1. Examination of interannual ocean CO 2 and air-sea CO 2 flux in the Western Gulf of Maine Douglas Vandemark 1, Joseph Salisbury 1, Jim Irish 1, Christopher Hunt 1, Shawn Shellito 1, Fei Chai 2 1 University of New Hampshire 2 University of Maine Contact: doug.vandemark@unh.edu Examination of interannual ocean CO 2 and air-sea CO 2 flux in the Western Gulf of Maine Douglas Vandemark 1, Joseph Salisbury 1, Jim Irish 1, Christopher Hunt 1, Shawn Shellito 1, Fei Chai 2 1 University of New Hampshire 2 University of Maine Contact: doug.vandemark@unh.edu RARGOM Gulf of Maine Symposium St. Andrews-by-the-Sea, New Brunswick October 4-9, 2009 Acknowledgments Thanks to all members of the R/V Challenger data collection team, GoMOOS and U. of Maine (N. Pettigrew and C. Roessler), NDBC, and to Drs. Jim Irish and Janet Campbell. This work was supported in part by the NOAA Coastal Services Center through an award to the UNH Center for Coastal Ocean Observation and Analysis (COOA); NOAA award NA16OC2740. Objectives Goals within the NACP program include developing a robust understanding of where, when and how carbon is transferred between reservoirs and then to monitor these exchanges. Carbon fluxes at the land-ocean boundary are one area of substantial uncertainty, and our research project is focused on issues related to both optimal sampling and mechanistic understanding of CO 2 air-sea fluxes in the coastal zone- including airshed and watershed control of coastal dynamics. This work provides an update of our coastal observation program in the Gulf of Maine addressing several key land-ocean-atmosphere carbon transport questions by intensive monitoring of the coastal ocean’s surface layer carbon dioxide in space and time. These questions include: Does the Gulf of Maine (temperate latitude, Northeastern U.S.) act as a net atmospheric carbon dioxide source or sink? Which controls dominate the Gulf’s annual cycle of CO 2 air-sea exchange? What is the magnitude of cross and alongshore variability in the surface layer carbon pool? Do large episodic terrestrial inputs of freshwater, nutrients, and carbonate species alter air-sea gas exchange in these coastal waters? What are the requisite temporal and spatial flux sampling scales for this region? Data collection Monthly shipboard data collected aboard UNH R/V Gulf Challenger, 2004-present Underway pCO 2 via fast-rate Weiss-type equilibrator system, sea surface temperature and salinity Atmospheric pCO 2 via the flow through system Profiles of TA and pH with depth as well as a suite that include pigments, zooplankton, and nutrients Ancillary physical fields from the GoMPOM model as well as USGS river discharge data (for cruise data visit: www.cooa.unh.edu )www.cooa.unh.edu Hourly/daily buoy data Met. & Wind speed from the NDBC and nearby coastal buoys Water Temp, Salinity, Chl_a, currents, light field data from GoMOOS buoys pCO2, dissolved oxygen, temperature, salinity from pCO 2 buoy near WB2 Hourly atmospheric CO 2 from several surrounding stations UNH R/V Gulf Challenger Fig 1 - Map of study region in the Western Gulf of Maine, and coastal shelf divisions (lower panel) versus depth and distance from the coastline for the cross- shore Wilkinson Basin transect – a single day cruise executed monthly since April 2004. Note the addition of the UNH/PMEL CO 2 time series station near WB2 starting in May 2006. The Kennebec/Androscoggin river system is a key landward influence upon the Western Gulf 4 where surface flow is predominantly North-South. Controls on pCO2 and interannual CO 2 flux estimates Addressing the data shown at left, a seasonal cycle in the seawater delta pCO 2 emerges with each year holding reasonably well to the five year average. The supporting hourly data from buoys and nearby met stations provide an excellent backdrop for examining controls at high temporal resolution and over 5 years. Notable observations: SST does not covary strongly with ΔpCO2 - a departure from shelf systems to the south Biology(see Chl data) and water column mixing (see temperature) appear to be key spring and fall controls on ΔpCO2 respectively - often requiring short time scale sampling to capture The F CO2 magnitude can be substantial yet a near net balance is found in the annual mean Surface salinity changes due to discharge are both seasonal and episodic - yet the relatively dry year 2004 is not exceptional in ΔpCO2 or F CO2 Δ pCO2 Time Series and Air-Sea Flux Variations 2004-2008 One benefit of the repeated cross-shore transects is their utility for examining and monitoring the seasonal and inter- annual cycles within this coastal ecosystem. Figures below illustrate a portion of our composite data set with the focus being station WB2 using monthly ship + supporting nearby station data to retrieve a bulk air-sea CO2 flux. Flo-thru oxygen, IOPs and CTD INNER MID-SHELF JEFFREY’S LEDGE DEEP BASIN Weiss-equilibrator Portsmouth NH Boston MA Portland ME UNH Monthly Wilkinson Basin Transect WB7 WB2 UNH & PMEL CO2 coastal buoy at 65 m depth Merrimack River Kennebec- Androscoggin Rivers Coastal Buoys NDBC/GoMOOS/UNH WB7 WB2 Net annual air-sea flux estimates 2004-2008 Inshore, Mid, and Offshore Stations A Carbon Source, but… While the Gulf is a productive ecosystem, our 5 year time series is showing the surface waters to be a weak net source with 2004 being the strongest year. But 2005-6 years were near neutral in C flux. The maximum annual swing for this region occurs between 2005 and 2004, amounting to a -1.1 TgC difference when averaged across the relevant area of the Gulf of Maine. Hourly-to-Daily Ancillary Data - Chlorophyll_a, Salinity, River Discharge Hourly estimate (monthly ship pCo2) Monthly avg. Monthly avg., ~5 yr climatology Buoy 44030- 01 m depth 50 m depth ship data 5 yr average Wanninkhof 1992 with hourly NDBC winds Buoy 44030- 01 m depth 50 m depth Five river average (USGS) Buoy 44030 - 03 m depth SeaWiFS - daily Multi-year air-sea CO 2 flux estimates – all stations Hourly estimates of the CO 2 air-sea flux are computed using the UNH cruise data and the nominal procedure for hourly mass flux estimation given as: F CO2 (molC/m2/hr) = α* k w (U) * (ΔpCO 2 )(1) where α is the gas solubility (a known quantity of T and S), kw the bulk gas transfer coefficient, and ΔpCO 2 = [pCO 2 seaside] – [pCO 2 airside], pCO 2 being the gas’ partial pressure at 1 atm. k w is estimated hourly with several algorithms (e.g. Wanninkhof, 1992; Wanninkhof and McGillis, 1999) – each in terms of an hourly or ‘steady’ 10m neutral stability anemometer wind speed. While atmospheric CO 2 measurements are typically collected aboard the Challenger we use a locally-modeled (see Padin et al., 2007) level for the airside level due to substantial (10-30 ppmv) daily variations that can limit their use is a one-per-month ΔpCO 2 estimate in the first panel shown at left. Temperature, salinity and pCO 2 data are interpolated from monthly values onto a 1 hour time step grid to estimate fluxes using Eq. 1. Atmos. sink Atmos. source CO 2 Flux inshore offshore fall mixing spring bloom Monthly sampling of pCO 2 vs. 3-hourly buoy sampling As shown on the map at far left, UNH has a buoy moored just east of WB2 collecting pCO 2 data using the NOAA/PMEL MAP-CO 2 system. The buoy has been deployed since 2006 and data below are being used to assess hourly to daily variations in the surface layer air and sea CO 2 levels. The range of daily buoy-observed variation about ship monthly-interpolated is significant, but the general agreement with that estimate is striking as well. The combined cruise and buoy data are being used to assess future sampling strategies for the wider Gulf, as well as the study of storm and discharge events. Annual FCO2Platform (molC/m2/yr)---------------- +0.02UNH/PMEL MAPCO2 buoy +0.17Monthly ship data 2007-2008 - 12 months 44030 44007 IOSN3