A mixed group of 441 marine invertebrates was collected in Southern California. After a large mortality event, numerous water quality parameters were evaluated. Copper was present at 33 μg/L, which is below the documented toxic level. No other toxins were identified. To investigate whether copper was the etiology for the mass mortality, purple sea urchins, Strongylocentrotus purpuratus, were used as sentinel species, as they were the most severely affected during the mortality event. Purple sea urchins were placed in multiple test systems of varying copper concentrations and died in periods of time proportionate to copper concentrations. Clinical signs, disease progression, and pathologic lesions were similar between test systems and the original mortality event. Copper caused disease and death in purple sea urchins at concentrations from 15 to 50 μg/L. The source of the copper toxicity was identified as sand filters contaminated by brass pump components.