Phytoplankton groups use different forms of nitrogen (N) and show distinct responses to physical drivers. This study aimed to resolve the N cycling roles of phytoplankton size classes and major taxa in a tidally influenced salt marsh. We used a whole ecosystem 15NO 3– tracer experiment to determine interactions among phytoplankton community structure, N biogeochemistry, and hydrodynamics over an 11-day mid/neap tidal cycle in a salt marsh creek, Plum Island, Massachusetts. During the study period (10 to 20 July, 2016) the creek experienced marsh platform flooding and non-flooding tides. Phytoplankton biomass and δ 15N were quantified for three size classes corresponding to microphytoplankton (microP, 20–200 µm), nanophytoplankton (nanoP, 3–20 µm), and picophytoplankton (picoP, < 3 µm). All three size classes showed minimal direct assimilation of the 15NO3– tracer; however, each size class also used distinct N sources. MicroP appeared to use internal N stores assimilated outside of the estuary throughout the experiment, despite a shift in taxonomic community composition from mid- (diatoms) to neap-cycle (dinoflagellates) as quantified by microscopy. NanoP appeared to switch from internal N stores to recycled 15NH 4+ over the mid/neap cycle, exhibiting maximal biomass and 15N use during the neap tide and flood tides therein. PicoP consistently used recycled N that was largely not derived from the 15N tracer, and they appeared to use different recycled sources during flood versus ebb tide. Our results suggest a dominant role of smaller phytoplankton size classes (picoP and nanoP) in salt marsh N cycling, and possibly increased N transfer through the microbial food web during neap tide. This study also demonstrates changing interactions among phytoplankton communities, physics, and N biogeochemistry over tidal cycles, showing that to understand estuarine planktonic N processing and growth, highly temporally resolved nutrient and physical conditions both within and outside of the estuary need to be considered.
|Advisor:||Stauffer, Beth A.|
|Commitee:||Hester, Mark W., Nelson, James A.|
|School:||University of Louisiana at Lafayette|
|School Location:||United States -- Louisiana|
|Source:||MAI 58/05M(E), Masters Abstracts International|
|Keywords:||Nitrogen source, Phytoplankton, Salt marsh, Size-specific uptake, Stable isotope, Tidal cycle|
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