Distributions of dissolved manganese and fluorescent dissolved organic matter in the Columbia River estuary and plume as determined by in situ measurement
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We deployed an instrument package consisting of a SeaBird 911 plus CTD, SeaTech 25-cm transmissometer, Chelsea AquaTracka fluorometer/nephelometer, Seapoint turbidity meter and ZAPS UV fluorometer/chemical sensor during a series of tows and lowerings within the Columbia River plume and estuary in 1994 and 1995. Simultaneous in situ measurements of Chi a, light attenuation, light scattering, and fluorescent dissolved organic matter (FDOM) or dissolved Mn collected along with CTD information provided new insight into the effects of mixing and non-conservative processes on riverine input to the coastal ocean. Inter-relationships among these parameters proved to be effective at delineating three coastal water masses: Columbia River Plume Water (CRPW), Shelf Deep Water (SDW), and Shelf Bottom Water (SEW). CRPW had variable amounts of FDOM, dissolved Mn, and Chi a as the plume mixed with coastal water. SEW was characterized by high concentrations of Chi a and low concentrations of dissolved Mn, suggesting a surface or near-surface source. The properties of SDW were intermediate between humic-rich plume water and turbid bottom water. FDOM displayed a quasi-linear and inverse relationship with salinity inside the estuary and nearshore coastal plume which supports previous work relating this parameter to the humic-rich terrestrial component of dissolved organic matter (DOM). There was, however, considerable evidence of lower FDOM levels within the estuarine turbidity maximum (ETM) and near the seaward boundary of the plume. There were also elevated levels of FDOM near the bottom at some locations on the shelf, suggesting that there may be a significant benthic flux of this material. Thus FDOM appears to be more reactive than previously thought when looked at in detail. Dissolved Mn also displayed non-conservative behavior in the estuary and across the plume showing significant input into surface waters. The sharpest Mn gradient on the shelf occurred near the seaward edge of the plume in an area of relatively high turbidity and Chi a, and relatively low FDOM. These distributions are consistent with the presence of a chemical front near the plume boundary where Mn-oxides are reduced during the oxidation of humic ligands. This process would release dissolved Mn and promote productivity in the turbid zone associated with the leading edge of the plume jet. This study demonstrates that chemical sensors can provide highly resolved, detailed information on trace substance distribution and variability in marine environments.