Sanctuary Integrated Monitoring Network
Monitoring Project

Nearshore water monitoring of oxygen and pH in southern Monterey Bay

Principal Investigator(s)

  • J. Ashley Booth
    University of California, Los Angeles
  • Curtis Deutsch
    University of California, Los Angeles
  • Brock Woodson
    Stanford University
Start Date: April 01, 2000
End Date: June 01, 2011

A decade-long time series recorded in southern Monterey Bay, California demonstrates that the shallow, near-shore environment (17 m depth) is regularly inundated with pulses of cold, hypoxic and low pH water. During these episodes, oxygen can drop to biologically threatening levels, and pH levels were lower than expected. Weekly water chemistry monitoring revealed that the saturation state of aragonite (the more soluble form of calcium carbonate) was often below saturation and had a moderate positive relationship with pH, however, analytical and human error could be high.

Pulses of hypoxia and low pH water with the greatest intensity arise at the onset of the spring upwelling season, and fluctuations are strongly semidurnal (tidal) and diurnal. Arrival of cold, hypoxic water on the inner shelf typically occurs 3 days after the arrival of a strong upwelling event and appears to be driven by upwelling modulated by internal tidal fluctuations. I found no relationship between the timing of low-oxygen events and the diel solar cycle nor with terrestrial nutrient input. These observations are consistent with advection of hypoxic water from the deep, offshore environment where water masses experience a general decline of temperature, oxygen and pH with depth, and inconsistent with biochemical forcing.

Comparisons with concurrent temperature and oxygen time series taken ~20 km away at the head of the Monterey Canyon show similar patterns but even more intense hypoxic events due to stronger semidiurnal forcing there. Analysis of the durations of exposure to low oxygen levels establishes a framework for assessing the ecological relevance of these events. Increasing oceanic hypoxia and acidification of both surface and deep waters may increase the number, intensity, duration and spatial extent of future intrusions along the Pacific coast. Evaluation of the resiliency of nearshore ecosystems such as kelp forests, rocky reefs and sandy habitats, will require consideration of these events.

To access the entire Master's Thesis, go to the link below.

Summary to Date

Continuous seawater measurements were recorded at two sites in Monterey Bay, one on the inner shelf at the Monterey Bay Aquarium (36.621°N, 121.899°W) and the other at the head of the Monterey Canyon at the Moss Landing Marine Laboratories (36.8025°N, -121.7915°W). Neither of the opportunistic data sets were not collected for scientific monitoring purposes, but rather were derived from water quality programs associated with animal husbandry.

The Moss Landing Marine Laboratories (MLML) has monitored temperature and
DO from their seawater intake pipe, located at 20 m depth ~300 m offshore of the MLML
shore lab. Since December 19, 2002, temperature and DO were recorded as part of an ‘industrial’ monitoring system every 5-munutes.

Monitoring Trends

  • The long duration of these water-monitoring programs made it possible to look at long-period oscillations, but the sampling period was not long enough to detect any trends on a decadal scale that might be related to climate change (Breaker and Ruzmaikin 2010).


For results see: Booth JAT. 2011. Hypoxic and low pH water in the nearshore marine environments of Monterey Bay, California: Characterizing a decade of oxygen and pH, and drivers of variability. Masters Thesis. Moss Landing Marine Laboratories, California State Universities. p. 124 Access it using the link below.

Study Parameters

  • carbonate chemistry
  • Dissolved oxygen
  • pH
  • Nitrites
  • Nitrates
  • Conductivity
  • Salinity
  • Temperature

Study Methods

See: Booth JAT. 2011. Hypoxic and low pH water in the nearshore marine environments of Monterey Bay, California: Characterizing a decade of oxygen and pH, and drivers of variability. Masters Thesis. Moss Landing Marine Laboratories, California State Universities. p. 124

At the Monterey Bay Aquarium, seawater from the adjacent kelp bed is drawn in through one of two intake pipes, the mouths of which are protected by coarse mesh screen. The intake lies at 17 meters depth, approximately 340 m offshore from the aquarium pump house and main facilities. Flow-through seawater measurements include temperature, dissolved oxygen (DO) and pH. Temperature has been recorded since September 21, 1995 by an AGM thermocouple at the pump house. Dissolved oxygen, conductivity, pH and carbonate chemistry were measured from the same water at a site inside the aquarium main facility. Dissolved oxygen has been recorded since April 1, 2000 with a Point Four OxyGuard Stationary Probe (Type 1). On September 24, 2009 a GLI Encapsulated LCP (Liquid Crystal Polymer) Differential pH Sensor with Internal Preamplifier and Glass Electrode was installed and calibrated using Fisher Scientific NBS buffers.

Discrete spot-check measurements of temperature, DO, and pH were taken once or twice per week since April 14, 1998, May 21, 1999, and June 18, 1996 respectively and have continued until the present (Table 1, Figure 11). Measurements were conducted using the outflow water of the flow-through monitoring program using a Hach IntelliCAL dissolved oxygen probe (optical) and liquid filled pH electrode (Figure 5). These data were used by MBA staff to assess the accuracy of the flow-through instruments. I used these data to serve as a quality control for the flow-through time series and to look at long-term patterns in pH. Temperature spot checks were taken inside the aquarium where the DO and pH sensors were located and were on average 1.33°C higher than the values from the thermocouple in the pump house. This warming was attributed to pipe friction.

On 27 May 2010, carbonate chemistry measurements (total carbon dioxide (TCO2) total alkalinity (TA), and carbonate (CO32-)) were added to the weekly spot check at MBA. Water was collected in airtight plastic bottles inside the aquarium where the DO and pH sensors are located (Figure 5). Samples were analyzed by the Gran titration method using a Metrohm 809 Titrando using Tiamo Titration Software. The probe on the titrator was calibrated monthly.

Figures and Images

Figure 1. Schematic of seawater intake plumbing and monitoring at the Monterey Bay Aquarium in Pacific Grove, California. Flow-through measurements record every 5 minutes and spot-check measurements were conducted weekly.

Figure 2. Schematic of cold, hypoxic (low oxygen) and low pH water transport into nearshore. Right panel illustrates the winter, non-upwelling season when hypoxic water is deep and the mixed layer is deep. The water advected inshore by internal tidal motions is similar to surface waters. The left panel shows upwelling conditions where deep, cold, hypoxic water is shallow enough for the internal tide to transport it inshore.

Figure 3. Schematic of average low DO/ low pH event in the nearshore of southern Monterey Bay (Monterey Bay Aquarium) over a 24 hour period at ~17 m depth. Center plot shows the DO/pH/temperature signal fluctuating with the semidiurnal internal tide. Additional axes indicated different oxygen units and the associated pH and temperature ranges. Far right labels show origin of the water sampled.