Rucheng Tian

Effects of pelagic food-web interactions and nutrient remineralization on the biogeochemical cycling of carbon: a modeling approach

Abstract The operation of the oceans biological CO2 pump depends on both the structure of the pelagic food web and remineralization processes in the water column. We have developed a novel pelagic ecosystem model to study the effects on carbon export of food-web interactions in the euphotic zone and remineralization processes over the entire water column. The one-dimensional model consists of 10 state variables that span the herbivorous and microbial food webs. It is forced by solar radiation, vertical mixing, and the nitrate concentration in deep water. According to the model, adjusted against a CJGOFS data set, up to 52% of the nitrate-based phytoplankton production is processed by the microbial food web before being exported from the euphotic zone. Remineralization of dissolved organic carbon and suspended particles in the water column is a key control on carbon export, and up to 77% of the total material exported from the euphotic zone is remineralized in a layer located between the bottom of the euphotic zone and the annual maximum depth of the surface mixed layer. Nitrification of ammonia released within this layer satisfies most of the biological demand for nitrate in the euphotic zone. This places limitations on the use of new production as usually determined at sea (i.e. based on the uptake of nitrate) to estimate carbon export towards the deep.

Iodine speciation: a potential indicator to evaluate new production versus regenerated production

Abstract Vertical profiles of iodide, iodate and total free iodine were determined monthly for one year (from July 1993 to June 1994) at the DYFAMED permanent station located in the northwestern Mediterranean Sea. Dissolved iodate and iodide were directly determined by differential pulse polarography and cathodic stripping square wave voltammetry, respectively. Iodate is the predominant species, ranging from 390 nM in surface waters to 485 nM in deep waters. Iodide is present in significant concentrations up to 80 nM in surface waters and from undetectable levels to several nanomolar (

Adaptive Coupled Physical and Biogeochemical Ocean Predictions: A Conceptual Basis

Physical and biogeochemical ocean dynamics can be intermittent and highly variable, and involve interactions on multiple scales. In general, the oceanic fields, processes and interactions that matter thus vary in time and space. For efficient forecasting, the structures and parameters of models must evolve and respond dynamically to new data injected into the executing prediction system. The conceptual basis of this adaptive modeling and corresponding computational scheme is the subject of this presentation. Specifically, we discuss the process of adaptive modeling for coupled physical and biogeochemical ocean models. The adaptivity is introduced within an interdisciplinary prediction system. Model-data misfits and data assimilation schemes are used to provide feedback from measurements to applications and modify the runtime behavior of the prediction system. Illustrative examples in Massachusetts Bay and Monterey Bay are presented to highlight ongoing progress.

Impact of high-frequency nonlinear internal waves on plankton dynamics in Massachusetts Bay

Author Posting.

Simulation and prediction of anthropogenic lead perturbation in the Mediterranean Sea

Abstract Simulations of Pb biogeochemical cycles were carried out based on a box model previously developed and tested by in-situ measurements. It has been demonstrated that the different evolutions of the fluxes determining the Pb budget lead to disequilibrium in Pb biogeochemical cycles in the Mediterranean Sea, particularly the anthropogenic perturbation governed by economical development and political regulations. Such a disequilibrium resulted in an increase in the concentration of lead between 1950 and 1976. Pb concentrations have decreased since 1976 in the Western Basin thanks to the application of limited Pb consumption regulations in the western countries. However Pb concentrations will increase again and the critical concentration for marine organisms will be reached by the middle of the next century if Pb consumption is not limited in the eastern countries. In relation to the exchange with the North Atlantic, the Mediterranean Sea acted as a sink for Pb before 1980, but since 1981, has become a source. Concerning the accumulation in the sediments, about half of the total external input was estimated to have been deposited between 1950 and 1992.

Sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a deep-water formation region

[1] We used a physical-biogeochemical model to examine the sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a subpolar, deep-water formation region. Documented changes in winter mixed layer depth between the late 1960s and the mid-1990s were used to construct scenarios of weak, moderate, and strong winter convection that drive the biogeochemical model. The model simulations suggest that the total biogenic carbon export (particle sinking flux + DOC export) is higher under strong winter convection (e.g., during the early 1990s) than under weak winter convection (e.g., during the late 1960s), by � 70% axcross the 200-m isobath and nearly double at 500 m and 1000 m depth. These large variations in total biogenic carbon export are essentially due to the response of DOC export to ocean climate conditions. Sensitivity analyses indicate that the variations in DOC export from the euphotic zone are due to the impact of the convection regime on the development of the microbial food web and on the bacterial consumption of DOC in surface waters. Although DOC downward fluxes within the mesopelagic zone (below � 500 m) are largely controlled by physical processes, the effect of convection on microbial dynamics can potentially amplify the year-to-year variations in the transport of DOC to the deep ocean due to convection. INDEX TERMS: 4215 Oceanography: General: Climate and interannual variability (3309); 4255 Oceanography: General: Numerical modeling; 4806 Oceanography: Biological and Chemical: Carbon cycling; 4840 Oceanography: Biological and Chemical: Microbiology; KEYWORDS: sensitivity analyses, carbon export, ocean climate, microbial food web, deep-water formation, Labrador Sea