Biogeochemical fluxes in Vietnamese Seas: Response to ENSO events and Holocene warming

• Applicant: Professor Dr. Martin Georg Wiesner
• Subject Area: Oceanography
• Term: from 2003 to 2010
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5400537

Off southern Vietnam strong coastal upwelling forced by the SW-monsoonal winds creates the biologically most productive off-shelf region in SE-Asian waters. At the height of this season, which alternates with a period of downwelling during the NE-monsoon, nearby rivers such as the Mekong attain their maximum runoff and discharge their load across the bordering shelf. This system is set out of phase by each ENSO event and expected to be seriously imparied in the future as these episodes are predicted to increase in both frequency and amplitude on top of a global warming trend. Preliminary time series data were optained for the 1998 El Nino, when both up- and downwelling was suppressed and riverine maximum discharges shifted into the (dry) NE-monsoon period. Results revealed (i) the presence of a low-diversity plankton community in response to anomalously high-temperature and low-nutrient surface waters with a strong nitrate limitation throughout the monsoonal year; and (ii) biogenic export production rates as low as those reported from the oligotrophic central South China Sea in ENSO-free years. In the lack of a detailed knowledge of the ecological and biogeochemical boundary conditions in the upwelling region, cause-and-effect amplitudes cannot be established and future perturbations of the system are difficult to assess. This proposal aims (i) to document, by time-series moorings, biogeochemical fluxes under La Nina and non-ENSO conditions in order to identify, qualitatively and quantitatively, ENSO-related anomalies; and (ii) to discriminate these signals from natural background changes by proxying the dynamics and efficiency of the upwelling system during the Holocene warming phase recorded in the sediments. Integration of the past and present-day scenarios will lead to a better understanding of the consequences of global environmental change in SE-Asia and will assist in tuning regional impact models of ocean ecosystem response to enhanced greenhouse conditions.

• DFG Programme: Research Grants
• International Connection: Vietnam
• Participating Persons: Dr. Birgit Gaye; Doan Nhu Hai; Nguyen Ngoc Lam, Ph.D.