Estuaries and Coasts

, Volume 33, Issue 4, pp 971–984

Inorganic and Organic Nitrogen Use by Phytoplankton Along Chesapeake Bay, Measured Using a Flow Cytometric Sorting Approach

  • Paul B. Bradley
  • Michael W. Lomas
  • Deborah A. Bronk

DOI: 10.1007/s12237-009-9252-y

Cite this article as:
Bradley, P.B., Lomas, M.W. & Bronk, D.A. Estuaries and Coasts (2010) 33: 971. doi:10.1007/s12237-009-9252-y


Two different approaches to measuring phytoplankton nitrogen (N) use were compared in late summer 2004 along the main axis of Chesapeake Bay. Uptake of 15N-labeled ammonium and nitrate and dual-labeled (15N and 13C) urea and dissolved free amino acids (DFAA) were measured in surface water samples from upper, mid, and lower bay stations. Two distinct methods were used to assess the relative uptake of N substrates by phytoplankton and correct for bacterial artifacts: (1) traditional filtration using Whatman glass fiber (GF/F) filters and (2) flow cytometric (FCM) sorting of chlorophyll-containing cells. The concentration of dissolved inorganic N (DIN) decreased with distance south along the bay, whereas dissolved organic N (DON) concentrations were relatively constant. Absolute N uptake rates measured using the traditional approach exceeded those of FCM-sorted phytoplankton, thereby suggesting the possibility of bacterial “contamination.” Ammonium was the dominant N form used throughout the transect, although FCM-sorted phytoplankton relied more on urea and DFAA as the ratio of DON/DIN increased toward the bay mouth. Overall, ammonium comprised 74 ± 17%, urea 10 ± 9%, DFAA 9 ± 7%, and nitrate 7 ± 12% of total measured N uptake by phytoplankton. Results suggest that bacteria relied primarily on DFAA and ammonium for N nutrition but also used N from urea at a rate similar to that of phytoplankton, whereas bacterial nitrate uptake was insignificant. On average, phytoplankton uptake of ammonium, urea, and DFAA was overestimated by 61%, 53%, and 135%, respectively, as a result of bacterial retention on GF/F filters.


Nitrogen uptakeFlow cytometryPhytoplanktonBacteriaDissolved organic nitrogenDissolved inorganic nitrogen

Copyright information

© Coastal and Estuarine Research Federation 2010

Authors and Affiliations

  • Paul B. Bradley
    • 3
  • Michael W. Lomas
    • 2
  • Deborah A. Bronk
    • 1
  1. 1.Virginia Institute of Marine Science, The College of William and MaryGloucester PointUSA
  2. 2.Bermuda Institute of Ocean SciencesSt. George’sBermuda
  3. 3.National Oceanic and Atmospheric AdministrationMarylandUSA