Importance of seasonal variability and coastal processes on estuarine manganese and barium cycling in a Pacific Northwest estuary

Dissolved and particulate manganese (Mn) and barium (Ba) within the Tillamook Bay estuary are examined over seasonal cycles under a range of river discharge conditions from January to December 1999. Based on estuarine distributions and box model calculations, Mn and Ba behavior within the estuary is best quantified by considering two dominant factors: interaction with suspended particulate material (adsorption/desorption reactions) and input from benthic sources. Seasonal differences in elemental concentration patterns are dominated by variations in the relative importance of these two variables. While Mn exhibits estuarine behavior typical for this element, Ba shows an uncharacteristic lower estuary source. We hypothesize that this source results from the benthic dissolution of ocean-derived particulate Ba. uxes of dissolved Mn and Ba from the river to the coastal ocean are related to river discharge rates with the high fluxes observed under winter conditions. These winter flux maxima occur despite estuarine removal processes that also occur under winter conditions. During all other seasons, box model calculations indicate that the total (dissolved plus particulate) river fluxes of Mn and Ba to the coastal ocean are enhanced by internal estuarine processes—i.e., by transport across the sediment–water interface. On an annual basis, dissolved Mn and Ba fluxes to the coastal ocean are enriched by 390% and 170%, respectively, over what is expected from riverine inputs alone. The results of this research underscore the importance of estuarine processes in modifying the riverine flux of these elements to the ocean and the necessity of temporal sampling in evaluating processes controlling elemental distributions and delivery to the coastal ocean. Measurements of both the dissolved and particulate phases are also essential for describing and modeling these elemental budgets.