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

Abstract

Rivers and estuaries along the central Oregon margin transport large amounts of fluvialand 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 2 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 plume zones were significantly different from communities outside the 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 terrestrially derived material. Peak heterotrophic respiration rates across sampling stations occurred at a local plume particle maximum, suggesting that particulate resources transported to coastal ocean waters 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.