Resolving Estuarine Nitrogen Use by Phytoplankton Communities Using a Whole Ecosystem Tracer Approach

The use of nutrients by diverse phytoplankton communities in estuarine systems, and their response to changes in physical and biogeochemical processes in these natural systems, is a significant ongoing area of research. We used a whole ecosystem 15 NO 3 − tracer experiment to determine the uptake of different nitrogen (N) forms in phytoplankton functional groups over a mid- to neap tidal cycle in a salt marsh creek in Plum Island Estuary, Massachusetts, USA. We quantified the biomass and δ 15 N for three groups corresponding to micro- (20–200 μm; microP), nano- (3–20 μm; nanoP), and picophytoplankton (< 3 μm; picoP). All three size classes showed distinct use of recycled N sources throughout the 11-day sampling period and minimal direct assimilation of the 15 NO 3 − tracer. MicroP consistently used high amounts of creek-derived 15 NH 4 + , even with a shift at neap tide from diatom- to dinoflagellate-dominated communities (including members of the harmful genus Alexandrium ). NanoP use of recycled 15 NH 4 + increased over the mid-neap tidal cycle, while picoP use decreased. Both biomass and NH 4 + use (as highest δ 15 N values) of all size groups were maximized during neap tide. This study demonstrates partitioning of recycled N use among size-based phytoplankton groups in the estuary, with distinct effects of tidal cycle on the nutrient uptake of each group, and with important implications for the roles of diverse phytoplankton communities in estuarine nutrient cycling.