1. 2006 OCRT Meeting, Providence Assessment of River Margin Air-Sea CO 2 Fluxes Steven E. Lohrenz, Wei-Jun Cai, Xiaogang Chen, Merritt Tuel, and Feizhou Chen

2. Introduction  A major objective of the U.S. Global Change Research Program Climate Change Science Program Strategic Plan and the North American Carbon Program is the application of satellite ocean color to characterize the spatial variability of air-sea CO 2 flux in the oceans adjacent to the North American continent

3. Introduction Despite their relatively small surface area, ocean margins may have a significant impact on global biogeochemical cycles, and potentially, in the global air-sea fluxes of CO 2

4. Introduction Margin ecosystems receive inputs of terrestrial organic and mineral matter and exhibit intense geochemical and biological processing of carbon and other elements Nowhere is this more evident that in regions influenced by large rivers

5. Introduction Recent studies (Lohrenz and Cai, 2006) in the northern Gulf of Mexico and elsewhere (Tsunogai et al., 1999; Ternon et al., 2000; Cai, 2003) demonstrate that enhanced biological production in large river plumes may lower surface pCO 2 levels such that these regions may exhibit a net surface influx of atmospheric CO 2 But characterizations have not been conducted over representative seasonal and discharge conditions

6. Introduction Systems are likely to differ depending on the both biological, chemical, and meteorological properties Quantification of the contributions of river- influenced margins to regional CO 2 fluxes is difficult due to the high degree of spatial and temporal variability in these regions

7. Introduction Satellite-based regional approaches (e.g., Lefevre et al., 2002; Ono et al., 2003; Olsen et al., 2004; Lohrenz and Cai, 2006) can be used to extend the spatial and temporal coverage for broad scale assessments of pCO 2 distributions and air-sea fluxes of CO 2 The primary objective of our research is to apply these approaches to the characterization of pCO 2 and air-sea fluxes of CO 2 in the river-influenced margin of the northern Gulf of Mexico

8. Outline Brief description of approach as applied to the Mississippi River outflow region Summary of initial findings –June 2003 –August 2004 –October 2005 Challenges and future directions

9. Approach

10. Approach In situ measurements of surface pCO 2 and environmental variables (T, S, chlorophyll) conducted

11. Approach Algorithm based on empirical relationships of in situ measurements of surface pCO 2 to environmental variables (T, S, chlorophyll) June 2003

12. Approach Apply algorithm to products retrieved from MODIS-Aqua imagery

13. Approach Produce pCO2 map June 2003

14. Approach Additional cruises in August 2004 and October 2005 August 2004October 2005

15. Approach PCA algorithms August 2004 (r 2 =0.858) October 2005 (r 2 =0.974)

16. Results Aug 2004 August 2004

17. Results

18. Results Aug 2004 October 2005

19. Results

20. Results Air-sea flux calculations: –CO 2 fluxes across the air-sea interface were calculated by the one-dimensional thin-film model: CO 2 Flux = k  [pCO 2 (w) – pCO 2 (a)] –where k is the gas transfer velocity and has the largest uncertainty in the flux calculation;  is the solubility of CO 2 at given temperature and salinity

21. Results Air-sea flux calculations: –Various sets of the gas transfer velocity, k, vs. wind speed relationships were used to provide a range of values to bracket the gas flux including: Liss and Merlivat, 1986; Wanninkhof and McGillis, 1999; Nightingale et al., 2000; McGillis et al., 2001

22. Results There was net uptake of CO 2 in June, but a net release in August and October October 2005 followed two major storm events accompanied by coastal flooding; strong winds Cruise Air-Sea Flux (mmol m -2 d -1 ) June 2003-(2-4) August 20046-10 October 200553-120

23. Challenges and Future Directions Challenges –Variability in conditions in coastal waters limits the generality of algorithms across temporal and spatial ranges –Remedy is to provide ample in situ data, including ship-based surveys and moored time-series –Couple ship-based operations with radiometric measurements to provide complementary data set

24. Challenges and Future Directions Future Directions –Continued characterization of Mississippi River system to capture range of seasonal and discharge conditions –Comparison of other river systems and shelf environments –Relate patterns to biogeochemical rates (primary production, net ecosystem metabolism) –Couple observations with ecosystem models to provide more informed understanding of carbon cycle controlling mechanisms

25. Acknowledgements Funding for this work was provided by NASA (NNG05GD22G and NNS04AB84H)