Severe hypoxia, which has resulted in mass mortality of fish and invertebrates, has been observed within the California Current system. Occurring in an upwelling system, hypoxia in the California coast reflects conditions in the ocean systems that deliver oxygen-poor water onto continental shelves: Hypoxia may represent climatic variability (e.g. wind stress anomalies), shifts in ocean circulation and marine ecological change. As typical to the semi-enclosed seas and estuaries, anthropogenic nutrient loading has increased hypoxia in the Alamitos Bay.