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HPLC and Stable Isotopic Measurements of Nitrogen Uptake by PhytoplanktonEPA Grant Number: U916135
Title: HPLC and Stable Isotopic Measurements of Nitrogen Uptake by Phytoplankton
Investigators: York, Joanna K.
Institution: Boston University
EPA Project Officer: Cobbs-Green, Gladys M.
Project Period: January 1, 2003 through January 1, 2006
Project Amount: $86,572
RFA: STAR Graduate Fellowships (2003) Recipients Lists
Research Category: Academic Fellowships , Aquatic Ecosystems , Fellowship - Aquatic Ecology and Ecosystems
The objectives of this research project are to: (1) refine our knowledge about forms and sources of nitrogen (N) assimilated by phytoplankton; (2) use isotopic signatures to define sources, losses, and transformations of nitrate (NO3) and ammonium (NH4) available to phytoplankton; and (3) provide new data on the fate of N as it travels through estuaries.
This work will be conducted in the Waquoit Bay, MA estuarine system, whose subestuaries have watersheds that vary in their degree of development, from Childs River's urbanized watershed to Sage Lot Pond's, which is primarily forested. These differences in land use result in differences in total N load to the estuary, differences in the relative amounts of NO3 and NH4 present in the estuarine water, and differences in the isotopic signatures of NO3 and NH4. Waquoit Bay can be considered as a regional experimental layout that will allow me to assess the relative importance of NO3 and NH4 in sites where their relative ratio varies more than an order of magnitude. I will apply newly developed techniques that combine stable isotopic and high performance liquid chromatography approaches to address the issue of preferential uptake of N forms by phytoplankton in the field. Previous work on this subject was based on experiments with laboratory cultures; I will use natural phytoplankton assemblages. To determine whether estuarine phytoplankton use NH4 or NO3, I will compare isotopic signatures (15N) of phytoplankton with 15N of NH4 and NO3. The 15N of NH4 and NO3 will vary down estuary, affected not only by uptake by phytoplankton, but also by mixing at tidal sites, denitrification, regeneration, and other biogeochemical processes. Because of this mix of influences, I will assess the uptake by phytoplankton within the context of denitrification and regeneration, the major processes contributing to the change in isotopic signature. To determine the importance of regenerated NH4, I will collect and incubate sediment cores. Denitrification will be assessed by the relative fractionation to heavier signatures in samples collected along a salinity gradient in all sampling locations.