MEPS 438:19-31 (2011)  -  DOI: https://doi.org/10.3354/meps09303

ABSTRACT: 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, a project was undertaken to further characterize the ETM protist assemblage by collecting and analyzing bottom and surface water samples throughout an ETM event in August 2007. Biogeochemical, microscopic and molecular (18S rRNA gene clone libraries) analyses were performed. The data confirmed that the majority of the particulate OM in ETMs is derived from chl a-poor particulate organic carbon tagged by DNA that is 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 into the OM pool of 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 estuarine 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. These ETMs are thus sites of effective transformation of freshwater, marine and estuarine OM inputs, as well as refugia for ecologically relevant protists.

KEY WORDS: Estuarine turbidity maxima · Columbia River estuary · Organic matter