Dissolved and particulate aluminum in the Columbia River and coastal waters of Oregon and Washington: Behavior in near-field and far-field plumes
Introduction
The Columbia River, originating some 820 m above sea level in British Columbia, is the largest river entering the northeast Pacific ocean, draining an area of approximately 665,000 km2 with an average discharge of 7300 m3 s−1 and annual river discharge of 260 km3 year−1(Barnes et al., 1972, Jay and Smith, 1990, Thomas and Weatherbee, 2006). A sustained maximum river flow generally occurs in May–June from interior snowmelt while shorter maxima occur during winter from storm events in the coastal basins. Minimum river flow occurs during August-September (Neal, 1972, Thomas and Weatherbee, 2006). During the late summer, the Columbia River accounts for ∼90% of the freshwater entering the sea between the Strait of Juan de Fuca and San Francisco Bay (Barnes et al., 1972).
The Columbia River plume forms as coastal seawater intrudes and mixes with river water at or near the mouth of the Columbia River estuary (Barnes et al., 1972). The Columbia River plume is a dominant hydrographic feature of the California Current system off the Washington and Oregon coasts and is observed as a shallow (∼2–20 m) surface lens of low-salinity water (Hickey et al., 1998). Because the chemistry of the river water and coastal seawater that mix to form the plume varies depending on time of year, tidal phase, and oceanographic conditions, the chemistry of the plume itself is expected to vary according to these changing conditions (Bruland et al., 2008). The Columbia River plume also plays a key role in the delivery of both macro- and micro-nutrients to offshore waters of Washington and Oregon (Hill and Wheeler, 2002, Lohan and Bruland, 2006, Aguilar-Islas and Bruland, 2006).
The movement of the plume is strongly influenced by seasonal variations in the local wind forcing. When dominant alongshore wind stress is to the south (upwelling conditions), Ekman transport is offshore and surface currents are to the southwest. Therefore, the plume separates from the coast and advects offshore and southward from the Columbia River mouth (Hickey et al., 1998). When dominant alongshore wind stress is to the north (downwelling conditions), Ekman transport is onshore and surface currents are to the north. This acts to keep the plume confined tightly along the Washington coast during strong northward wind events and more over the mid-Washington shelf during weaker wind events (Thomas and Weatherbee, 2006). Reversal of the wind forcing can lead to sudden changes between these two modes. Recent studies show that episodic relaxation or reversal of the northerly winds during summer can allow the Columbia River plume to strongly influence the Washington and Oregon coast with plumes moving north and south of the mouth of the Columbia River simultaneously (Garcia Berdeal et al., 2002).
Sources of aluminum (Al) to the world oceans include riverine inputs, atmospheric inputs, and, to a much lesser degree, dissolution from sediments while scavenging onto particle surfaces is regarded as the predominant mechanism for removal of Al. Oceanic levels of dissolved Al are found at trace concentrations, ranging from less than 0.50 nM in intermediate/deep waters of the North Pacific to >25 nM in surface and deep waters of the eastern North Atlantic where high eolian dust input from the Saharan desert is observed (Hydes, 1979, Orians and Bruland, 1986, Kramer et al., 2004, Measures et al., 2008). Higher surface water Al concentrations have been observed in the oligotrophic subtropical gyres of the ocean basins while lower concentrations have been observed in the more productive boundary regions (Orians and Bruland, 1986, Johnson et al., 2003, Measures et al., 2005). Concentrations of dissolved Al in riverine waters are highly variable, ranging from ∼50 nM to >1 μM (Hydes and Liss, 1977, Mackin and Aller, 1984a, Morris et al., 1986, Upadhyay and Sen Gupta, 1995, Takayanagi and Gobeil, 2000).
Conomos and Gross (1972) investigated the physical, geochemical, and biological processes governing suspended particulate material in the Columbia River estuary and nearby coastal ocean. Concentrations of riverine suspended particulate material varied from 8 to 40 mg L−1, of which 85–95% consisted of lithogenous particulate material. Assuming 8.2% Al by weight (Taylor, 1964) in crustal material, this equates to greater than 20 μM particulate Al. This suspended lithogenous particulate material consisted largely of feldspar and quartz with the aluminosilicates biotite [(K(Mg,Fe)3AlSi3O10(OH)2] and muscovite [Al2Si4O10(OH)2] dominating the larger particle size fractions (Conomos and Gross, 1972). Being that the high suspended particulate load of the river is dominated by aluminosilicates, it follows that the Columbia River is likely a major source of dissolved and particulate Al as well as silicic acid to coastal waters of Washington and Oregon. In contrast, California Current waters are low in both dissolved Al (<1 nM Al; Orians and Bruland, 1986) and silicic acid (∼2–4 μM; Hill and Wheeler, 2002, Aguilar-Islas and Bruland, 2006).
This study provided a unique opportunity to investigate dissolved and particulate Al and silicic acid distributions during both upwelling and downwelling conditions in May/June 2006 in the Columbia River, the river plume, and the adjacent coastal waters of Oregon and Washington. This paper describes the distributions of dissolved and particulate Al and silicic acid both within the Columbia River and estuary as well as in near- and far-field plumes both north and south of the Columbia River. The behavior of Al and silicic acid in river–ocean mixing within the estuary and during advection of the plume away from the river mouth is also discussed.
Section snippets
Sample collection and filtration
Seawater samples were collected aboard the R/V Wecoma off the Oregon and Washington coasts from May 21, 2006 to June 21, 2006 during the RISE-4W cruise, the last of five cruises associated with the RISE (River Influence on Shelf Ecosystems) program (see Bruland et al., 2008, for more detail). Surface (∼1 m) sampling was conducted using an underway clean surface pump “fish” system described in detail elsewhere (Bruland et al., 2005, Aguilar-Islas and Bruland, 2006, Lohan and Bruland, 2006).
Downwelling/northward plume transects
A period of strong south winds (∼12 m s−1) persisted from May 22, 2006 through May 27, 2006 and remained downwelling-favorable through the end of the month. This resulted in a narrow Columbia River plume moving northward and hugging the Washington coast over this time period. The locations of the downwelling northward plume surface transects (T1, T4, and T6) are shown in Fig. 1. Surface transect T4 was across the near-field plume close to the mouth of the Columbia River while transects T1 and T6
Comparison with previous data
Dissolved Al data from the Columbia River during May 2006 show significant differences from previously published results of river and estuarine studies. In a study of dissolved Al in the Zaire and Niger River plumes, Van Bennekom and Jager (1978) measured dissolved Al concentrations of 1.0–1.6 μM in the Zaire River and significantly lower values of 0.1–0.2 μM in the relatively higher suspended particulate matter (SPM) waters of the Niger River. In the Zaire River estuary, an increase in dissolved
Conclusions
The Columbia River is a significant source of dissolved and particulate Al to the coastal waters of Oregon and Washington with both dissolved Al and particulate Al being orders of magnitude greater in Columbia River plume waters than in Al-deplete California Current waters. Likely due to a combination of salt-induced flocculation of colloidal Al within the Columbia River estuary and Al scavenging onto particle surfaces in the turbidity maximum within the estuary, dissolved Al showed a
Acknowledgements
We thank the Captain and crew of the R/V Wecoma for their assistance on this research expedition. We deeply appreciate the efforts of Bettina Sohst for the nutrient analyses, Geoffrey Smith for help with the sampling, and Barbara Hickey as the chief scientist on-board. We are also appreciative of the efforts of two anonymous reviewers whose comments were constructive and greatly improved this manuscript. This work was supported by RISE program, funded by National Science Foundation CoOP
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