Protist 18S rRNA gene sequence analysis reveals multiple sources of organic matter to turbidity maxima of the Columbia River estuary

The Columbia River estuary is traditionally considered a detritus-based ecosystem fueled in summer by organic matter (OM) from expired freshwater diatoms. Since Estuarine Turbidity Maxima (ETM) are sites of accumulation and transformation of this phytoplankton-derived OM, to further characterize the ETM protist assemblage, we collected in August 2007 bottom waters throughout an ETM event, as well as surface water during the peak of bottom turbidity, and performed biogeochemical, microscopic and molecular (18S rRNA gene clone libraries) analyses. These data confirmed that the majority of the particulate OM in ETMs is derived from chlorophyll a-poor particulate organic carbon tagged by DNA too damaged to be detected by molecular analysis. The phylogeny of the diatoms uncovered in our clone libraries demonstrated that the fresh, labile particulate OM fraction of the ETM has a marine origin. In addition, the detection of DNA from dead Myrionecta rubra cells from red tide bloom decay in the pre-ETM bottom waters suggests a transient autochthonous input to the OM pool in estuarine bottom waters. Furthermore, the discovery of Katablepharis sequences associated with the ETM event during a period when this flagellate is not abundant in the estuary water column indicates that ETMs enable the retention of this microeukaryote within the estuary system. These findings challenge the traditional view of freshwater-derived detritus-based turbidity maxima, and imply multiple sources of OM to the Columbia River ETMs, which are thus sites of effective transformation of freshwater, marine and estuarine OM inputs, as well as refugium areas for ecologically relevant protists.