Seasonal advection of Pacific Equatorial Water alters oxygen and pH in the Southern California Bight

Abstract

Chemical properties of the California Undercurrent (CU) have been changing over the past several decades, yet the mechanisms responsible for the trend are still not fully understood. We present a survey of temperature, salinity, O2, pH, and currents at intermediate depths (defined here as 50–500 m) in the summer (30 June to 10 July) and winter (8–15 December) of 2012 in the southern region of the Southern California Bight. Observations of temperature, salinity, and currents reveal that local bathymetry and small gyres play an important role in the flow path of the California Undercurrent (CU). Using spiciness (π) as a tracer, we observe a 10% increase of Pacific Equatorial Water (PEW) in the core of the CU during the summer versus the winter. This is associated with an increase in π of 0.2, and a decrease in O2 and pH of 30 μmol kg−1 and 0.022, respectively; the change in pH is driven by increased CO2, while total alkalinity remains unchanged. The high‐π, low‐O2, and low‐pH waters during the summer are not distributed uniformly in the study region. Moreover, mooring observations at the edge of the continental shelf reveal intermittent intrusions of PEW onto the shelf with concomitant decreases in O2 and pH. We estimate that increased advection of PEW in the CU could account for approximately 50% of the observed decrease in O2, and between 49 and 73% of the decrease in pH, over the past three decades.

Evaluating the promise and pitfalls of a potential climate change–tolerant sea urchin fishery in southern California

“…For growth analyses, hydrographic data for the study area were obtained during a multidisciplinary research cruise carried out off the San Diego coast line on board the R/V Melville from 8 to 15 December 2012 (see Nam et al , 2015 for July 2012 data and seasonal results). A single profile of salinity, temperature, pressure, and dissolved oxygen (DO) at 1-m resolution was generated from the surface to 1051 m (32.6901°N, −117.5306°W) using a Sea-Bird Electronics, Inc., CTD instrument (SBE9) and DO sensor (SBE43).…”

“…A single profile of salinity, temperature, pressure, and dissolved oxygen (DO) at 1-m resolution was generated from the surface to 1051 m (32.6901°N, −117.5306°W) using a Sea-Bird Electronics, Inc., CTD instrument (SBE9) and DO sensor (SBE43). Discrete water samples were collected every 50–100 m of water depth and analysed for DO and pH following methods described by Nam et al (2015). In brief, oxygen samples were analysed following standard Winkler titration procedures (Dickson, 1996), and pH samples were analysed spectrophotometrically at 20° C using a custom automated system with m-cresol purple without further purification (Nam et al , 2015).…”

“…Discrete water samples were collected every 50–100 m of water depth and analysed for DO and pH following methods described by Nam et al (2015). In brief, oxygen samples were analysed following standard Winkler titration procedures (Dickson, 1996), and pH samples were analysed spectrophotometrically at 20° C using a custom automated system with m-cresol purple without further purification (Nam et al , 2015). Reported in situ pH was calculated from measured pH and dissolved inorganic carbon in CO2SYS (van Heuven et al , 2011) using dissociation constants from Lueker et al (2000).…”

“…Hydrographic data (depth, DO, temperature, and in situ pH) were measured from a single vertical profile in December 2012 (32.69012° N, −117.53061° W) (Nam et al , 2015). …”

Biodiversity response to natural gradients of multiple stressors on continental margins

“…CTD casts from that margin indicate that inter-seasonal differences in environmental parameters converge at more than 200 m depth [15], which is common on most margins (e.g. [33]). In our dataset, only approximately 7% of stations are above 200 m depth, and thus the impact of seasonal variability is expected to be limited.…”