Browsing by Subject "Carbon cycle"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    High-resolution electrospray ionization mass spectrometry for molecular level characterization of dissolved natural organic matter in the Lake Superior watershed.
    (2010-08) Steinbring, Carla Jean
    Recent studies have begun to explore the molecular-level link between terrestrial and aquatic dissolved organic matter (DOM) in rivers and estuaries and their receiving oceans or lakes. This is of interest because of DOM‟s roles in carbon, nitrogen and phosphorus cycles and its reactivity with trace metals and anthropogenic organic molecules. These recent studies, primarily in brackish or salt-water systems, have shown that allochthonous components of DOM contain more aromatic compounds, while autochthonous components contain more aliphatic compounds. Here we extend these techniques to a temperate oligotrophic large lake (Lake Superior). Samples from the lake and watershed, including swamp, creek, river, near-shore lake, and offshore lake sites are compared using Fourier transform ion cyclotron resonance mass spectroscopy. In addition, replicate analyses on the instrument allow us to study reproducibility of the instrument method. Results are analyzed using cluster analysis, non-metric multidimensional scaling, Van Krevelen diagrams, and carbon versus mass diagrams. We find interesting similarities and differences between sites based on hydrological proximity of sites, storm events, and terrestrial impact.
  • Thumbnail Image
    Item
    The integral role of phytoplankton stoichiometry in ocean biogeochemical dynamics
    (2019-11) Tanioka, Tatsuro
    Photosynthesis by ocean algae (phytoplankton) contributes roughly half of the earth's net carbon production. Organic matter produced using carbon dioxide in the atmosphere not only supports marine food webs, but also acts as a climate stabilizer, because carbon is subsequently transported to the deep ocean and stored there for thousands of years. Attempts to model global marine biological production and its impacts on global biogeochemical cycles often assume a constant elemental stoichiometry of carbon, nitrogen, and phosphorus in phytoplankton biomass. This ratio, known as the Redfield ratio, was determined on the basis of an analysis of many samples of marine plankton collected over 70 years ago. This notion is well established in the oceanographic community but there is no clear physiological justification for why the C:N:P ratios in phytoplankton should strictly follow the Redfield ratio. Many recent studies revealed that C:N:P ratio of particulate organic matter can deviate significantly from the Redfield Ratio with some noticeable spatial and temporal variability. Studies suggest that factors such as nutrient availability, light, and temperature play a crucial role in modifying C:N:P ratio of phytoplankton. In this dissertation, I investigate the roles of marine phytoplankton stoichiometry in the global marine biogeochemical dynamics by combining meta-analysis, numerical modeling, and remote sensing. I propose a mechanistic framework for predicting C:N:P in phytoplankton under different environmental conditions and I incorporate this framework into an Earth System Model to show their effects on global carbon cycle. I also present results on how the change in elemental composition of phytoplankton could affect the feeding behavior of zooplankton as well the ecosystem stoichiometry. Finally, I show that C:N:P is closely tied to the rate at which oxygen is consumed during organic matter remineralization and I propose that the change in phytoplankton stoichiometry could ameliorate the rate of marine deoxygenation. In summary, C:N:P of phytoplankton is flexible and will play key roles in future global ocean biogeochemical dynamics.
  • Thumbnail Image
    Item
    Modeling the phenological response to climate change and its impact on carbon cycle in Northeastern U.S. forests
    (2015-03) Xu, Hong
    By controlling the timing of leaf activities, vegetation phenology plays an important role in regulating photosynthesis and other ecosystem processes. As driven by environmental variables, vegetation phenology has been shifting in response to climate change. The shift in vegetation phenology, in turn, exerts various feedbacks to affect the climate system. The magnitude of phenological change and the feedbacks has yet been well understood. The goal of this dissertation is to use phenological model with remote sensing and climate data to quantify historical and future trends in leaf onset and offset in northeastern U.S. forests, and use a dynamic ecosystem model, Agro-IBIS, to quantify the impact of phenological change on terrestrial carbon balance. This dissertation has three major parts. First, six phenological metrics based on remotely sensed vegetation index were evaluated with ground phenological observation in Agro-IBIS. Second, a modified phenological metric was used to parameterize a set of phenological models at regional scale; one model for each of leaf onset and offset were selected to examine historical trends; Agro-IBIS simulations were run to quantify the impact of phenological trends on ecosystem productivities. Finally, downscaled climate projections from global climate models under two emission scenarios were used to drive phenological models to predict the trends in leaf onset and offset in the 21st century; and the impact of photoperiod on leaf onset were particularly examined. The results of this study suggest that remotely sensed phenological metrics can be used to improve phenological models with evaluation and adjustment; advancement of leaf onset and delay of leaf offset in the past have increased productivities and could potentially mitigate the warming temperature in the future; lack of physiological understanding of the driving factors of phenology such as photoperiod could result in large uncertainties in phenological projections.