Climatic and Tidal Forcing of Hydrography and Chlorophyll Concentrations in the Columbia River Estuary
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- Roegner, G.C., Seaton, C. & Baptista, A.M. Estuaries and Coasts (2011) 34: 281. doi:10.1007/s12237-010-9340-z
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Hydrographic patterns and chlorophyll concentrations in the Columbia River estuary were compared for spring and summer periods during 2004 through 2006. Riverine and oceanic sources of chlorophyll were evaluated at stations along a 27-km along-estuary transect in relation to time series of wind stress, river flow, and tidal stage. Patterns of chlorophyll concentration varied between seasons and years. In spring, the chlorophyll distribution was dominated by high concentrations from freshwater sources. Periods of increased stream flow limited riverine chlorophyll production. In summer, conversely, upwelling winds induced input of high-salinity water from the ocean to the estuary, and this water was often associated with relatively high chlorophyll concentrations. The frequency, duration, and intensity of upwelling events varied both seasonally and interannually, and this variation affected the timing and magnitude of coastally derived material imported to the estuary. The main source of chlorophyll thus varied from riverine in spring to coastal in summer. In both spring and summer seasons and among years, modulation of the spring/neap tidal cycle determined stratification, patterns of mixing, and the fate of (especially freshwater) phytoplankton. Spring tides had higher mixing and neap tides greater stratification, which affected the vertical distribution of chlorophyll. The Columbia River differs from the more tidally dominated coastal estuaries in the Pacific Northwest by its large riverine phytoplankton production and transfer of this biogenic material to the estuary and coastal ocean. However, all Pacific Northwest coastal estuaries investigated to date have exhibited advection of coastally derived chlorophyll during the upwelling season. This constitutes a fundamental difference between Pacific Northwest estuaries and systems not bounded by a coastal upwelling zone.
KeywordsChlorophyll concentrationHydrographyUpwellingColumbia River estuaryNortheast Pacific
The Columbia River estuary is the interface between a high-volume river and a wind-forced coastal upwelling zone. River and ocean end members exhibit significant variability in forcing functions that influence estuarine circulation (Bruland et al. 2008; Chawla et al. 2008). The fluvial end member varies seasonally and interannually in the timing and magnitude of volume flux, with maximum flows during the spring freshet in May or June and a relatively large interannual variation in total flow caused by runoff of winter snow pack (Mote et al. 2003). Flow patterns are further modified by operation of Columbia River dams and flood control structures.
The ocean end member is influenced by coastal wind stress that forces upwelling and downwelling conditions on the shelf and nearshore zone (Huyer 1983; Hickey 1989). Wind stress controls variability in cross-shelf gradients of biophysical properties and the water masses present at the estuary mouth (Roegner et al. 2002; Dale et al. 2008). Large variation in the direction, duration, and intensity of coastal wind stress occurs over seasonal and interannual timescales. Estuarine circulation is further forced by spring/neap variation in the mixed semidiurnal tides (amplitudes between 1.0 and 3.5 m), which modulate stratification and mixing processes (Jay and Smith 1990; Chawla et al. 2008). During spring tides, increased tidal mixing favors salt wedge formation with intermediate tidal intrusion length, while neap tides foster stratification and increased salinity penetration into the estuary. These baroclinic and barotropic forces generate dynamic circulation patterns with horizontal currents of 1 to 2 m s−1 and a hydrologic residence time from one to several days (Jay and Smith 1990).
In turn, these factors affect advection of phytoplankton and biogeophysical cycling in the Columbia River estuary. Previous studies have focused mainly on the riverine end member of the system, which has been shown to be a significant source of phytoplankton to the estuary. Freshwater diatoms dominate most riverine samples, and the species composition resembles that of eutrophic lakes (Haertel et al. 1969; Prahl et al. 1998). Chlorophyll concentrations are large and maximal during the spring diatom bloom and typically range between 15 and 30 mg m−3 (Prahl et al. 1998; Sullivan et al. 2001). Concentrations during summer are reduced to 5 to 10 mg m−3 and reach minimal levels in winter. Nutrients (N, P, and Si) rarely became limiting to phytoplankton production in the river due to numerous point sources of nutrient addition coupled with short residence times of water masses (Prahl et al. 1998; Sullivan et al. 2001; Bruland et al. 2008). Along the tidal fluvial section (about 230 rkm from Bonneville Dam to the estuary), chlorophyll concentrations tend to increase with distance downstream as the standing stock of phytoplankton accumulates (Small et al. 1990). However, these freshwater cells lyse on contact with 3–5-psu salinity water (Lara-Lara et al. 1990), and much of the organic material entering the estuary may be biologically processed in estuarine turbidity maxima (ETM) (Baross et al. 1994; Prahl et al. 1997; Small and Prahl 2004). During moderate- and high-flow conditions, the system tends to export nutrients and particulate organic matter via the Columbia River plume (Klinkhammer et al. 1997; Bruland et al. 2008). During low-flow conditions that typify summer conditions, salinity intrusion and residence times are maximized and ocean-derived nutrients and phytoplankton can be advected to the estuary (Haertel et al. 1969; Lara-Lara et al. 1990).
The effect of variation in wind stress on the ocean end member and on hydrographic properties of the Columbia River estuary has received less attention than the river end member. Phytoplankton production on the Oregon continental shelf is dependent on light intensity and wind-forced upwelling of subsurface nutrients (Huyer 1983; Hickey 1989; Henson and Thomas 2007). Upwelled water is colder (<10°C), saltier (>31 psu), and higher in nitrate (>30 μmol L−1) than surface water and, when upwelled into the photic zone, stimulates phytoplankton growth. Winds become favorable for upwelling after a seasonal switch in atmospheric pressure regimes known as the “spring transition” (Huyer et al. 1979). Timing of the spring transition varies between March and July, and this has major implications for ocean ecology, as early spring transitions generally presage good ocean conditions for the coastal food chain (Peterson and Keister 2003; Kosro et al. 2006; Barth et al. 2007) and fisheries (Logerwell et al. 2003; Shanks and Roegner 2007). Upwelling is a dynamic process that is highly sensitive to the frequency, intensity, and duration of wind events (Austin and Barth 2002), which in turn are driven by the position and propagation of atmospheric pressure systems (Strub et al. 1987; Henson and Thomas 2007). Winds during the March through October upwelling season often oscillate between upwelling and downwelling or relaxation conditions at 2 to 6 day weather and 20-day intraseasonal periods, resulting in pulses of productivity in the coastal ocean (Bane et al. 2005). A rapid shoreward movement of the surface mixed layer and the phytoplankton it contains can follow relaxation from upwelling winds (Roegner et al. 2002; Dale et al. 2008).
The present study expands previous investigations in the river-dominated Columbia River estuary. We examine the response of estuarine hydrographic parameters to varied climatic forcing during spring and summer 2004 to 2006, and we focus on the role of local wind stress on the distribution of chlorophyll within the estuary. Our observations confirm the riverine source of chlorophyll during the spring phytoplankton bloom, as well as the role of river flow on modulating the magnitude of freshwater phytoplankton transport. We further show that the vertical distribution of riverine chlorophyll is sensitive to the spring/neap tidal modulation. On a seasonal scale, we demonstrate that wind-induced upwelling controls the advection of high salinity and coastally derived chlorophyll to the estuary. On an interannual scale, we conclude that the magnitude, origin, and spatial distribution of chlorophyll in the estuary are largely controlled by climatic variation in both river flow and coastal wind stress.
Materials and Methods
Time Series Measurements
Variation in climatic forcing affecting estuarine hydrography was assessed with time series of river flow, coastal wind stress, and maximum daily estuarine salinity. Time series were compiled for March through September of 2004 to 2006. Daily river flow (Q, m3 s−1) and temperature were recorded at Bonneville Dam (rkm 234; http://www.cbr.washington.edu). To explore interannual and seasonal variation in river forcing and river temperature, we compared scatterplots of anomalies from the 10-year mean daily temperature and flow at Bonneville Dam each year from March through September.
Coastal wind velocity vectors were measured at NOAA buoy 46029 (46.12° N, 124.51° W) or when necessary to bridge data gaps from NOAA buoy 46041 (47.34° N, 124.75° W) (www.ndbc.noaa.gov). Hourly wind vectors were converted to mean daily alongshore wind stress (τN, N m−2). Positive (northward) wind stress induces downwelling, and negative (southward) wind stress drives upwelling. We computed a time series of cumulative daily northward wind stress (ΣτN) from March through September to compare the relative timing and magnitude of the net upwelling and downwelling periods between years. Within the estuary, we used the tidal daily maximum salinity (Smax, psu) recorded at CORIE station Red 26 (rkm 7.5; 46.21° N, 123.95° W) as a measure of oceanic input (www.ccalmr.ogi.edu/CORIE). Linear regression with nearby stations was used to fill data gaps when necessary. Time series of these variables were constructed for each of the 3 years to examine seasonal and interannual variations in climatic forcing. We used cross-correlation to discern the effect of mean daily northward wind stress on detrended maximum daily estuarine salinity (at lags −7 to 0 day). Spectral analysis was used to assess possible spring/neap tidal signals in the salinity time series, enabling a 2-day resolution.
For interannual comparisons of the frequency, duration, and intensity of wind events, we identified individual upwelling and downwelling weather patterns and periods of low wind stress (LWS) (calm periods) from low-pass-filtered wind stress time series (3-day running average). We based identifications on a wind stress threshold τΝ of ±0.03 N m−2, which has been observed to instigate changes in sea surface temperature values at Pacific Northwest (PNW) coastal sites (Roegner et al. 2007). Upwelling events were defined as periods of at least 3 days of consecutive τΝ ≤ −0.03 N m−2, downwelling events as τΝ ≥ 0.03 N m−2, and periods of LWS between ±0.03 N m−2. We determined the number, duration, and total stress of upwelling and downwelling periods and the duration of LWS periods, for the March through September period of each year.
Water Property Measurements
Indices of Biophysical Parameters
Chlorophyll and physical variables were transformed into indices to investigate the influence of climatic and tidal forcing on regulating chlorophyll delivery to the Columbia River estuary. Salinity was used to delimit the ocean and river end members of the estuarine continuum. The freshwater source was designated by salinity ≤ 4 psu, and the ocean source was assigned to salinity ≥29 psu. These values encompass some mixing of end members within the estuary. For each cruise, the mean chlorophyll concentration was calculated for all bins within these salinity ranges. This produced paired variables for each date sampled: chlorophyll in ocean water (COW) and chlorophyll in river water (CRW). We then created a source ratio (SR) = CRW/COW, where SR > 1 indicates chlorophyll enrichment in river water and SR < 1 the converse. From the river flow data, we created a cumulative stream flow index (SF) as the mean of the mean daily river flow at Bonneville Dam for the previous 4 days. This time frame accounts for advection of chlorophyll downstream and into the estuary. Flow conditions were categorized based on the 10-year historical data from Bonneville Dam as “low” (<4,000 m3 s−1), “medium” (4,000 ≥ SF ≤ 8,000 m3 s−1), and “high” (>8,000 m3 s−1). We used the 4-day cumulative northward wind stress (CWS, N m−2) as an index of forcing from upwelling/downwelling winds. CWS, which could have positive or negative values, was created by summing the mean daily northward wind stress for the previous 4 days to account for “spin up” of wind stress on ocean conditions. CWS was categorized as “weak” (|CWS| < 0.2), “moderate” (0.2 ≥ |CWS| ≤ 0.4), or “strong” |CWS| > 0.4 upwelling/downwelling conditions. Finally, we used lunar stage to index the spring/neap cycle (SN, dimensionless). Values ranged from 0 (new moon, spring tide) to 1 (full moon, spring tide), with 0.5 denoting neap periods. We then plotted log10 SR against day of year (DoY), SF, CWS, and SN to examine time and each physical forcing on advection of chlorophyll to the estuary. We also plotted CRW by COW to look for seasonal changes in chlorophyll intensification. Principal components analysis was used to evaluate the relative variance explained by these indices. Finally, we determined the date of the spring transition to upwelling conditions based on sea-level records as in Shanks and Roegner (2007) and created a time index (DoT) where the date of the transition for a particular year was normalized to zero, and days sampled before that date were negative and those after were positive. We then regressed log10 SR by DoT to estimate the time response of chlorophyll in the estuary to the spring transition.
Time Series of Physical Data
Frequency, duration, and intensity (Στ, N m−2) of wind events during March through September, 2004–2006
The 2005 study period was dominated by downwelling winds or low wind stress from March through June and delayed but strong upwelling thereafter (Fig. 4). Cumulative wind stress remained constant and positive from April until commencement of upwelling and declined steadily during the summer upwelling period; however, it did not reach zero until mid-September. There were four upwelling events that summed to 68 days, four downwelling events totaling 55 days, and low wind stress of 91 days (including week-long periods in April–May and June). The total number of upwelling days, total upwelling intensity, and total days of downwelling plus low wind stress were similar in 2004 and 2005; however, the timing and periodicity of wind events were very different (Table 1).
In contrast, 2006 was a year of strong upwelling (Fig. 5). Wind stress turned negative in mid-April and remained in upwelling mode throughout the measurement period except for three low- to moderate-intensity downwelling events. Cumulative wind stress was high and positive during much of the spring and turned negative at the beginning of August. There were five upwelling events lasting 117 days with a total intensity twice that of 2004 or 2005 (Table 1). There were six downwelling events of moderate total intensity extending for 58 days (similar to 2005) and only 39 days of low wind stress.
Seasonal wind stress patterns influenced trends in maximum estuarine salinity and the intrusion of high-salinity water into the estuary. In general, Smax values remained below 31 psu during spring downwelling conditions and increased coincident with upwelling intensity. In 2004, intrusions of high-salinity water were first detected in June and repeated incursions were in phase with upwelling pulses (Fig. 3). There were 22 of 212 days (10.4%) where Smax ≥ 31 psu. In 2005, maximum estuarine salinity was depressed until the end of June and increased and remained above 31 psu for 56 of 186 days (30.1%) during the long summer period of persistent upwelling (Fig. 4). In 2006, high-salinity water first entered the estuary following the first main upwelling period in mid-May (Fig. 5). Salinity declined sharply during periods of sustained downwelling of moderate intensity and high flows in May and July and otherwise oscillated around 31 psu for the remainder of the measurement period. There were 50 of 131 days (38.2%) where Smax ≥ 31 psu, but equipment failure in late summer precluded a full accounting.
Highest two correlations and associated lags (day) from cross-correlation of mean daily northward wind stress versus maximum daily estuarine salinity for dates given
1 Mar to 30 Sep
15 Jun to 30 Sep
Water Column Structure and Chlorophyll Distribution
T/S and C/S Diagrams
Regression statistics for temperature–salinity (T/S) and chlorophyll–salinity (C/S) relationships measured during hydrographic surveys
C/S diagrams reveal a complex pattern of chlorophyll distribution in the estuary (Fig. 7). During spring 2004 and 2005, high chlorophyll concentrations (up to 40 mg m−3) were present in river water, and concentrations decreased with increasing salinity. In 2006, spring samples exhibited reduced and more variable concentrations of chlorophyll in river water (5 to 20 mg m−3). These patterns are consistent with freshwater diatom blooms being transported into the estuary (Haertel et al. 1969; Prahl et al. 1998), where they lyse in contact with salt water (Lara-Lara et al. 1990). The year 2006 was a high-flow year, which may have limited the accumulation of diatom cells in river water relative to the moderate-flow conditions during 2004 and 2005. After June, riverine chlorophyll levels were generally <6 mg m−3, and the incidence of salinity >31 psu increased. This high-salinity water often contained enhanced chlorophyll concentrations (10 to 15 mg m−3) relative to fluvial levels (e.g., Fig. 7, e, k, l, q–u). However, the relative contribution of the oceanic source of chlorophyll to the estuary varied, presumably due to variation in production and delivery of ocean phytoplankton to the coast. An additional pattern of high chlorophyll concentration was found in mesohaline salinities (approximate range 5 to 20 psu) during late summer (Fig. 7, k, r–u). This signal was caused by blooms of the “red water” producing mixotrophic ciliate Myrionecta rubra, which commonly form dense and persistent accumulations in the CRE during late summer and autumn (Herfort et al. 2010; Roegner, unpublished data).
Regression statistics in C/S diagrams did not indicate simple conservative mixing characteristics (Table 3). While slopes were generally negative in spring and positive after June, consistent with a switch in delivery of chlorophyll from river to ocean end members, correlation coefficients varied greatly. The large deviations from linearity and the low coefficients occurred in part when large concentrations of Myrionecta were present in the mesohaline water during late summer.
Indices of Biophysical Parameters
Data distribution in relation to cumulative stream flow (SF) showed that most samples were collected at low- or medium-flow periods, and only three dates occurred during high-flow periods (Fig. 8b). Ocean advection dominated in summer low-flow periods; riverine sources occurred over a wider range of SF but were highest in low- to medium-flow periods during spring.
Surveys were also made over a range of 4-day cumulative wind stress values (Fig. 8c), with 12 cruises during upwelling conditions and nine during downwelling. Both cruises that failed to encounter salinity of >29 psu were during downwelling periods and medium or high flow. CWS values during upwelling ranged up to −0.5 N m−2, while downwelling wind stresses were clustered below 0.3 N m−2, with three dates in the “strong” downwelling category. Note that both upwelling and downwelling values occurred in both spring and summer time periods. Upwelling conditions generally resulted in enhancement by the ocean end member (in 67% of cases), while during downwelling the converse trend was observed (57% of cases).
The SN was not sampled evenly: 71% of the 21 cruises were made during spring tide conditions (Fig. 8d). No obvious pattern of SR with SN was observed.
Principal component eigenvalues for factors 1 to 3
Day of year
4-d cumulative wind stress
Chlorophyll in river water
Chlorophyll in ocean water
River Sources and Stream Flow
The time series of the source ratio SR indicates a strong seasonal trend in river versus ocean sources of chlorophyll to the Columbia River estuary (Fig. 9). During spring, chlorophyll transport to the estuary was dominated by fluvial processes. High chlorophyll concentration of <4 psu in water indicates a freshwater origin (Fig. 7), and values of the SR were >1 from March through June (Figs. 8a and 9), indicating a river-dominated source. We did not ascertain the phytoplankton assemblage during this study. However, previous work has shown that freshwater diatoms dominate riverine phytoplankton dynamics (Haertel et al. 1969; Lara-Lara et al. 1990; Sullivan et al. 2001). Small et al. (1990) found a seasonal abundance peak during the spring diatom bloom in March and April, followed by decreased levels through autumn (Fig. 7). On the Oregon shelf, light is limiting to phytoplankton growth in winter, and the minimum light intensity threshold of 21 W m−2 does not occur until mid-March (Henson and Thomas 2007). Light limitation may also be a primary factor limiting algal biomass in the river during winter (Fig. 7; Sullivan et al. 2001).
Stream flow appeared to modulate the magnitude of the spring bloom. The chlorophyll concentration in the high-flow year 2006 was reduced compared to levels in 2004 and 2005 (Figs. 3, 4, 5, and 7). The presumed mechanism is that algal biomass accumulates during transit in low-flow conditions but is minimized when rapidly advected through the system in high flows (Prahl et al. 1998). Maximum spring bloom concentrations exceeded 40 mg m−3 in 2004 and 2005, compared to 20 mg m−3 in 2006. However, note that chlorophyll concentrations during spring generally remain relatively high even during high-flow years (>15 mg m−3; Small et al. 1990; Prahl et al. 1998).
The fate of this fluvial phytoplankton depends on stream flow and tidal factors. Freshwater cells tend to lyse when subjected to salinities of 3–5 psu (Lara-Lara et al. 1990), and the resultant material is converted to detrital particulate and dissolved organic matter. Where this salinity-based transformation takes place is related to flow velocity, stage of tide, and the spring/neap stratification–destratification (Fig. 6). While tidal freshwater stations in the lower Columbia River are exemplified by a well-mixed water column, estuarine stations can be highly stratified. During ebb tides, high-flow periods with minimal salinity intrusion within the estuary, and neap periods of high stratification, freshwater phytoplankton can be transported directly into the coastal ocean in the Columbia River plume (Fig. 6). In contrast, during flood tides, spring tides of increased mixing, and periods of reduced flow, transformations occur within the estuary and often in ETM zones (Baross et al. 1994). ETM form and migrate along the river channel based on salinity and velocity characteristics (Small and Prahl 2004; Jay et al. 2007). Thus, under some hydrographic conditions, the river can supply fluvially derived organic matter directly to the coast; during other periods, that material is processed within the estuary itself. Tidal and stream flow factors thus modulate the fate of fluvial phytoplankton.
Ocean Sources and Wind Stress
The average date of the spring transition, calculated for a 34-yr period as in Shanks and Roegner 2007, is 7 April ± 24 days (DOY 97). The spring transitions in 2004 and 2006 were within 2 weeks of the mean date (3 April and 22 April), while the transition in 2005 was delayed until around 23 May, over two standard deviations from the mean. As defined by Kosro et al. (2006), the “biological spring transition” lags the physical spring transition by about a month (35 ± 27 days) and usually occurs in mid-May; in 2005, some biological systems did not respond until August (Barth et al. 2007). This lag was also pronounced in our study during 2005 (Fig. 4); downwelling conditions dominated during May and June, while sustained upwelling occurred from late July through August and was associated with relatively high imported chlorophyll levels (15–20 mg m−3; Fig. 7, k, l). These results also conform to studies of shelf productivity conducted by Hickey et al. (2006), who found that shelf chlorophyll concentrations remained below 5 mg m−3 until July 2005 and increased to around 15 mg m−3 in August. While the consequences of this delay in productivity to shelf ecosystems have been discussed (Barth et al. 2007), the effects on the estuarine ecosystem are unknown.
The frequency, duration, and intensity of local wind events also differed substantially between years (Table 1), and this affected patterns of maximum estuarine salinity, which is the tracer for upwelled water. The year 2004 was characterized by a high number of alternating upwelling and downwelling periods, and 2005 was dominated by downwelling conditions or low wind stress until August, while 2006 had relatively sustained upwelling conditions after the spring transition in March, aside from ∼3 weeks of moderate downwelling-favorable conditions in late May and June. Inspection of the time series of τΝ and Smax reveals that estuarine salinity was highly sensitive to fluctuations and trends in coastal wind stress (Figs. 3, 4, and 5). Time series exhibited rapid fluctuations of Smax with changes from upwelling to downwelling conditions. The largest salinity decreases occurred when strong downwelling events coincided with periods of high stream flow. Using cross-correlation, the best fit of the two time series occurred at lags of −3 to −1 day, indicating a rapid response of Smax to τΝ, with correlations becoming stronger later in the season (Table 2). When upwelling occurred early (2004 and 2006), Smax increased in the estuary earlier than when the spring transition was delayed (2005). These data fit the paradigm of wind stress influencing the type of water masses present near the estuarine mouth, with increasing salinities occurring with strong upwelling wind events and a general trend of increasing salinity in the estuary as the upwelling season progresses.
Wind stress fluctuations also affected chlorophyll concentration in the ocean end member (Figs. 3, 4, and 5). Cruises during periods of active upwelling often exhibited enhanced chlorophyll concentrations in water >29 psu (Fig. 7, e, k, l, q, r, u). During downwelling or before “spin up” of coastal water, lower concentrations and lower salinities were present in the ocean end member (Fig. 7, d, f, i, j, p). Samples in May and June were commonly collected during low wind stress or downwelling conditions during the study years, even when the spring transition had occurred. After the onset of sustained upwelling, slopes of data points in C/S diagrams were usually positive, indicating higher chlorophyll in higher-salinity water (Table 3). However, transitions from upwelling to downwelling conditions quickly affected estuarine chlorophyll distributions (Figs. 4 and 7, d, e).
Maximum chlorophyll concentrations in high-salinity water were 10 to 15 mg m−3, levels that are similar to those observed on the Washington shelf during upwelling events (Roegner et al. 2002; Hickey et al. 2006) but much lower than concentrations in the river during spring diatom blooms. The fate of coastal phytoplankton in the estuary is uncertain. However, data from benthic diatom surveys indicate that freshwater forms dominate most areas surveyed and that estuarine and marine species were limited to peripheral bays such as Baker Bay (Amspoker and McIntire 1986). It is likely that coastal inputs were proportionally more important to the ecology of the Columbia River estuary before enhancement of fluvial phytoplankton by the hydrosystem (Sullivan et al. 2001).
Most Pacific Northwest coastal estuaries have small watersheds with seasonal rainfall patterns and alternate between terrestrial and oceanic nutrient supply (Colbert and McManus 2003; Sigleo and Frick 2007; Brown and Ozretich 2009). In the winter rainy season, coastal estuaries may provide important sources of nutrients to the ocean during periods of high flushing (Hill and Wheeler 2002). In summer, these estuaries generally exhibit low stream flow, depleted fluvially derived nutrients, and short hydraulic residence times. This results in a decreased importance of riverine and terrestrial sources of organic and inorganic material to estuarine systems. Alternatively, during summer periods, nutrients and phytoplankton generated on the continental shelf during upwelling events can be transported to the coast during subsequent wind relaxation events and then into estuaries through tidal advection (Roegner and Shanks 2001; Roegner et al. 2002; Banas et al. 2007). Oceanic phytoplankton species have been found in estuarine water samples (Haertel et al. 1969; Newton and Horner 2003), and stable isotope analysis indicates that marine-derived nutrients can be incorporated into estuarine benthic food webs (Ruesink et al. 2003; Sigleo et al. 2005). Shellfish toxicity and fishery closures along the Washington coast have also been linked to wind-driven transport of the harmful algal bloom diatom Pseudo-nitzschia spp. to the coast (Trainer et al. 2002). Hydrographic and biogeochemical links between upwelling dynamics and coastal estuaries have been found in a number of estuaries in Washington (Pearson and Holt 1960; Roegner et al. 2002; Hickey et al. 2002) and Oregon (Haertel et al. 1969; de Angelis and Gordon 1985; Roegner and Shanks 2001; Colbert and McManus 2003; Hickey et al. 2002; Brown and Ozretich 2009), as well as further south in California (Martin et al. 2007; Caffrey et al. 2010). It should be noted that these examples of allochthonous coastal production fuelling estuarine dynamics in Pacific Northwest estuaries are contrary to the paradigm of river-borne nutrients driving autochthonous primary and secondary production within Atlantic and Gulf coast estuaries.
The Columbia River estuary differs from other Pacific Northwest coastal systems in its prodigious freshwater export and high fluvial phytoplankton production. Nutrients do not appear to become limiting to riverine phytoplankton (Prahl et al. 1998; Sullivan et al. 2001; Bruland et al. 2008), and river water chlorophyll concentrations can be maintained at relatively high levels (>6 mg m−3) throughout summer months. Chlorophyll levels can be extremely enriched during the spring bloom. This material likely fuels secondary production in the estuary during transformation in ETM (Baross et al. 1994; Prahl et al. 1997; Small and Prahl 2004) or in the coastal ecosystem after export to the ocean. High primary production in the fluvial zone is thought to be an anthropogenic effect of dam construction, and conditions in the unregulated river were likely characterized by a greatly reduced riverine phytoplankton component and an increase in terrestrial detritus (Sullivan et al. 2001). But despite extensive management of the Columbia River hydrosystem, climatic factors still resulted in large variation in stream flow during our 3-year study.
As in other Pacific Northwest coastal estuaries, the ocean end member contributes ocean-derived chlorophyll and probably nutrients to the Columbia River estuary during summer. Ocean sources dominate estuarine chlorophyll patterns during upwelling periods, but the local wind patterns that control the magnitude and timing of ocean input also exhibit significant interannual variation. One might surmise that strongly pulsed upwelling/relaxation cycles would result in the greatest cumulative transport of ocean phytoplankton to the estuary (as in 2004) because this would favor repeated cycles of ocean production and subsequent advection to the coast. In contrast, sustained upwelling (as in the summers of 2005 and 2006) may lead to the enhanced inwelling of inorganic nutrients and autotrophic production within the system, since phytoplankton in newly upwelled water at the coast may require time in the photic zone to become densely concentrated (Roegner et al. 2002). As a result of Ekman transport, blooms during active upwelling usually develop tens of kilometers from the shore (Small and Menzies 1981).
An interesting comparison with low-discharge PNW estuaries (i.e., not the Columbia) can be made with the particularly well-studied Galician Rias Baixas of NW Spain (Alvarez et al. 2005). Reports indicate that phytoplankton in these systems can have either autochthonous or allochthonous origins, based partially on species type (diatom versus dinoflagellate) and their relative motility (Fermin et al. 1996; Bravo et al. 2010). In situ growth of diatoms fueled from inwelled nutrients commonly occurs in the Rias when downwelling conditions follow an upwelling episode (Figueiras et al. 2002). The ensuing stratification and reduced flushing tends to stimulate production that fuels a large mussel aquaculture industry (Alvarez-Salgado et al. 1996). However, advection of phytoplankton from the coast also occurs during relaxation or downwelling, typically in late summer, and can include toxic red-tide-forming dinoflagellates (Bravo et al. 2010; Escalera et al. 2010).
A comparison of the number and range of average daily freshwater discharge from estuarine systems bordering the four main upwelling Large Marine Ecosystems
River discharge range (m3 s−1 d−1)
In the Columbia River estuary, seasonal variation in stream flow and wind stress determined the balance of riverine or ocean end members dominating estuarine chlorophyll patterns. Stream flow influenced delivery of freshwater and riverine phytoplankton to the estuary, while variation in local wind stress controlled upwelling and the delivery of high-salinity water and coastally derived phytoplankton to the estuary. Stratification, mixing, and the spatial distribution of water properties and chlorophyll were influenced by the spring/neap tidal cycle. Due to the water retention afforded by snow pack in the Cascade Mountains, there is a much larger and longer freshwater influence on hydrography and chlorophyll production in the Columbia than smaller systems that drain the low-lying Coast Range. The magnitude of stream flow and the initiation, frequency, duration, and intensity of upwelling-favorable winds are climatically controlled and exhibit large interannual variability. Variations in these forcings have strong ecological ramifications for riverine, estuarine, and ocean ecosystems in the Pacific Northwest and likely along other eastern boundary current coastlines.
We appreciate the efforts of Capt. Gene Bock and crew of R/V Forerunner and thank Michael Wilkin for troubleshooting glitches in oceanographic instrumentation. We are also grateful to three reviewers who helped clarify the manuscript. Reference to trade names does not imply endorsement by the National Oceanographic and Atmospheric Administration or Oregon Health and Science University. This research was supported by the Bonneville Power Association.