Coastal upwelling supplies oxygen-depleted water to the Columbia River estuary

Abstract

Low dissolved oxygen (DO) is a common feature of many estuarine and shallow-water environments, and is often attributed to anthropogenic nutrient enrichment from terrestrial-fluvial pathways. However, recent events in the U.S. Pacific Northwest have highlighted that wind-forced upwelling can cause naturally occurring low DO water to move onto the continental shelf, leading to mortalities of benthic fish and invertebrates. Coastal estuaries in the Pacific Northwest are strongly linked to ocean forcings, and here we report observations on the spatial and temporal patterns of oxygen concentration in the Columbia River estuary. Hydrographic measurements were made from transect (spatial survey) or anchor station (temporal survey) deployments over a variety of wind stresses and tidal states during the upwelling seasons of 2006 through 2008. During this period, biologically stressful levels of dissolved oxygen were observed to enter the Columbia River estuary from oceanic sources, with minimum values close to the hypoxic threshold of 2.0 mg L(-1). Riverine water was consistently normoxic. Upwelling wind stress controlled the timing and magnitude of low DO events, while tidal-modulated estuarine circulation patterns influenced the spatial extent and duration of exposure to low DO water. Strong upwelling during neap tides produced the largest impact on the estuary. The observed oxygen concentrations likely had deleterious behavioral and physiological consequences for migrating juvenile salmon and benthic crabs. Based on a wind-forced supply mechanism, low DO events are probably common to the Columbia River and other regional estuaries and if conditions on the shelf deteriorate further, as observations and models predict, Pacific Northwest estuarine habitats could experience a decrease in environmental quality.

Figure 1. Columbia River estuary sample stations.

Circles show transect stations; triangles indicate anchor stations. The inset identifies the regional location.

Figure 2. Time series of physical drivers and O₂-S scatterplots during 29 July through 6 November 2006.

A. Tidal (left axis) and spring-neap (right axis) cycles. Circles denote cruise dates. B. Bars: Low-pass filtered mean daily northward wind stress (τ_N, N m⁻²). Gray shaded sections denote upwelling events defined as ≥3 consecutive days of τ_N < −0.03 (dotted line). Triangles and letters indicate the timing of hydrographic cruises. C. O₂-S scatterplots for designated cruises. Oxygen values are expressed as both concentration (mg L⁻¹) and as percent saturation. Red horizontal line is the 6 mg L⁻¹ criterion. Grey horizontal line is the 100% saturation level. Black dashed vertical line denotes upwelled water at 31 psu. Regression statistics are in Table 1.

Figure 3. Time series of physical drivers and O₂-S scatterplots 30 April through 7 September 2007.

A. Tidal and spring-neap cycles. Circles denote cruise dates. B. Bars: Low-pass filtered mean daily northward wind stress (τ_N, N m⁻²). Gray shaded sections denote upwelling events defined as ≥3 days of consecutive τ_N < −0.03 (dotted line). Triangles and letters indicate the timing of hydrographic cruises. C. O₂-S scatterplots for designated cruises. Oxygen values are expressed as both concentration (mg L⁻¹) and as percent saturation. Red horizontal line is the 6 mg L⁻¹ criterion. Black dashed vertical line denotes upwelled water at 31 psu. Regression statistics are in Table 1.

Figure 4. Time series of physical drivers and O₂-S scatterplots 29 June through 29 July 2008.

A. Tidal and spring-neap cycles. Circles denote cruise dates. B. Bars: Low-pass filtered mean daily northward wind stress (τ_N, N m⁻²). Gray shaded sections denote upwelling events defined as ≥3 days of consecutive τ_N < −0.03 (dotted line). Triangles and letters indicate the timing of hydrographic cruises. C. O₂-S scatterplots for designated cruises. Oxygen values are expressed as both concentration (mg L⁻¹) and as percent saturation. Red horizontal line is the 6 mg L⁻¹ criterion. Black dashed vertical line denotes upwelled water at 31 psu. Regression statistics are in Table 1.

Figure 5. Scatter plots and regression statistics.

A. Maximum salinity (S_max) by cumulative northward 4-d wind stress (CWS); B. Minimum oxygen concentration (O_2min) by cumulative 4-d wind stress; C. Regression coefficients from O_2min-S_max regression by cumulative 4-d wind stress. Black = 2006; blue = 2007; red = 2008.

Figure 6. Across-channel transects of vertical water column salinity and dissolved oxygen concentration during Autumn 2006.

Salinity, top panels. Oxygen concentration, bottom panels. Plots A–C occurred during upwelling conditions, and plots D and E during downwelling conditions. Plots A, D, and E occurred near high water, while Plots B and C occurred on the same date but different stages of the tide. Salinity isopleths are 2 psu. Oxygen isopleths are 1 mg L⁻¹, Supersaturated = blue; normoxic = white; stressed =  red. Bottom contours are shown in black. Triangles designate location of measurements.

Figure 7. Across-channel transects of vertical water column salinity and dissolved oxygen concentration during May 2007.

Salinity, top panels. Oxygen concentration, bottom panels. Plots A and B depict downwelling and C and D show upwelling conditions. For each date, two transects were made as indicated in the header. Isopleth and fill designations are as in Figure 6.

Figure 8. Along-channel channel transects of vertical water column salinity and dissolved oxygen concentration during Autumn 2006.

Salinity, top panels. Oxygen concentration, bottom panels. Cruises shown in Plots A and C occurred during upwelling, while cruises in Plots B and D occurred during downwelling conditions. Isopleth and fill designations are as in Figure 6.

Figure 9. Anchor station time series of vertical water column salinity and dissolved oxygen concentration during August 2007.

Salinity, top panels. Oxygen concentration, bottom panels. Plots A and B show upwelling during spring tides, which contrasts with Plots C and D that show downwelling during neap tides. Isopleth and fill designations are as in Figure 6.

Figure 10. Anchor station time series of vertical water column salinity and dissolved oxygen concentration during July 2008.

Salinity, top panels. Oxygen concentration, bottom panels. The plots contrast spring tides (A and B) from neap tides (C and D) during a strong upwelling event. Isopleth and fill designations are as in Figure 6.