Iron Speciation and Cycling in the Eastern Tropical North Pacific  

     The Eastern Tropical North Pacific is a unique and relatively under-studied area of the oceans, with interesting features such as a prominent water column suboxic zone which in some locations penetrates into the lower reaches of the euphotic zone.  In the proposed study area off southern Baja, distinct regions of biological and chemical stratification occur in the mixed layer, thermocline, and near-surface suboxic zone.  Preliminary data described in this proposal indicates that the bioavailability of iron may strongly influence the phytoplankton community at some levels in this layered system.  The proposed research will undertake the first exploration of this unique oceanographic region for the purposes of characterizing the concentration and organic speciation of dissolved iron, and the influence exerted by iron availability on the composition of the phytoplankton community.  Complementary mechanistic experiments will also be carried out on seawater and particulate samples obtained in the region to study processes related to the photochemical reactivity and production of iron-binding ligands in the marine environment.   

     Trace metal sampling and on-deck incubation experiments will be carried out on two previously funded cruises off southern Baja and Manzanillo/Acapulco in 2003.  The competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE/ACSV) method of Wu and Luther 1995 and Witter and Luther 1998 will be employed to determine the concentration and speciation of dissolved iron.  Incubation experiments will involve trace metal clean techniques and addition of iron and major nutrients to assess the response of the phytoplankton community at different levels in the euphotic zone to iron addition.  Mechanistic experiments will employ CLE/ACSV techniques to study the photochemical destruction of natural ligands, as well as production of ligands from naturally-occuring particulate microenvironments in the water column, such as the fecal pellets of macrozooplankton grazers.  For these latter studies, structural and chromatographic methods (HPLC, LC-MS, UV/Vis spectrophotometry) will also be employed to determine the nature of the Fe-binding ligands produced.  It is anticipated that this field and mechanistic study of iron speciation and cycling in the unique environment of the Eastern Tropical North Pacific will contribute to our general understanding of the upper ocean biogeochemistry of iron by elucidating the interactive influences between iron availabilty and planktonic community composition in this region, and addressing current questions regarding the sources and sinks of iron binding ligands in seawater.  These studies will provide a key link between field measurements of iron speciation and laboratory studies of iron cycling.

     This proposal will advance discovery, promote learning, and enhance infrastructure for scientific research and education in several ways.  Support is requested for graduate students to participate in the proposed research and to attend a national meeting.  In addition, an REU Supplement is requested which is designed to establish a collaboration with the Southampton College of Long Island University, a non-Ph.D. granting institution, by funding a research internship for an undergraduate student from Southampton College to work at Scripps Institution of Oceanography on the proposed project.