A prototype instrument array known as REOS1, for remote electrooptical sensor, provided automated surveillance of a nuisance algal population in an open drinking-water reservoir. The high-resolution data base provided by moored sensors enabled reservoir managers to detect algal blooms before water quality was aesthetically compromised. A remote data link also provided immediate feedback on the efficacy of algicide treatments in the reservoir. A simple decision matrix included in this article shows how REOS data can be used to diagnose the biological status of an open reservoir and to guide daily operations. Includes 5 references, table, figures.