Marine nutrients like dissolved nitrate or silica are the focus of our research. Our main emphasis concerns inorganic nutrients in upper ocean waters, where most oceanic photosynthesis occurs even though nutrient concentrations are often very low. The challenge is to detect changes in those concentrations as precisely as possible and then try to find explanations for the changes. We conduct detailed surface surveys with highly sensitive nutrient measurements in patches of coastal surface waters labeled with tracers. Nutrient concentrations in these waters are usually well down into the nanomolar range. Principal nutrients under investigation are nitrate, nitrite, and ammonium ion. Initial findings are that nitrate and nitrite in these coastal surface waters show only slight variations from the averages over an annual cycle, but that, in sharp contrast, ammonium ion concentrations can vary up to many-fold times higher than what appears to be normal background ammonium levels. We are currently investigating likely causes for this vastly different behavior of ammonium ion relative to the other main nitrogen-bearing inorganic nutrients. A major part of the fieldwork is to develop a version of our high-sensitivity nutrient sensor that will function in an autonomous underwater vehicle. It will allow us to determine the lateral shapes of patches of coastal water with ammonium enrichments, among other things. The AUV will greatly help the search for explanations. The other part of our research concerns comparisons of and temporal trends in nutrient concentrations within permanently anoxic ocean waters. Multi-year trends are being measured in the Cariaco Basin along the Venezuelan continental margin, and those results are compared to nutrient data from the Black Sea and other anoxic regions in the ocean. The objective of this work is to understand the pathways by which anoxia can alter chemical processes in the sea.
Articles
Si Cycle in the Cariaco Basin, Venezuela: Seasonal Variability in Silicate Availability and the Si : C : N Composition of Sinking Particles (with Robert C. Thunell, Claudia Benitez-Nelson, Frank E. Muller-Karger, Laura Lorenzoni, Mary Scranton, Ramon Varela, and Yrene Astor), Global Biogeochemical Cycles (2008)
A 9-year time series of water column and sediment trap measurements was used to examine...
Red Tides in the Gulf of Mexico: Where, When, and Why? (with John J. Walsh, J. K. Jolliff, B. P. Darrow, J. M. Lenes, S. P. Milroy, A. Remsen, D. A. Dieterle, Kendall L. Carder, F. R. Chen, Gabriel A. Vargo, Robert H. Weisberg, Frank E. Muller-Karger, Eugene Shinn, K. A. Steidinger, Cynthia A. Heil, C. R. Tomas, J. S. Prospero, T. N. Lee, G. J. Kirkpatrick, T. E. Whitledge, D. A. Stockwell, T. A. Villareal, A. E. Jochens, and P. S. Bontempi), Journal of Geophysical Research - Oceans (2006)
Independent data from the Gulf of Mexico are used to develop and test the hypothesis...
Phytoplankton Response to Intrusions of Slope Water on the West Florida Shelf: Models and Observations (with John J. Walsh, Robert H. Weisberg, D. A. Dieterle, R. Y. He, B. P. Darrow, J. K. Jolliff, K. M. Lester, Gabriel A. Vargo, G. J. Kirkpatrick, T. T. Sutton, A. E. Jochens, D. C. Biggs, B. Nababan, Chuanmin Hu, and Frank E. Muller-Karger), Journal of Geophysical Research - Oceans (2003)
Previous hypotheses had suggested that upwelled intrusions of nutrient-rich Gulf of Mexico slope water onto...
Simulation of Carbon-Nitrogen Cycling During Spring Upwelling in the Cariaco Basin (with John J. Walsh, D. A. Dieterle, Frank E. Muller-Karger, R. Bohrer, W. P. Bissett, R. J. Varela, R. Aparicio, R. Diaz, R. Thunell, G. T. Taylor, M. I. Scranton, and E. T. Peltzer), Journal of Geophysical Research - Oceans (1999)
Coupled biological-physical models of carbon-nitrogen cycling by phytoplankton, zooplankton, and bacteria assess the impacts of...
Nutrient Provinces in the Sea: Concentration Ratios, Reaction Rate Ratios, and Ideal Covariation, Journal of Geophysical Research - Oceans (1992)
Global distributions of the ratios of the concentrations of nitrate + nitrite (= [N]) and...