MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Publlshed December 1 Uncoupling of bacteria and phytoplankton during a spring diatom bloom in the mouth of the Yellow Sea Byung Cheol Chol, Joong-Ki choi2, Chang-Soo chung3, Gi Hoon Hong3 '~epartment of Oceanography, Seoul National University, Shin-lim Dong, Kwan-ak Gu, Seoul 151-742, Republic of Korea 'Department of Oceanography, Inha University, Young-hyeon Dong, Inchon 402-751. Republic of Korea 3Korea Ocean Research & Development Institute. Ansan, PO Box 29, Kyunggi Do 425-600, Republic of Korea ABSTRACT: 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 pro- duction 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. Bacte- rial 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 phyto- plankton. Further, strong tidal mixing seemed to facilitate the bacterial use of sinking carbon in the aphotic zone. KEY WORDS: Uncoupling . Bacteria . Phytoplankton . Yellow Sea . Diatom bloom INTRODUCTION Bacteria are a significant component in microbial food webs and biogeochemical cycles in marine ecosystems (Azam et al. 1983, Sherr & Sherr 1988, Azam & Smith 1991, Ducklow & Carlson 1992).Bacter- ial abundance correlates positively with phytoplank- ton biomass (Bird & Kalff 1984, Ducklow & Carlson 1992), and bacterial production with phytoplankton production (Fuhrman & Azam 1982, Cole et al. 1988, Billen et al. 1990) for large scales of space and time. However, these close relationships have not been often reported for small scales of space and time (Pomeroy & Deibel 1986, Andersen 1988). Major causes for the uncoupled relationships are thought to be low water temperature (Pomeroy & Deibel 1986), low substrate (Pomeroy et al. 1991),or food web structure (Andersen 1988). Recently, hydrodynamic conditions have been pro- posed to determine the energy flow and material cycle in the sea (Legendre & Fevre 1989). In stratified water a 'microbial loop' type of food web appeared, while a shorter 'classical' type of food web appeared to domi- nate along the deep mixed layer (Kiarboe et al. 1990). Different food chains developed as a result of differ- ences in tidal mixing in the English Channel (Holligan et al. 1984).In our study area (along a transect from the southwestern tip of the Korean Peninsula to the axis of the Yellow Sea) tidal current is strong. Due to the dissi- pation of the tidal energy (tidal range of 3 to 5 m), a high concentration of suspended particulate matter is maintained in the shallow region throughout the year. O Inter-Research 1994 Resale of full article not permitted