DURING an investigation of the marine phytoplankton in a shallow Rhode Island coastal salt pond1, an abundant benthic diatom flora occurred throughout the water column on May 3, 1961. This community was probably roiled up from the bottom during the vertical mixing characteristic of the water-mass at that time (Table 1) ; a distinct halocline persisted prior to this date. The water-mass was 1.5–2.0° C warmer than in mid-April and while the bottom (6 ft.) salinity decreased by 2.5 parts per thousand, the mean water-column salinity decreased by only 1 part per thousand. Similarly, gross changes were not observed in other environmental factors studied : dissolved oxygen, nitrate, nitrite, silicate and phosphate.