Quantifying microzooplankton communities along open ocean salinity gradients

Using FlowCam microscopy and particle analysis, we characterized microzooplankton and their prey items across the ocean salinity gradients of two large river plumes: The Amazon River in the Western Tropical North Atlantic, and the Mississippi River in the Northern Gulf of Mexico. We quantified and identified these organisms with a focus on grazer populations at various depths ranging from plume to open ocean ecosystems. Samples were taken at a total of ten stations utilizing niskin bottles attached to a conductivity/temperature/depth (CTD) rosette in addition to near surface samples collected from the flowthrough sampling system at set intervals during transit. Samples were also taken from nutrient amendment experiments carried out on surface water samples. Microplankton communities were observed in both environmental samples and nutrient amendment experiments. Among the more notable grazers observed across the salinity gradient were various genera of Cyclopoida and Harpacticoida. Grazers were likely responding to high abundance of phytoplankton genera including Trichodesmium, Rhizosolenia, and Skeletonema. Overall, grazer communities were most dense in the surface plume waters and at lower salinities.