 O/o of primary production, suggesting that bacteria and phytoplankton were not closely coupled during the study period. The uncoupling seemed to be unrelated to phytoplankton community structure. Strong tidal mixing, which would mix the organic matter produced in the euphotic zone into the deeper water column, possibly coupled with mass sinking of phytoplankton to the bottom seemed to cause low substrates level and unbalanced growth for bacteria in the euphotic zone and thereby the observed uncoupling of bacteria from phytoplankton. Further, strong tidal mixing seemed to facilitate the bacterial use of sinking carbon in the aphotic zone.