A free‐floating drifter was designed to directly determine in situ photochemical production rates, photolysis rates, and light fluxes in seawater. This drifter consisted of six trays that were suspended in series from a single buoy line. The trays were constructed so that the attached quartz vessels, containing filtered seawater, were exposed to both downwelling and upwelling irradiation. The quartz vessels were sealed at both...