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 determi ned by in situ measurement with a fiber optic spectrometer. We interpret th ese data to represent fluorescence from the humic material associated with terrestrial dissolved organic matter (DOM). We were able to convert FDOM da ta to quantitative estimates of fluorescent dissolved organic carbon (FDOC) by calibrating our instrument against organic carbon measured by high temp erature combustion. The concentration of FDOC in the Columbia River is 140- 180 mu M, in agreement with DOC data from several previous investigations a nd our own high-temperature measurements. This result supports the view tha t humic fluorescence can be used as an analog for terrestrial DOC in some c ircumstances. The distribution of FDOM in the estuary is nonlinear when plo tted against salinity indicating a source within the estuary. The Aux of DO C 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 emb ayments, and in situ production associated with the estuary turbidity maxim um (ETM). FDOM-absorbance demonstrates that substantial shifts in fluoresce nce efficiency occur only in the ETM. This result together with other recen t studies suggests that microbial attack of particulate organic carbon in t he ETM may be the primary source of excess FDOC in the Columbia estuary. On ce FDOM leaves the estuary it mixes into the coastal ocean conservatively w ith little indication of removal. The presence of three water masses explai ns 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 terre strial material into the ocean. Copyright (C) 2000 Elsevier Science Ltd.