Two methods to alleviate the problem of fouling of moored flow tube optical instruments are presented. A chemical method diffuses a concentrated solution of bromine into the flow tube between sampling periods, creating a toxic environment for microorganisms. An optical method removes a baseline value from the red peak of chlorophyll a. Three spectral absorption meters equipped with the chemical system were deployed in the southeastern Bering Sea from March to September 1993. For a 40-m instrument the system prevented biofouling for the entire deployment, while an 11-m instrument was free of contamination for approximately 3.5 months. Reasonable estimates of in situ chlorophyll a were obtained from all three instruments by the subtraction of the baseline.¿ 1997 American Geophysical Union