Oxygen declines and the shoaling of the hypoxic boundary in the California Current

Abstract

[1] We use hydrographic data from the California Cooperative Oceanic Fisheries Investigations program to explore the spatial and temporal variability of dissolved oxygen (DO) in the southern California Current System (CCS) over the period 1984 -2006. Large declines in DO (up to 2.1 mmol/kg/y) have been observed throughout the domain, with the largest relative DO declines occurring below the thermocline (mean decrease of 21% at 300 m). Linear trends were significant (p < 0.05) at the majority of stations down to 500 m. The hypoxic boundary ($60 mmol/kg) has shoaled by up to 90 m within portions of the southern CCS. The observed trends are consistent with advection of low-DO waters into the region, as well as decreased vertical oxygen transport following near-surface warming and increased stratification. Expansion of the oxygen minimum layer could lead to cascading effects on benthic and pelagic ecosystems, including habitat compression and community reorganization. Citation: Bograd, S. J., C. G.

Under Pressure: Climate Change, Upwelling, and Eastern Boundary Upwelling Ecosystems

“…Here, decreased pH of shelf waters has been associated with strong upwelling at interannual scales (Feely et al, 2008), though longer-term data are generally not available. Over longer periods, decadal-scale changes in ventilation and source-water properties have likely resulted in decreased oxygen concentrations and shoaling of the oxygen minimum layer in the California (Bograd et al, 2008;McClatchie et al, 2010) and Benguela Systems (Monteiro et al, 2008;Salvanes et al, 2015). Such changes, however, have also been associated with known modes of decadal variability in oceanatmosphere processes (Deutsch et al, 2005;Chhak and Di Lorenzo, 2007).…”

Impacts of hypoxia on the structure and processes in pelagic communities (zooplankton, macro-invertebrates and fish)

“…The intensification of hypoxic conditions in the water column is an issue that emerged only recently, according to investigations in different upwelling systems (Bograd et al, 2008;Monteiro et al, 2008;Stramma et al, 2008). The data source for the Benguela system contains many gaps but shows a similar trend (Fig.…”

“…Increasingly hypoxic conditions and the expansion of the OMZ could lead to cascading effects on benthic and pelagic ecosystems, including habitat compression and community reorganization (Bograd et al, 2008). Mesopelagic organisms such as crustaceans and myctophid fishes that live in the upper boundary region of the OMZ or perform diel vertical migrations to feed in the surface layer at night may be affected (Childress and Seibel, 1998), with consequent impacts on their epipelagic prey species.…”

Response of seafloor ecosystems to abrupt global climate change

“…The global ocean inventory of oxygen is predicted to decline between 1% and 7% by the year 2100, and modeling predictions reveal extensive oceanic deoxygenation, on thousand-year timescales, under "business-as-usual" carbon emission scenarios (2). Modern oceanographic time series already document rapid loss of [O 2 ] in interior ocean waters over the last 4 decades (3,4), although this trend is complicated in regions where [O 2 ] demand is decreased through the slackening of trade winds (5). As oxygen levels in the ocean decrease and the already extensive oxygen minimum zones (OMZs) expand, the volumetric habitat for aerobic respiration is reduced, presumably resulting in a fundamental reorganization of marine communities.…”