Amplification of hypoxic and acidic events by La Niña conditions on the continental shelf off California

Abstract

[1] Low-oxygen and low-pH events are an increasing concern and threat in the Eastern Pacific coastal waters, and can be lethal for benthic and demersal organisms on the continental shelf. The normal seasonal cycle includes uplifting of isopycnals during upwelling in spring, which brings low-oxygen and low-pH water onto the shelf. Five years of continuous observations of subsurface dissolved oxygen off Southern California, reveal large additional oxygen deficiencies relative to the seasonal cycle during the latest La Niña event. While some changes in oxygen related to the isopycnal depression/uplifting during El Niño/La Niña are not unexpected, the observed oxygen changes are 2-3 times larger than what can be explained by cross-shore exchanges. In late summer 2010, oxygen levels at mid-depth of the water column reached values of 2.5 ml/L, which is much lower than normal oxygen levels at this time of the seasons, 4-5 ml/L. The extra uplifting of isopycnals related to the La Niña event can explain oxygen reductions only to roughly 3.5 ml/L. We find that the additional oxygen decrease beyond that is strongly correlated with decreased subsurface primary production and strengthened poleward flows by the California Undercurrent. The combined actions of these three processes created a La Niña-caused oxygen decrease as large and as long as the normal seasonal minimum during upwelling period in spring, but later in the year. With a different timing of a La Niña, the seasonal oxygen minimum and the La Niña anomaly could overlap to potentially create hypoxic events of previously not observed magnitudes.

Response of O<sub>2</sub> and pH to ENSO in the California Current System in a high-resolution global climate model

“…Coastal upwelling ecosystems, such as the California Current System (CalCS), support some of the world's most productive fisheries due to the seasonal, wind-driven upwelling of nutrient-rich waters (e.g., Chavez and Messié, 2009;Messié et al, 2009). The upwelled waters fuel biological production in the CalCS but are also characterized by lower O 2 and lower pH than the surrounding surface waters (e.g., Bograd et al, 2008;Nam et al, 2011). Although the CalCS is accustomed to seasonal fluctuations in O 2 and pH, several observational studies sug-gest that both have decreased in the last decades, particularly within 100 km of the coast (e.g., Bograd et al, 2008;Feely et al, 2008;Nam et al, 2011;Bednaršek et al, 2014;Bednaršek and Ohman, 2015).…”

“…The upwelled waters fuel biological production in the CalCS but are also characterized by lower O 2 and lower pH than the surrounding surface waters (e.g., Bograd et al, 2008;Nam et al, 2011). Although the CalCS is accustomed to seasonal fluctuations in O 2 and pH, several observational studies sug-gest that both have decreased in the last decades, particularly within 100 km of the coast (e.g., Bograd et al, 2008;Feely et al, 2008;Nam et al, 2011;Bednaršek et al, 2014;Bednaršek and Ohman, 2015). Specifically, Bograd et al (2008) found a shoaling of the hypoxic threshold, which is typically considered to be ∼ 60 µmol kg −1 (Gray et al, 2002;Diaz and Rosenberg, 2008;Vaquer-Sunyer and Duarte, 2008), of up to 90 m in the southern CalCS between 1984and 2006.…”

“…Source waters for upwelling in the CalCS also tend to be anomalously warm, shallow, and fresh during El Niño (Jacox et al, 2015b), resulting in similarly warm and fresh anomalies at the surface. During an ENSO cold event (La Niña), these processes are typically reversed, with nearshore surface waters being anomalously low in O 2 and pH due to a shoaling of the isopycnal surfaces (Nam et al, 2011).…”

“…Several regional modeling studies have shown the connection between ENSO and the vertical transport, water column density, origins, and properties of upwelled water (Jacox et al, 2014(Jacox et al, , 2015b, and demonstrated the relative importance of remote versus local forcing on both the physics and the biogeochemistry of the CalCS (Frischknecht et al, 2015;Jacox et al, 2015a;Frischknecht et al, 2017). During the 2010/2011 La Niña, Nam et al (2011) observed decreases in O 2 and pH in the upwelling region along the coast that were 2-3 times larger than expected solely due to the cross-shore shoaling of the isopycnal surfaces. They found that the additional reduction of O 2 was related to decreased subsurface primary production and a short-term strengthened poleward flow of the California Undercurrent.…”

Technical Note: Controlled experimental aquarium system for multi-stressor investigation of carbonate chemistry, oxygen saturation, and temperature

“…Some turbulence was caused by the recirculation of the seawater for equilibration. Target pH and oxygen levels were chosen based on values recorded at a location near Scripps (Nam et al, 2011). For this experiment, low pH and high oxygen levels were paired in one treatment, and high pH and low oxygen levels in the other, to attempt to understand organismal responses to the individual parameters.…”