Natural intrusions of hypoxic, low pH water into nearshore marine environments on the California coast

A decade-long time series recorded in Central California demonstrates that a 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 physiologically stressful levels, and pH can reach values that potentially result in dissolution of calcium carbonate. Pulses of the greatest intensity arose at the onset of the spring upwelling season, and fluctuations were strongly semidiurnal and diurnal. Arrival of cold, hypoxic water on the inner shelf appears to be driven by tidal-frequency internal waves pushing deep, upwelled water into nearshore habitats. We found no relationship between the timing of low-oxygen events and the diel solar cycle. These observations are consistent with the interpretation that hypoxic water is advected shoreward from the deep, offshore environment where water masses experience a general decline of temperature, oxygen and pH with depth. Analysis of the durations of exposure to low oxygen concentrations establishes a framework for assessing the ecological relevance of these events, but physiological tolerance limits to such hypoxic events are not well documented for most near-shore organisms expected to be impacted.