Nitrogen dynamics in water column and sediments, nutrient-organism interactions in coastal ecosystems, relationships between nitrogen inputs and oxygen removal in coastal environments such as the “Dead Zone,” Dissimilatory nitrate reduction to ammonium (DNRA) as a mechanism to retain available N in coastal regions.

Research Interests

Nitrogen cycling is important in coastal and estuarine ecosystems because nitrogen is transformed to a variety of forms, by autotrophic and heterotrophic processes in the water and sediments, and is a nutrient that often limits the growth of phytoplankton and other plants in these relatively shallow systems. Internal regeneration is an important process providing nitrogen to plants whereas denitrification causes available nitrogen to be lost from estuarine and coastal ecosystems. Major goals are to understand and quantify the biotic mechanisms and rates of nitrogen remineralization and the fate of nitrogen compounds in the water and at the sediment-water interface. This information is needed to understand nitrogen biogeochemistry and to quantify the relative importance of internal nitrogen regeneration processes compared to inputs from other sources in these productive ecosystems. Nitrogen regeneration and transformation rates in the water and at the sediment-water interface are measured with 15-N as an isotopic tracer. Ammonium concentrations and isotope ratios are measured by high performance liquid chromatography. Denitrification rates are determined using a Membrane Inlet Mass Spectrometer. Nitrogen cycling studies have been conducted in Texas coastal ecosystems, Florida Bay, the Mississippi River Dead Zone, The Great Lakes, Lake Taihu, China, and other shallow coastal environments and wetlands.

Selected Publications

Dagg, M.J, J.W. Ammerman, R.M.W. Amon, W.S. Gardner, R.E. Green, and S.E. Lohrenz.  2007.  Water column processes influencing hypoxia in the northern Gulf of Mexico: A synthesis.  Estuaries and Coasts 30:735-752.

McCarthy, M.J., P.L. Lavrentyev, L. Yang, L. Zhang, Y. Chen, B. Qin, and W.S. Gardner.  2007.   Nitrogen dynamics relative to microbial food web structure in a subtropical, shallow, well-mixed, eutrophic lake (Taihu Lake, China).  Hydrobiologia (Special issue on Lake Taihu) 581: 195-207.

Gardner, W.S., M.J McCarthy, S. An, D. Sobolev, K.S. Sell, and D. Brock.  2006.  Nitrogen fixation and dissimilatory nitrate reduction to ammonium (DNRA) support nitrogen dynamics in Texas estuaries.   Limnol. Oceanogr. (Special issue on Eutrophication) 51:558-568.

Jochem, F.J., M.J. McCarthy, and W.S. Gardner.  2004.  Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000.  J. Plankton Res. 1265-1275.

Gardner, W.S., P.J. Lavrentyev, J.F. Cavaletto, M.J. McCarthy, B.J. Eadie, T.H. Johengen, and J.B. Cotner. 2004.  The distribution and dynamics of nitrogen and microbial plankton in southern Lake Michigan during spring transition 1999-2000.  J. Geophys. Res. (Electronic publication).

An, S. and W.S. Gardner.  2002.  Dissimilatory nitrate reduction to ammonium (DNRA) as a nitrogen link, versus denitrification as a sink in a shallow estuary (Laguna Madre/Baffin Bay, Texas).  Mar. Ecol. Prog. Ser. 237:41-50.

An, S., W.S. Gardner, and T. Kana.  2001.  Simultaneous measurement of denitrification and nitrogen fixation using isotope pairing with membrane inlet mass spectrometry analysis.  Appl. Environ. Microbiol.  67:1171-1178.

Gardner, W.S., L. Yang, J.B. Cotner, T.H. Johengen, and P.J. Lavrentyev. 2001. Nitrogen dynamics in sandy freshwater sediments (Saginaw Bay, Lake Huron). J. Great Lakes Res. 27:84-97.

Gardner, W.S., J.F. Cavaletto, H.A. Bootsma, P.J. Lavrentyev, and F. Troncone.  1998.  Nitrogen cycling rates and light effects in tropical Lake Maracaibo, Venezuela. Limnol. Oceanogr. 43(8):1814-1825.

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