Behavior of terrestrial dissolved organic matter at the continent-ocean boundary from high-resolution distributions

This paper presents distributions of fluorescent dissolved organic matter (FDOM) in the Columbia River estuary and associated coastal plume as determined by in situ measurement with a fiber optic spectrometer. We interpret these data to represent fluorescence from the humic material associated with terrestrial dissolved organic matter (DOM). We were able to convert FDOM data to quantitative estimates of fluorescent dissolved organic carbon (FDOC) by calibrating our instrument against organic carbon measured by high temperature combustion. The concentration of FDOC in the Columbia River is 140-180 mu M, in agreement with DOC data from several previous investigations and our own high-temperature measurements. This result supports the view that humic fluorescence can be used as an analog for terrestrial DOC in some circumstances. The distribution of FDOM in the estuary is nonlinear when plotted against salinity indicating a source within the estuary. The Aux of DOC from the river was 1040 mols C sec(-1) during our October sampling period. Mass balance calculations show that estuarine source(s) added 14% to this amount. Potential sources of this material are groundwater, intertidal embayments, and in situ production associated with the estuary turbidity maximum (ETM). FDOM-absorbance demonstrates that substantial shifts in fluorescence efficiency occur only in the ETM. This result together with other recent studies suggests that microbial attack of particulate organic carbon in the ETM may be the primary source of excess FDOC in the Columbia estuary. Once FDOM leaves the estuary it mixes into the coastal ocean conservatively with little indication of removal. The presence of three water masses explains most of the variability of FDOM on the shelf. There is a reversal in the slope of the FDOM-salt relationship between Columbia River plume water and deeper shelf water masses. We interpret this bilateral distribution to be the net result of two conservative processes: primary dilution of dissolved fluvial material with coastal surface water and mixing of refractory terrestrial material into the ocean. Copyright (C) 2000 Elsevier Science Ltd.