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Mid-water oceanic depths in coastal upwelling regions often contain oxygen deficient waters, called oxygen minimum zones (OMZs). These hypoxic ( 60 i M O2) conditions can have adverse effects on ecosystems, as many organisms cannot survive in low O2 conditions. This study investigated the OMZ in the topographically isolated Santa Monica Basin (SMB), California, a recipient of high nutrient input. The last survey of this area ~35 years ago, reported a pervious 350 year expansion of the SMB OMZ. In order to assess the OMZ since the last evaluation, sediment cores from 12 stations were retrieved by a multicorer from O2-ventilated (60 M O2) to near-anoxia (~4 M O2) regions along two depth-transects ranging from water depths between 71 and 907 m. The sediment porewater and supernatant water of the cores were analyzed for sulfate (SO42-), nitrate (NO3-), phosphate (PO43-), ammonium (NH4+), total sulfide, dissolved iron (Fe (II)), total alkalinity (TA) and bacterial sulfate reduction. The two deepest stations (907 and 893 meters, ~5 i M O2) exhibited down-core accumulation of NH4+ and TA, while also displaying enhanced rates of sulfate reduction close to the sediment surface; these patterns are all evidence of low oxygen conditions in the overlying water column. Shallower stations upslope (starting at 777 m water depth) featured increasing signs of bioturbation and bioirrigation effects in the geochemical profiles of NH4+, TA, PO43- and Fe (II). Low sulfate reduction rates (areal rates range from 0.13-0.86 mmol m-2 d-1) were detected at all stations. These results were compared with data separate from this thesis, including: 210Pb lamination analyses, the presence and activity of macrofauna at the seafloor, and iron speciation analyses. According to the stations sampled, we could not identify a definite spreading or reduction of the OMZ at the seafloor since the last survey of the SMB was done ~35 years ago.
The biogeochemical cycling of nutrients and oxygen in the coastal ocean is of high importance given that anthropogenic nutrient inputs have doubled the preindustrial nutrient load to water bodies. In particular, excess nitrogen (N) causes eutrophication and hypoxia in the coastal ocean, which is further exacerbated by increased stratification due to global warming. Owing to relatively shallow waters in coastal regions, processes within the sediment and water column play equally important roles in key biogeochemical cycles. To understand the factors that regulate the nutrient and oxygen cycle in seasonally hypoxic coastal basins, this thesis employs high-resolution field observations conducted in Bedford Basin, coupled with numerical modeling, to investigate the impact of various geochemical, biological, and physical drivers. In Chapter 2, field observations of weekly CTD casts and measured benthic oxygen uptake were used to develop and parameterize a numerical model to understand the development of hypoxia in Bedford Basin and quantify different sources and sinks of oxygen in three contrasting years. Chapter 3 is focused on understanding the annual development of the nitrogen cycle based on weekly timeseries of geochemical parameters and phylogenetic marker genes in bottom waters over four consecutive years. Measured geochemical and biological parameters were incorporated into a box model to simulate the nitrification dynamics and identify the controlling factors. Through this approach, a novel mechanism of nitrification was identified whereby strong physical mixing dilutes the resident nitrifier biomass leading to delayed and decoupled nitrification. Weak physical mixing during winter may have the reverse effect. In Chapter 4, benthic biogeochemical processes were studied through seasonal measurements of organic matter remineralization rate, benthic fluxes, sediment geochemical profiles, along with reaction-transport modeling. Overall, this thesis studied the development of hypoxia in a coastal basin, nutrient cycles in the water column and sediment, and the physical and biological drivers of coupled biogeochemical cycles.
Proceedings of the NATO Advanced Research Workshop, held in Yalta, Crimea, Ukraine, 4-8 October 2003
Oceans account for 50% of the anthropogenic CO2 released into the atmosphere. During the past 15 years an international programme, the Joint Global Ocean Flux Study (JGOFS), has been studying the ocean carbon cycle to quantify and model the biological and physical processes whereby CO2 is pumped from the ocean's surface to the depths of the ocean, where it can remain for hundreds of years. This project is one of the largest multi-disciplinary studies of the oceans ever carried out and this book synthesises the results. It covers all aspects of the topic ranging from air-sea exchange with CO2, the role of physical mixing, the uptake of CO2 by marine algae, the fluxes of carbon and nitrogen through the marine food chain to the subsequent export of carbon to the depths of the ocean. Special emphasis is laid on predicting future climatic change.
Biological processes in the oceans play a crucial role in regulating the fluxes of many important elements such as carbon, nitrogen, sulfur, oxygen, phosphorus, and silicon. As we come to the end of the 20th century, oceanographers have increasingly focussed on how these elements are cycled within the ocean, the interdependencies of these cycles, and the effect of the cycle on the composition of the earth's atmosphere and climate. Many techniques and tools have been developed or adapted over the past decade to help in this effort. These include satellite sensors of upper ocean phytoplankton distributions, flow cytometry, molecular biological probes, sophisticated moored and shipboard instrumentation, and vastly increased numerical modeling capabilities. This volume is the result of the 37th Brookhaven Symposium in Biology, in which a wide spectrum of oceanographers, chemists, biologists, and modelers discussed the progress in understanding the role of primary producers in biogeochemical cycles. The symposium is dedicated to Dr. Richard W. Eppley, an intellectual giant in biological oceanography, who inspired a generation of scientists to delve into problems of understanding biogeochemical cycles in the sea. We gratefully acknowledge support from the U.S. Department of Energy, the National Aeronautics and Space Administration, the National Science Foundation, the National Oceanic and Atmospheric Administration, the Electric Power Research Institute, and the Environmental Protection Agency. Special thanks to Claire Lamberti for her help in producing this volume.