Element Stoichiometry, New Production and Nitrogen Fixation

Introduction Over the decade and a half since planning for the Joint Global Ocean Flux Study (JGOFS) began, a number of shifts, both subtle and profound, have occurred in certain paradigms of biological and chemical oceanography. Nowhere have greater changes taken place than in the way we view the stoichiometry of elements in the ocean and the processes that influence these patterns. We started this era with a conception of new production that focused on nitrate, linked to other elements in a simple, generally stable ratio in the ocean. We now know that elemental ratios vary more than we thought. We also know far more about the importance of iron as a limiting nutrient and its effects on elemental stoichiometry. Assumptions about systems in steady state have given way to the recognition that nothing is constant except change. Relationships between nutrient fluxes and climate conditions over a broad spectrum of time scales have become apparent. These changes in our perceptions accord biological processes a more significant role in the exchange of carbon between oceanic and atmospheric pools and open the door to the evaluation of possible options for the active sequestration of carbon dioxide (CO2). They also provide a springboard for research in the post-JGOFS era that can elucidate these processes and predict their role in the future behavior of ocean ecosystems and elemental cycles. Marine nitrogen and phosphorus cycles are inextricably linked to the carbon cycle in the ocean. Pioneering research efforts by Hildebrand W. Harvey, L.H. N. Cooper and Alfred C. Redfield established a robust quanttafive ratio of carbon to nitrogen to phosphorus (C:N:P) in the cells of phytoplankton and particulate matter in the laboratory and in both surface and deep ocean waters. It became known as the "Redfield Ratio," specifying 106 moles C to 16 moles N to 1 mole P. The observed biochemical, molecular and enzymatic similarities of most planktonic organisms led Redfield and his colleagues to a unifying theory that, while it has greatly simplified efforts to model the complex dynamics of nutrient fluxes in the sea, may have inadvertently stifled the examination and explanation of non-conforming evidence. The Geochemical Ocean Sections (GEOSECS) field program in the 1970s produced the first high-quality global data on nutrient and transient tracer distributions. Analysis of these data sets confirmed the global distribution of relatively constant nutrient ratios. Although absolute ratios often varied from the canonical Redfield values, changes in nutrient levels along isopycnal or neutral density surfaces in the deep sea usually retained ratios near those specified by Redfield (Anderson and Sarmiento, 1994). Because of the apparent consistency of the Redfleld ratio throughout the ocean, knowledge of one element allowed investigators to calculate the concentration of other elements, and by inference, the processes that controlled them. Another paradigm that governed ways of thinking about nutrient fluxes when JGOFS began was the "new production hypothesis. " Introduced by Dugdale and Goering (1967), this formulation distinguished between the recycling of a limiting nutrient within a system and its introduction from outside the system. For nitrogen in the euphotic zone, this approach was used to distinguish between the recycling of ammonium and the uptake of nitrate, introduced through upwelling from deep waters (Figure la). Eppley and Peterson (1979) extended this conceptual analysis to show the balance between imports and exports in a steady-state ocean and to begin to identify regional patterns in the pro-