Effects of eutrophication on the biogeochemical cycling of carbon, oxygen, sulfur and energy in coastal marine ecosystems
Year: 1989
Type: dissertation
Abstract: The effects of eutrophication on the biogeochemical cycling of the major elemental cycles, carbon, oxygen, and sulfur and on energy flow, were investigated in experimental marine ecosytems (mesocosms). Eutrophic environmental conditions were created within mesocosms by daily addition of varying amounts and sources of nutrients and sewage sludge. Inorganic nutrient loading resulted in an increase in the magnitude and duration of winter-spring phytoplankton populations and production with a correlated increase in sedimentation and accumulation of carbon in surface sediments. Net ecosystem production during the winter and spring was followed by net system heterotrophy during the summer and fall with a concomitant decrease in sedimentary carbon concentrations. Carbon sedimentation and accumulation (up to 19 mol C m$\\sp{-2}$) in surface sediments stimulated sediment oxygen consumption and sulfate reduction metabolism. Sulfate reduction increased by an order of magnitude in carbon enriched sediments and was the dominant pathway for benthic carbon remineralization in all the mesocosms. However, the fraction of benthic metabolism using sulfate for a terminal electron acceptor did not increase along the nutrient addition gradient when annual integrated estimates of metabolism were compared. Whole system carbon and oxygen budgets showed the sediments to be an important and influential component in eutrophic marine ecosystems. A significant fraction of allochtonous sewage carbon was remineralized via sulfate reduction in the sediments. This led to the dissociation of carbon cycling from oxygen and energy flow in the system due to an increased role of sulfur biogeochemistry. Accumulation of solid sulfides in the sediments from sulfate reduction during the experiment explained the discrepancy between carbon dioxide and oxygen consumption at the sediment water interface (respiratory quotients $>$ 2) and represented a large storage of energy originally contained in organic matter. Oxygen dynamics in eutrophic marine systems were regulated by phytoplankton cycles and sulfur cycling in the sediments. Initiation of hypoxia (oxygen less than 2 mg l$\\sp{-1}$) was caused by rapidly declining phytoplankton populations and a large imbalance between day oxygen production and night oxygen consumption in the water column. However, high sulfate reduction rates and the accumulation of hydrogen sulfide in surface sediments was found to be influential in the demise of the macrofauna and the precipitation and maintenance of oxygen concentrations less than 1 mg l$\\sp{-1}$.
DOI: 10.23860/diss-3557
Author: Sampou, Peter Andre
Institution: University of Rhode Island
Language: English
Subfield: Oceanography
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
OpenAlex ID: W10546492