Document Type


Degree Name



Institute of Environmental Health


Oregon Health & Science University


Shallow, off-channel sloughs in the lower Columbia River provide critical habitat to 13 stocks of threatened or endangered juvenile Pacific salmon (Oncorhynchus spp.). We examined water quality parameters (temperature, dissolved oxygen and nutrients) and phytoplankton dynamics in three off-channel habitats (Whites Island, Campbell Slough, and Franz Lake Slough) that differ in terms of their inherent hydrogeomorphic features in the spring and summer months between 2011 and 2015. Both abiotic and phytoplankton data were analyzed in the context of varying flow conditions at seasonal and inter-annual time scales. The years of study included two that had higher–than-average discharge (2011, 2012), two that had near-average discharge (2013, 2014), and one that had below-average river discharge (2015), thus providing an excellent spectrum of conditions for investigation.

During low-discharge periods, off-channel habitats differed from the mainstem in terms of water temperature, dissolved oxygen dynamics, and nutrient concentrations. Phytoplankton species succession varied from year to year as well as on a seasonal basis, with more dramatic shifts observed at poorly connected sites during low water years. Through Non-metric Multidimensional Scaling (NMDS), significant variations among phytoplankton assemblages were observed that depended on seasonal patterns of river discharge and relative connectivity of off-channel habitats to the mainstem.

High abundances of cyanobacteria (up to 450,000 cells mL-1) were observed during the summer months at the two sites with slower water exchange with the mainstem (Campbell Slough and Franz Lake Slough). Known toxin-producing taxa, including Microcystis sp., Dolichospermum sp. and Merismopedia sp., dominated the cyanobacteria assemblages. Canonical Analysis of Principal Coordinates (CAP) indicated that water temperature and dissolved inorganic phosphorus (DIP) were major factors driving the variance among phytoplankton assemblages. Moderate positive correlations between cyanobacteria and DIP and moderate negative correlations between cyanobacteria and N:P were observed during times of low flow.

The data indicate that despite the fact that the mainstem Columbia possesses low dissolved nutrient concentrations and has good water quality relative to many other big river systems, shallow water habitats appear to be vulnerable to periods of poor water quality in the summer months.




School of Medicine

Available for download on Sunday, December 30, 2018