Use of an inert tracer sulfur hexafluoride in studying the initiation and growth of red-tide bloom, gas exchange process, and the possible roles of mixotrophic dinoflagellates in dimethylsulfide and dimethylsulfoniopropionate cycles

DoctorThis thesis consists of four chapters pertaining to the applications of sulfur hexafluoride (SF6) to studies of red-tide bloom and gas exchange rate, and the roles of mixotrophic dinoflagellates on dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) cycles. First chapter describes application of a Lagrangian method to investigate the cause of harmful algal blooms in the southern coastal waters of Korea. Based on the SF6Lagrangian study and the analysis of historical data, in situ growth of Cochlodinium polykrikoides within the SF6labeled water accounts for only a fraction of the total cell increase. The probable mechanism for the overall increase is that much of the cell accumulation in the early phase of the red tide bloom is due to the input of C. polykrikoides cells by lateral mixing of inshore waters with the alongshore current that contains high C. polykrikoides cell density. The second chapter presents gasexchange results obtained using the dual tracer (SF6 and 3He) method for a period of 5 days at the study site. The resulting gasexchange rate was approximately 4.5 cm h-1 at a mean wind speed of 3.9 m s-1, meaning that DMS emission rate in the coastal area was comparable to that in the open ocean regardless of tidal current. The third chapter describes the diurnal cycles of total (dissolved and particulate) DMSP (DMSPT) and DMS. The most probable mechanism controlling the diel DMSP and DMS cycles is vertical migration of the mixotrophic dinoflagellate Gymnodinium impudicum. Another interesting observation is the increase in DMS emission during the evening, probably because of bacterial production or direct release of DMS by G. impudicum as a defence mechanism, or both. The fourth chapter concerns the retention of DMSP in phototrophic dinoflagellates the arises from mixotrophy. In this study, the cellular concentration of DMSP in Karlodinium veneficum (mixotrophic growth) fed on either DMSP-rich Amphidinium carterae (phototrophic growth only) or DMSPpoor Teleaulax sp. (phototrophic growth only) were measured for 79 days. The results indicate that the DMSP content of prey species directly affects the retention of DMSP by mixotrophic dinoflagellates, and that mixotrophic dinoflagellates produce dimethylsulfide not only through DMSPlyase activity but also indirectly through mixotrophy