Nutrient cycling in the Columbia River Basin involves the transport of nutrients through the system, as well as transformations from among dissolved, solid, and gaseous phases, depending on the element. The elements that constitute important nutrient cycles include macronutrients such as nitrogen (as ammonium, nitrite, and nitrate), silicate, phosphorus, and micronutrients, which are found in trace amounts, such as iron. Their cycling within a system is controlled by many biological, chemical, and physical processes.
The Columbia River Basin is the largest freshwater system of the Pacific Northwest, and due to its complexity, size, and modification by humans, nutrient cycling within the system is affected by many different components. Both natural and anthropogenic processes are involved in the cycling of nutrients. Natural processes in the system include estuarine mixing of fresh and ocean waters, and climate variability patterns such as the Pacific Decadal Oscillation and the El Nino Southern Oscillation (both climatic cycles that affect the amount of regional snowpack and river discharge). Natural sources of nutrients in the Columbia River include weathering, leaf litter, salmon carcasses, runoff from its tributaries, and ocean estuary exchange. Major anthropogenic impacts to nutrients in the basin are due to fertilizers from agriculture, sewage systems, logging, and the construction of dams.
Nutrients dynamics vary in the river basin from the headwaters to the main river and dams, to finally reaching the Columbia River estuary and ocean. Upstream in the headwaters, salmon runs are the main source of nutrients. Dams along the river impact nutrient cycling by increasing residence time of nutrients, and reducing the transport of silicate to the estuary, which directly impacts diatoms, a type of phytoplankton. The dams are also a barrier to salmon migration, and can increase the amount of methane locally produced. The Columbia River estuary exports high rates of nutrients into the Pacific Ocean; with the exception of nitrogen, which is delivered into the estuary by ocean upwelling sources.
Description
The Columbia River basin is a major watershed and the largest river in the Pacific Northwest region of North America. Extending from southern British Columbia to northern Nevada, the watershed includes seven American states and two Canadian provinces and drains an area of about 260,000 square miles. The Columbia River stretches 1,620 miles in length until its discharge into the Pacific Ocean near Astoria, Oregon. The average annual discharge rate of the Columbia River changes due to climate and land-use variability, but generally ranges from 120,000 to 260,000 cubic feet per second. Finally, the population of the Columbia River Basin within the United States is about 4.6 million people (per the 2000 census).
The water cycle in the Columbia River is dependent on the relation between water flow and topography in the basin. Within the United States, only the Missouri-Mississippi River system has annual runoff greater than that of the Columbia River. The amount of water that the river carries each year is determined by the precipitation, sunlight, and air temperature in the basin, which varies from year to year. West of the Cascade Range, most of the winter precipitation falls as rain, but in the Cascade Mountains and eastward, precipitation through winter is snow. Snowmelt in the mountains begins to reach the river during early to mid-spring. About 30 percent of the streamflow in the Columbia River occurs between January and March (primarily from rainfall) and about 30 percent occurs between April and June (from a combination of rainfall and snowmelt).
Natural processes Coastal upwelling, river discharge, tidal mixing, estuarine circulation, climate oscillation, and remineralization, are sources or sinks for the Columbia River basin nutrient budget. Due to these transport processes the Columbia River Estuary provides large nutrient sources to the adjacent sub-arctic Northeast Pacific coastal surface water.
Nitrogen is added to rivers through many natural processes, such as the decomposition of leaf litter and organic matter. Nitrogen gas is the most abundant molecule in earth's atmosphere, comprising about 78 percent of the total composition of air, however not typically a large source of nitrogen to the river. This form of nitrogen, dinitrogen gas (N2), is inert and biologically unavailable to most living organisms. However, some bacteria and archaea can utilize nitrogen fixation to convert dinitrogen into ammonia or other compounds accessible to organisms.
Finally, nitrogen load at the Columbia River mouth is about 2-20 times larger than that at the Canadian border. This gradient in nitrogen distribution is partly a result of the inputs of the Columbia's major tributaries, the Snake River and the Willamette River. The Snake and Willamette rivers together contribute about 50-80 percent of the nitrogen load in a given year, coming from activities within those respective watersheds.
Weathering and runoff Weathering is the breakdown of rocks, soil, and minerals through contact with water, the atmosphere, and biological organisms, converting solid phase minerals to dissolved phase compounds. This process can introduce nutrients, most notably phosphorus, into the Columbia watershed. Both chemical and physical weathering occur, usually together, and this coupling tends to accelerate the other. Precipitation varies across the basin, influencing the amount of weathering and subsequent runoff that transports this material into the basin.
Organic matter such as leaves may also naturally fall into the waterways through runoff. Over time, this material is respired, releasing assimilated nutrients into the environment.
Salmon impacts on nutrient cycling An important source of nutrients, such as nitrogen and phosphorus, for rivers in the Pacific Northwest is the spawning of salmon and subsequent death and decomposition of fish up-river. Every fall, ocean salmon at the end of their life cycle swim upriver to spawn before they die. The remineralization of their organic matter releases significant nitrogen loads into the river, although recent decades have observed decreased salmon runs and subsequently less impact on the total nitrogen budget.
Salmon are an anadromous family of fish that rear in freshwater, migrate to the ocean as juveniles, and then return to the freshwater as adults to lay their eggs and die in what is called a salmon run. Their death and subsequent decomposition releases substantial amounts of nitrogen and lower amounts of phosphorus into the system, boosting the productivity of local headwaters and serving as a vector of material transport across the ocean to watershed interface. Studies have estimated salmon in some parts of the Columbia river contribute up to 60 percent of nitrogen to other trophic levels.
The regional ubiquity of salmon as a recreational and commercial fishery has led to increased attention on local populations within the Columbia River system. Annual salmon harvests are among the highest-earners of the Pacific Northwest fishery economy; in the lower Columbia River specifically, salmon account for over 84 percent of commercial fish landings.
While areas accessible to salmon have been reduced over the past century through the construction of dams and the modification of river channels, this group of fish historically inhabited almost 13,000 miles of the Columbia River basin. Efforts from fishery management and state agencies, as well as the listing of several resident salmon stocks under the protection of the Endangered Species Act, has led to the re-establishment of accessibility to some parts of the Columbia River system for salmon. The regional return of salmon runs restores an important segment of local nutrient cycles, particularly that of nitrogen and phosphorus, to the system.
Estuarine and ocean exchange The Columbia River Estuary is the most downstream portion of the river that experiences ocean tides, generally defined as the furthest reach of the Columbia River plume to the Bonneville Dam. Here, a transition zone occurs where freshwater discharge from the river meets and mixes with Pacific Ocean saltwater. In this region, physical processes that affect nutrients are the circulation (entrance/exit, movement) of specific water masses with their nutrient load, the magnitude of tidal flow distributing ocean waters inland, and bottom sediment exchange affecting nutrient particle mobility.
With the exception of nitrogen, the Columbia River estuary nutrient budget tends to be dominated by nutrient transport out of the system. During periods of high river flow, generally April to June, the estuary has a daily average residence time of less than a day. Meanwhile, at times of lowest river discharge, typically September to October, this turnover time lengthens to about three days. The turnover time can also vary due to the tidal cycle, and affect the amount of oceanic exchange in the estuary; on the whole the residence time is fairly short. Through a more rapid transit time of waters within the river, local primary production tends to be lower as autotrophic communities are quickly flushed out of the system. Since there is less subsequent biological uptake of nutrients, there is a high rate of nutrient transport out of the Columbia River and into coastal waters.
The upwelled waters of the Pacific Ocean have very high concentrations of dissolved nutrients due to the history of biological respiration that has occurred throughout the water masses' time in circulation through the deep ocean. Biological respiration is the consumption of organic matter and concurrent release of nutrients contained in that biomass. This process is an important source for nutrients that other organisms require. Examples of these organisms include phytoplankton and algae, whose communities are often limited in growth by the availability of specific nutrients, most commonly nitrogen which is the primary limiting nutrient the Columbia River. In addition, to nitrogen, iron, and silicate cycling in the estuary can impact local and coastal ocean phytoplankton communities.
Nitrate Due to coastal upwelling, ocean sources of nitrate (an oxidized form of nitrogen) exceed river sources by a magnitude of roughly 3 to 1, and is a primary source of nitrogen for the Columbia River Estuary. This is evident within the estuary by the positively correlated relationship of nitrate availability and salinity, the latter an identifying characteristic of highly saline marine waters. As a result of differences in delivery to the system, nitrate is the primary limiting nutrient in the estuary. Consequently, during seasonal periods of downwelling when less ocean water is mixed into the estuary, local nitrate concentrations may be very low. Thus, nutrient cycling and primary production in the estuary are heavily linked with the seasonal winds off the Oregon and Washington coasts that control local ocean upwelling and downwelling.
Phosphate Although phosphorus is of interest in the mid-river, especially in and around reservoirs throughout the Columbia River Basin as discussed below, it is not typically a limiting nutrient for biological communities within the estuary. As a result, there is little available literature on phosphate cycling within the estuary.
Iron