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Winter river plumes shape community composition and activity of heterotrophic microorganisms on the Oregon Coast

B. Kieft*, B. C. Crump, A. E. White, M. A. Goñi, R. S. Mueller

*Corresponding author:

ABSTRACT: Rivers and estuaries along the central Oregon margin transport large amounts of fluvial- and terrestrial-derived materials into the coastal ocean during the winter season, which can become trapped in a nearshore coastal current by local density gradients and wind forcing. The influence of these substantial and persistent allochthonous inputs on wintertime biological activity in the Oregon coastal region is not well understood. We compared prokaryotic communities inside and outside of two buoyant coastal river plumes off the central Oregon coast in order to understand the relationship between plume conditions and the distributions of prokaryotic populations that form the base of the wintertime coastal food web by transforming carbon and nitrogen compounds. Both free-living and particle-associated communities inside nearshore plumes zones were significantly different from communities outside of plume influence. Particulate organic matter concentrations correlated with the distribution of several Bacteroidetes populations with established roles in complex organic matter degradation in coastal ecosystems. Plume conditions also correlated with marine Gammaproteobacteria that are known to degrade terrestrial-derived material. Peak heterotrophic respiration rate across sampling stations occurred at a local plume particle maximum, suggesting that particulate resources transported to coastal ocean by river plumes may be used or transformed by co-localized heterotrophic microorganisms. Taken together, the associations between river plume resources and prokaryotic populations implicated in organic matter turnover suggest that microbes in Oregon coastal ecosystems use allochthonous resources that are transported into the coastal ocean during winter and that these resources help shape the coastal food web during the winter season.