1. Impact of Watershed Characteristics on Surface Water Transport of Terrestrial Matter into Coastal Waters and the Resulting Optical Variability:An example from the Penobscot River, Maine C. Roesler and H. Franklin, School of Marine Sciences, Univ. of Maine Darling Marine Center, Walpole, ME; G. Aiken, USGS Boulder, CO; T. Huntington, USGS, Augusta, ME; A. Barnard and C. Orrico, WET Labs Inc., Philomath, OR INTRODUCTION Coastal waters are biogeochemically, physically, and therefore optically complex as a result of the commingling of waters arising from terrestrial, freshwater and marine ecosystems. Separating the influences of these three ecosystems on the optical properties of the resulting mixture is challenging, particularly given the variability within each. The goal of this project was to identify optical proxies for particulate and dissolved organic Carbon components in the Penobscot River, indicative of watershed land coverage. We use these proxies, applied to moored optical observations to quantify carbon pools entering the coastal Gulf of Maine Waters. METHODS Optical Proxies Discrete water samples were collected approximately monthly from ~20 stations in the Penobscot River Watershed (Fig. 1). Water samples were analyzed for particulate organic carbon, total suspended solids, and absorption; dissolved organic carbon, fluorescence and absorption Discrete optical observations of chl and colored dissolved organic matter (CDOM) fluorescence (F chl, F CDOM ) and particle backscattering (b b ) were made with WETLabs ECO-BBFL2 sensors synchronous with water sample collection Optical proxy relationships between optical observations and biogeochemical properties were based upon nearly four years of observations (Figs. 2-5) Moored Sensors WET Labs ECO-BBFL2 sensors (F chl, F CDOM and b b ) were deployed in the lower Penobscot River during ice-free season in 2005-2008 (Fig. 1C), comparable hourly optical observations are obtained from the GoMOOS moorings (Fig. 1D) since 2001 Optical proxy relationships were applied to the moored optical time series, yielding hourly particulate and dissolved carbon concentration estimates Total organic carbon flux was computed from moored observations and river discharge observations; conservative behavior was modeled and compared to observed values to predict non-conservative behavior RESULTS Optical Proxies F CHL was a reasonable proxy for chl absent cyanobacterial bloom and high CDOM; additional correction for CDOM is necessary to be as robust as absorption (Fig. 2). POC was well predicted by TSS and demonstrated dependence on season and landuse, the b b proxy for POC was flow and season dependent (Fig. 3) DOC was well predicted by both a CDOM and F CDOM. Corrections for in situ fluorescence quenching are being investigated to improve relationship. Tributaries draining bogs demonstrated significantly reduced fluorescence quantum yield (Fig. 4). Carbon Time Series from Moored Sensors Total organic carbon flux was computed from hourly optical observations (F chl, F CDOM and b b ) calibrated with optical proxy relationships, and river discharge (Fig. 5) The DOC is the largest fraction of organic carbon input to Penobscot Bay, followed by POC and algal carbon (Fig. 5B) Nearly 50% of river DOC is lost in the estuary during high flux periods, >90% is conserved during low flux periods (Fig. 6) Riverborne DOM impacts significantly the ocean color retrieved chl concentration due to similar absorption ratios (Fig. 7) Fig. 1. Map of Maine with the Penobscot River Watershed highlighted, B. Major tributaries and drainage basins, each dominated by a specific coverage type (e.g. bog, agriculture, forest). C. Monthly sampling stations within the watershed (USGS number and km from Penobscot Bay indicated). Moored optical sensors are deployed at West Enfield, Sunkhaze Stream, Passadumkeag River and Eddington (blue symbols). D. False color SeaWiFS chl image of Penobscot Bay and Eastern Maine Coastal Current showing location of GoMOOS optical moorings (E, F, I). Acknowledgements We gratefully acknowledge support from the NASA IDS Program and the Office of Naval Research Environmental Optics Program. Fig. 2. Chlorophyll Proxies. A. I n situ F CHL vs. extracted chl concentration, stations with high CDOM (red) or cyanobacteria blooms (blue) indicated; recent results indicate correction for CDOM required even in lower CDOM waters. B. Phytoplankton absorption at 676 nm vs. extracted chl concentration. Fig. 3. Particulate Organic Carbon Proxies. A. Relationship between POC and total suspended solids (TSS) by station. B. Relationship between in situ b b and POC. The backscattering POC proxy observations are distinguished by month (and indirectly by discharge). High discharge intervals are associated with low POC sediments, particularly the spring discharge compared to fall (seasonal relationships incorporated into POC discharge calculations (Fig. 6). Fig. 4. Dissolved Organic Carbon Proxies. Optical proxies for DOC based upon (A) in situ F CDOM and (B) a CDOM at the excitation wavelength 370 nm; symbols as in Fig. 3A. Tributaries draining bogs shown as red symbols. Fig. 5. Carbon Time Series A. Total Organic Carbon Flux and B. associated algal, non-algal particles and dissolved fractions computed from organic carbon-specific optical proxy relationships applied to in situ moored optical observations. DOC dominates TOC flux, non-algal POC contributes significantly during rain events and blooms. C D E F A B C D 2005 Fig. 6. Tracing Carbon Hourly DOC estimates from Eddington (green) and buoy F (blue). Marine DOC endmember computed from high salinity observations at F (black). Modeled conservative DOC at buoy F (cyan) based upon endmember mixing and observed salinity. Estimated DOC loss in estuary (red) from observed – modeled is maximal during high flux periods, near zero during low flux periods (3 and 11 day transit times, respectively). Fig. 7. DOM impacts on Ocean Color A. Phytoplankton and CDOM absorption with OC4v4 chl channels indicated, depicting similar absorption ratios. B. Daily MODIS OC3v5 chl vs. chl derived from calibrated chl fluorescence observations at Buoy E. Note MODIS estimates are rarely less than 2 mg/m 3 due to residual CDOM contamination. C. Annual cycle in MODIS and Buoy E estimated chl for 2005 shows MODIS overestimation, particularly during periods of low chl periods and/or high river discharge of DOM.