The Variability of Partial Pressure of Carbon Dioxide (pCO2) in a River-Influenced Coastal Upwelling System: A Case of the Northeast Pacific Coast

The Northeast Pacific coastal ocean, as a typical river-influenced coastal upwelling system, is characterized by significant variability of sea surface partial pressure of carbon dioxide ( p CO ₂ , <200 to >1000 μatm). This study reviewed the pCO₂ variability and its underlying controlling mechanism in this highly dynamic region by bringing together previous scientific findings and historical data. The large pCO₂ variability reflects the complex interactions between physical processes (riverine input and coastal upwelling) and the biological responses to the nutrient transportation associated with these physical processes, while temperature and air-sea gas exchange play a minor role in affecting pCO₂. Both the river water and upwelled subsurface water are characterized by higher concentrations of pCO₂ and nutrients when compared to the coastal surface water. The presence of high chlorophyll-a and low pCO₂ in river plumes and areas adjacent to upwelling locations showed the intense biological CO₂ uptake. The influences of riverine input and coastal upwelling thus mainly depend on the competing effect of high background pCO₂ of river water and upwelled subsurface water vs. the biological dropdown of pCO₂ resulting from the riverine- and upwelling-associated nutrient supplies. The strength of upwelling-favorable wind plays an important role in the pCO₂ variability by affecting the intensity of coastal upwelling, with stronger wind speed causing more intense upwelling. The long-term pCO₂ increasing rate in the Northeast Pacific coast is observed to be lower than that in the North Pacific open ocean.

• pCO2 Variability
• Coastal Upwelling
• River-Influenced Coastal Systems
• Northeast Pacific Coast
• Climate Change

1. Introduction

Since the industrial revolution, the carbon dioxide (CO₂) in the atmosphere has increased from 280 ppm to the present level of 415 ppm (January 2021) due to anthropogenic activities (Keeling et al., 2004; Takahashi et al., 2009; IPCC, 2013; Takahashi et al., 2014). The rapid increase in atmospheric CO₂ concentration significantly affects the earth’s climate as CO₂ is one of the most significant greenhouse gases (IPCC, 2013). The ocean exchanges CO₂ with the atmosphere at the air-sea interface, absorbing about 25% of the anthropogenic carbon emissions annually and lowering the atmospheric CO₂ concentration (Takahashi et al., 2002; Sabine et al., 2013). When CO₂ is taken up and transported into the ocean interior, it is isolated from the atmosphere on time scales of hundreds and thousands of years. It is projected that about 90% of anthropogenic CO₂ would end up in the ocean on a thousand-year time scale (Sabine et al., 2004), which mitigates the CO₂-induced global climate change. On the other hand, the oceanic uptake of anthropogenic CO₂ is acidifying the ocean and changing the carbonate chemistry of the seawater. This leads to ocean acidification as “the other CO₂ problem” (Doney et al., 2009a), which could have several negative consequences for ocean life, especially for calcifying organisms like corals and coccolithophores (Feely et al., 2004; Doney et al., 2009a; Takahashi et al., 2014; George, 2017; Fassbender et al., 2018; Jiang et al., 2019; Gattuso & Hansson, 2011). As atmospheric pCO₂ is relatively steady, the air-sea CO₂ exchange is mainly determined by the seawater pCO₂ variability. The surface ocean pCO₂ variability is strongly influenced by changes in temperature (Takahashi et al., 2006; Bates, 2007; Takahashi et al., 2009; Rödenbeck et al., 2013; Sutton et al., 2017), physical processes (Friederich et al., 2002; Hales et al., 2005; Fiechter et al., 2014) and biological activities (Thomas et al., 2005; Zhai et al., 2005; Yoshikawa Inoue et al., 2017). Understanding the distribution and variability of sea surface pCO₂ is thus critical for accurately quantifying the carbon budget (Bates et al., 1998; Sabine et al., 2004; Dai et al., 2009; Evans et al., 2013; Jiang et al., 2013; Fiechter et al., 2014).

Despite the small surface area (~7% of the sea surface), coastal oceans are one of the most dynamic and biologically productive ecosystems playing an important role in global carbon biogeochemical cycles (Gattuso et al., 1998; Cai, 2003; Chen & Borges, 2009; Evans et al., 2011; Hales et al., 2012; Chen et al., 2013; Evans et al., 2013). The coastal oceans serve as a connection between the terrestrial and open ocean ecosystems, buffering the effects from land sources before they impact oceanic systems (Thomas et al., 2005; Chen & Borges, 2009). The strong physical-biogeochemical interactions result in large spatial and temporal variations of sea surface pCO₂ in coastal oceans, making it challenging to quantify carbon fluxes in coastal environments. Estimates based on available data showed that the coastal ocean could either act as a sink (DeGrandpre et al., 2002; Hales et al., 2005; Borges et al., 2006; Chen and Borges, 2009; Hales et al., 2012; Jiang et al., 2013; Turi et al., 2014; Evans et al., 2019; Jiménez-López et al., 2019) or source (Friederich et al., 2002; Borges et al., 2005; Wang et al., 2005; Xue et al., 2012; Robbins et al., 2018; Li et al., 2020) for atmospheric CO₂ depending on the dominant controlling mechanism of sea surface pCO₂ in a specific system.

The Northeast Pacific coast, as a river-influenced coastal upwelling system, is a fascinating region for studying the coastal pCO₂ variability. It was characterized by highly complex physical processes, including coastal currents, river input, and wind-driven upwelling. Furthermore, riverine- and upwelling-associated nutrient supplies promote high biological productivity. The interactions between physical and biological processes result in significant variations of sea surface pCO₂ on different temporal and spatial scales (Friederich et al., 2002; Gan & Allen, 2005; Chavez & Messié, 2009; Hales et al., 2012; Evans et al., 2013; Fassbender et al., 2018). On that note, this study brings together existing knowledge and data to provide a comprehensive review of the pCO₂ variability in this study region using the following outlines.

• The major physical and biological processes in the Northeast Pacific coast.

• Spatiotemporal pCO₂ dynamics in the Northeast Pacific coast and the driving factors.

• Possible future changes affecting pCO₂ variability in the study region.

2. Environmental Settings of the Northeast Pacific Coast

2.1. Circulation Features

The Northeast Pacific coast is located to the west of the North American continent, which is greatly influenced by the California current system (Figure 1). The key circulation features of the California current system are the equatorward traveling California Current, the poleward traveling California Undercurrent, and the seasonal poleward Davidson Current (Lynn & Simpson, 1987). The California Current is an all-year-round shallow, wide (60 to 900 km), and relatively slow current, which carries the nutrient-rich, cold, high-latitude North Pacific water from the coast of British Columbia southward to the southern Baja California coast (Lynn & Simpson, 1987; Zaytsev et al., 2003; Turi et al., 2014). The California Undercurrent comes along as a subsurface flow between 100 and 300 m over the continental slope and transports saline, warm, equatorial waters northward (Chelton, 1984; Lynn & Simpson, 1987). The Davidson Current (also named as the Inshore Countercurrent) is a seasonal flow during fall and winter (Lynn & Simpson, 1987). It flows over both the slope and shelf, transporting upper ocean shallow waters, which largely come from the California Current waters

with some alterations by coastal processes. Hayward and Venrick (1982) discovered significant variations in phytoplankton biomass and productivity in the California Current. They stated that fluorescence or sea surface chlorophyll values could be used as measures of the biological status of oceanic habitats but should be used with caution due to changes in spatial or temporal relationships (Hayward & Venrick, 1982).

2.2. Riverine Inputs

Rivers are a significant component of the global carbon cycle because they can influence carbon dynamics not just in wetlands but also in coastal areas where river flows are emptied (Ran et al., 2015). Rivers are the main pathways transporting terrestrial organic matter, particles, and human-derived materials to the coastal oceans (Sharples et al., 2017). River water exports not only carbon but also nitrogen, phosphorus, and silica, which are the potentially limiting nutrients for phytoplankton growth. As a result, riverine nutrients promote the phytoplankton biomass and biological productivity of the coastal ocean, especially in the river plume where both nutrients and light are favorable for phytoplankton growth (Hickey & Banas, 2003; Jiang et al., 2019). The Columbia River is the largest river in terms of discharge running into the Pacific from America (Figure 1), with an annual average flow of around 7500 m³·s⁻¹ near the mouth (Kammerer, 1990). The discharge of Columbia River is characterized by a seasonal maximum at the start of summer corresponding to the peak snowmelt (i.e., May to June, 10,000 m³·s⁻¹) and a minimum during winter (i.e., December to March, 300 m³·s⁻¹) (Evans et al., 2013). Another minor freshwater input to the Northeast Pacific coast is from the Smith River (Figure 1), with an annual average discharge rate of 106 m³·s⁻¹ (U.S. Geological-Survey, 2014).

2.3. Wind Pattern and Coastal Upwelling

The wind circulation is an important forcing for the movement of surface water in the California current system. The season reversal wind pattern is also closely related to the vertical mixing in this region. During spring and summer, when the wind travels southwards alongshore toward the equator, surface water is moved offshore, resulting in coastal upwelling (King et al., 2011). On the contrary, downwelling favorable winds (northwards along the coast) are observed in autumn and winter (Hickey & Banas, 2003). The California current is one of the five main coastal currents linked to high upwelling regions (Friederich et al., 2002; King et al., 2011). According to Evans et al. (2011), this upwelling event usually starts in April and lasts until October, the same period when the wind is traveling southwards. The upwelling brings cool, saline, CO₂-rich, and nutrient-rich water to the surface layer. The upwelling-derived nutrients promote the growth of sea plants, from microscopic phytoplankton to massive kelp forests. These plants (primary producers) constitute the core of the food web that comprises highly productive fisheries, large populations of sea mammals (dolphins, whales), and sea birds. This biologically active ecosystem can stretch up to 500 kilometers from the coast. Huyer et al. (2005) used the differences in alongshore currents, prevailing wind speed to differentiate two coastal upwelling domains in the Northeast Pacific coast: north and south of the Cape Blanco at 42.9˚N. Both upwelling regions were characterized by southwards traveling wind in summer (Friederich et al., 2002; Huyer et al., 2005; King et al., 2011), with stronger wind speeds and higher coastal upwelling indexes observed in the southern region around 42˚N (Figure 2) (Huyer et al., 2005). The coastal upwelling index has proven effective in tracking the strength of wind forcing and upwelling in the California Current System (Bograd et al., 2009). Several studies have used this index to identify the impact of ocean variability on the reproductive performance of many fish and aquatic species in coastal upwelling regions (Jacox et al., 2018). Coastal upwelling in the California Current system is usually reduced during the occurrence of El Nino, leading to a massive drop in primary productivity. This interruption in the marine food web affects the nearby coastal communities where fishers and government depend mainly on revenues from the fishery industry (Friederich et al., 2002).

3. Spatio-Temporal Variation of pCO 2 and Its Controlling Mechanisms in the Northeast Pacific Coastal Waters

3.1. Influences of Riverine Input on pCO 2 Variability

Due to anthropogenic influences and the degradation of terrestrial organic matter (Chen & Borges, 2009; Chen et al., 2013), most rivers have a pCO₂ concentration higher than that in the atmosphere with a large CO₂ emission potential

(Evans et al., 2013; Ran et al., 2015; Kokic et al., 2018). Although the Columbia River carried a large amount of nutrients into the coastal ocean, the biological response was limited in the nearshore high-turbidity area with low salinity (<5 psu, Figure 3) due to low light availability (Borges et al., 2006; Guo et al., 2009). Cruise surveys showed that the seasonal variability of pCO₂ in the Columbia estuary was about 280 to 700 μatm, and it acted as a CO₂ source for the atmosphere in most seasons except for late summer (Figure 3). The low pCO₂ occurring in summer was related to the fact that warm temperature favors photosynthesis of phytoplankton, as well as the high discharge providing more nutrients due to the peak of snowmelt freshet (Evans et al., 2013). Because of the alleviation of light limitation in conjunction with the persistence of riverine nutrients during the mixing between freshwater and seawater, high chlorophyll-a concentrations and biological production rates are generally observed in the mid-salinity river plumes where nutrients and light are favorable for phytoplankton growth (Figure 3). This biological uptake significantly draws down pCO₂, leading to strong undersaturation (<200 μatm). Therefore, the influences of riverine input on pCO₂ variability in the Northeast Pacific coast depend on the competing effect between the heterotrophic condition of the freshwater and the autotrophic CO₂ uptake in the river plume.

3.2. Influence of Coastal Upwelling on pCO 2 Variability

In the Northeast Pacific coast around April to October, equatorward winds drive Ekman transport, causing upwelling of subsurface water (Evans et al., 2011). As illustrated in Figure 4(a), the subsurface waters are rich in dissolved inorganic carbon (DIC) and have very high pCO₂ reaching or exceeding 1000 μatm (Chavez et al., 2007; Feely et al., 2008). pCO₂ values of about 700 μatm have been found in nearshore upwelling outcrops off the California-Oregon-Washington coast (Hales et al., 2005). Moreover, Feely et al. (2008) reported the upwelling of corrosive “acidified” waters (pH < 7.7) onto the region’s shelf deteriorating the acidification status in the Northeast Pacific coast (Figure 4(b)). Upwelling not only brings high pCO₂ signals but also transports nutrients from depth to the surface to promote phytoplankton growth. Due to the delayed biological response to the upwelling-induced nutrient supply, elevated Chl-a concentrations were generally observed in areas adjacent to the upwelling center. When the upwelled water moves off and along the shore, as observed by van Geen et al. (2000), pCO₂ can be quickly drawn down to undersaturation levels (<300 μatm) below the atmospheric equilibrium. Therefore, the influences of coastal upwelling on pCO₂ variability in the Northeast Pacific coast mainly depend on the balance between the upwelling-induced DIC transportation (increase pCO₂) and the subsequent biological CO₂ uptake stimulated by the upwelling-associated nutrient supply.

3.3. Surface Water pCO 2 Variability and Its Controlling Mechanisms

Previous studies have proved that one of the most significant controls governing the sea surface pCO₂ variability in the open ocean is sea surface temperature (SST), and pCO₂ generally showed a positive correlation with SST (Hales et al., 2005; Hales et al., 2012; Xue et al., 2012; Sutton et al., 2017; Yoshikawa Inoue et al., 2017; Ogundare et al., 2021). However, the pCO₂ and SST distributions in the Northeast Pacific coast (Figure 5(a)) indicated that factors other than SST played the dominant role in modulating pCO₂ while the temperature effect was minor. To further investigate the non-thermal controlling factors on pCO₂ variability, we applied temperature normalization on the surface pCO₂ following Takahashi et al. (2006).

npCO₂ = pCO_2,SSTobs × exp (0.0423 × (SST_mean - SST_obs)) (1)

where SST is sea surface temperature, pCO_2,SSTobs denotes the pCO₂ observed at in situ SST (SST_obs), and SST_mean refers to the mean SST. The temperature-normalized npCO₂ represents the pCO₂ changes caused by non-thermal processes such as air-sea CO₂ exchange, physical mixing, and biological activity (Figure 5(b)).

The variations of pCO₂ and npCO₂ were clearly related to different water masses with characteristic temperature and salinity (Figure 5). The riverine signals characterized by lower salinity (SSS < 30, Figure 5) and high chlorophyll concentration (Figure 6) were mainly observed in the area adjacent to the river mouth of the Columbia River (Huyer et al., 2005). Fassbender et al. (2018) also reported a similar observation in the low salinity Columbia River Plume 30 - 90 km from shore, which is advected ashore by the Ekman transportation and towards the south by the coastal jet in summer. The observed pCO₂ values at the salinity range of 22 - 30 were mostly below the atmospheric level (Figure 5), which suggests the dominance of biological CO₂ uptake in the river plume over the high background pCO₂ of freshwater as discussed in Section 3.1. However, high pCO₂ values (up to 780 μatm) were sometimes observed, which may be a result of a delayed biological response to riverine-borne nutrients because of the high discharge rate or sustained high-turbidity conditions. The newly upwelled subsurface waters, observed at the upwelling centers (Figure 6), were characterized by low SST (<10˚C), high salinity (>32 psu), high pCO₂ (>500 μatm), and high npCO₂ (>500 μatm) (Figure 5). On the contrary, high Chl-a concentration in areas adjacent to upwelling showed the phytoplankton growth response to the upwelling-associated nutrient supply, which significantly draw down pCO₂ leading to strong undersaturation (Figure 6). Overall, the Northeast Pacific coast is a river-influenced coastal upwelling system characterized by large variations in sea surface temperature (SST, 8˚C - 18˚C), sea surface salinity (SSS, 22 - 34), and pCO₂ (120 to 912 μatm). Previous findings reported the Northeast Pacific coastal region acted as a sink for atmospheric CO₂ (Hales et al., 2005; Chen & Borges, 2009; Hales et al., 2012; Turi et al., 2014; Evans et al., 2019).

3.4. Long-Term Seawater pCO 2 Variability

Several investigators estimated similar pCO₂ increasing rates in the North Pacific open ocean as 1.3 ± 0.2 μatm·yr⁻¹ by Takahashi et al. (2006), 1.39 ± 0.18 μatm·yr⁻¹ by Yoshikawa-Inoue et al. (2014), and 1.36 ± 0.16 μatm·yr⁻¹ by Wong et al. (2010). These rates were generally in pace with the increasing rate of atmospheric pCO₂ of 1.5 μatm·yr⁻¹ estimated by Takahashi et al. (2009). Despite the existence of previous pCO₂ investigations in the Northeast Pacific coastal region (Friederich et al., 2002; Gan & Allen, 2005; Hales et al., 2012; Evans et al., 2013; Fassbender et al., 2018), the long-term changing rate of pCO₂ in the Northeast Pacific coast is still with significant uncertainties because of undersampling in the highly dynamic system. The closest estimates available are those of Wong et al. (2010) and Takahashi et al. (2006). The former estimated the

pCO₂ increasing rate of change in Line P (48 - 49˚N, 130 - 125˚W) to be 0.57 ± 0.36 μatm·yr⁻¹, and the latter revealed a range from 0.2 ± 0.8 μatm·yr⁻¹ to 1.0 ± 0.5 μatm·yr⁻¹ along the North American west coast. The average long-term pCO₂ increasing rate in the Northeast Pacific coastal ocean seems to be lower than that in the North Pacific open ocean.

4. Future Changes in CO 2 Dynamics of the Region

Possible future changes in biological and physical processes can potentially cause negative and positive feedbacks affecting the carbonate system in the Northeast Pacific coast. An increase in SST due to climate change tends to increase seawater pCO₂ by reducing the solubility of CO₂ in seawater (Doney et al., 2009b; Takahashi et al., 2009). The circulation pattern of the northwesterly winds may enhance more intense vertical upwelling (Bogden & Edwards, 2001; Xiu et al., 2018). Increased upwelling will bring more DIC (to increase sea surface pCO₂) and nutrients (to enhance the biological dropdown of pCO₂) (Jiang et al., 2014). Moreover, the circulation pattern may be affected by the impact of more frequent strong El Niño events, which deepens the thermocline and subsequently suppresses the upwelling of nutrient-rich subsurface waters, which are needed by phytoplankton to draw down CO₂ (Wang et al., 2019). The interplay between intense upwelled CO₂-rich waters and photosynthetic reactions is vital in estimating the shelf’s overall sink-source status. Additionally, anthropogenic activities such as marine eutrophication, fisheries overexploitation, and potential geotechnical projects could have drastic future consequences for coastal carbon biogeochemistry (Doney et al., 2009b). For example, the building of dams has been known to massively drop river discharge volumes, trapping suspended solids and nutrient-rich water, which subsequently alters the river-borne carbon dynamics (Ran et al., 2015).

Data Availability

SST, SSS, and pCO₂ data are publicly available at Coastal Ocean Data Analysis Product in North America (CODAP-NA, Version 2021; NCEI Accession 0219960) https://www.ncei.noaa.gov/access/ocean-carbon-data-system/oceans/Coastal/north_america_west.html, Chlorophyll-a data was downloaded from Aqua MODIS, NASA https://polarwatch.noaa.gov/catalog/chl-aqua/download/.

Acknowledgements

This research was funded by the National Key Research and Development Program of China (2016YFA0601404), the Bureau of Science and Technology of Zhoushan, Zhejiang (2019C81037), and NSFC-Zhejiang Joint Fund for the Integration of Industrialization and Informatization (No. U1709201). The authors acknowledge NOAA’s North America Coastal Ocean Data Analysis program and Aqua MODIS, NASA for the availability of public data. Addey, C. I. acknowledges the support of the China Scholarship Council during his postgraduate study. The authors are grateful to the reviewers for their comments which helped improve the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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