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Study Assesses Impacts To Columbia River Estuary’s Basic Food Supply, Zooplankton, Phytoplankton
Posted on Friday, May 02, 2014 (PST)

Invasive species, warming and changes to the natural flow of the Columbia River are impacting the timing and presence of zooplankton and phytoplankton, a basic food supply, in the Columbia River estuary, according to a recent study.


It’s been 20 years since the last assessment and, according to the study authors, much more needs to be researched to determine the trophic (nutritional) impacts, but the largest changes to plankton presence is during periods in late summer when the river flows in the lower Columbia River estuary can be as low as 2,000 cubic meters per second. Spring flows during snowmelt exceed 10,000 cubic meters per second.


“I would hesitate to extend any specific results to other river estuaries,” said study author Joanne Breckenridge, a PhD candidate in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia and also from the School of the Environment at Washington State University. “Distinct wet- and dry-season zooplankton assemblages are not uncommon in other estuaries, but the changed assemblage composition that we saw is likely a result of shipping activity and potentially also modification of the natural flow cycle of the river.”


She added that the San Francisco Estuary would be another, more extreme, example of a zooplankton community that has undergone a large change in composition. Much of the change in that estuary, according to the report, are due to the presence of invasive species that arrived in San Francisco Bay as ballast water of ocean going ships. These changes have been linked to pelagic fish declines, the article says.


“Plankton assemblage variability in a river-dominated temperate estuary during late spring (high flow) and late summer (low flow) periods,” the article that describes these changes to plankton presence and timing in the Columbia River estuary was recently published in Estuaries and Coasts, DOI: 10.1007/s12237-014-9820-7. The final publication is available at


The authors are Breckenridge; S.M. Bollens and G. Rollwagen-Bollens, who designed the study, both from the School of Environment at Washington State University; and Dr. G.C. Roegner of NOAA Fisheries Northwest Fisheries Center at the Point Adams Research Station in Hammond, Ore.


“Plankton are important because they are at the base of the pelagic food web,” said Breckenridge.  “Zooplankton are fed upon by fish and clams, among others.”


While scientists have not identified a direct relationship between the presence of plankton and fish health, it is known that young sockeye salmon eat zooplankton while in the estuary. (See CBB, April 25, 2014, “Research Cites Local Ocean Conditions, Feeding Habits as Reasons for Recent Large Sockeye Returns,”


The extent of the Columbia River estuary is river kilometers 20 to 44, which is the saline reach of the river, but that reach varies with high and low flow. In addition to changes to the natural river flow caused by dredging and diking and the presence of dams upstream of the estuary, environmental conditions such as climate change, are also impacting the timing of spring plankton blooms.


Precipitation is increasingly falling as rain rather than snow, according to the article. While water from the Columbia River and its tributaries has historically been from snow melt, climate change will likely “result in an increase in winter discharge, a decrease in summer discharge, and maximum discharges occurring earlier in the year. Earlier peak discharges could affect the timing and/or magnitude of spring plankton blooms. Decreased summer discharges will increase water residence time in the estuaries of these rivers and increase the potential for local warming.”


This and other previous studies in the Columbia River and other estuaries acknowledge that “the timing and magnitude of freshwater discharge, through its influence on salinity, temperature, mixing, nutrient delivery, and residence time (among other factors), may play an important role in determining species composition within estuaries.”


Residence time, according to Breckenridge, is the time that water will remain in the estuary, typically one to nine tidal cycles. “Plankton, by definition, are unable to swim against currents,” she said. “Estuarine plankton species generally have behaviors to keep themselves from being swept out to sea, but their ability to do so can depend on how quickly water is being swept through the estuary.”


During sample collection, the authors found that zooplankton and to a lesser extent phytoplankton presence differed between high and low flow periods. This is a normal occurrence and has been found in other estuaries, such as the San Francisco estuary, the Pearl River estuary in China and Darwin Harbour in Australia.


“The abundance of freshwater phytoplankton appeared to peak throughout the estuary in the late spring,” the article says. “As freshwater phytoplankton decreased over the low-flow period, the marine contribution to the assemblage became prominent, particularly toward the mouth of the estuary.”


This indicates a shift from river to “marine-derived” phytoplankton between high and low flow periods.


“We speculate that reduced vertical mixing and increased residence time that have occurred since large-scale regulation of the flow cycle began in the 1960s may now favor motile phytoplankton species over diatoms during periods of low flow,” the article says.


Invasive copepods (zooplankton) are present in the Columbia River estuary at lower levels than that found in the San Francisco estuary. Either that is because the invasion is more recent than  in San Francisco and may still be increasing in numbers, or some aspects of the Columbia River estuary, such as  its high flows and short residence time, make it a more difficult environment to colonize, according to the article.


Further research is needed, the article says, especially about the ecological impacts of changes in copepod species dominance and of the massive blooms that likely did not exist with the free flowing river.

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