In a mangrove lagoon on Isabela Islands, Galapagos, there was a pronounced Schlieren effect about 0.5 m from the water surface. The lagoon was strongly stratified with regard to salinity and chlorophyll concentration. The low salinity surface water supported considerable production when incubated for 2 days, indicating that rapid loss of surface water to the sea probably accounted for the low standing crop of chlorophyll.