Date

2016-9-23

Author

Dişa, Deniz

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The ocean has a crucial role in global carbon cycle. Marine ecosystems are responsible for storing the carbon within the ocean body by means of uptaking atmospheric carbon into the ocean, transforming it into organic carbon through photosynthesis and transporting to the profound depths of the ocean. Playing a significant role in the marine food webs, grazing on plankton and providing nutrient to ecosystem by its metabolic activities, fish is thought to have a considerable impact on carbon export. For this reason, having regard to its increasing trend especially after 1950s, fishing is expected to impact carbon cycle directly by changing the fish biomasses. However, how fish impacts the biogeochemistry of marine ecosystems is not known clearly and to be assessed quantitatively. In this regard, this study aims to analyze the impact of fish and fisheries on marine biogeochemical processes by setting up an end-to-end model that simulates lower and higher tropic levels of marine ecosystems simultaneously. For this purpose, a biogeochemical model, which simulates lower tropic level dynamics (e.g. carbon export, nutrient cycles) and an ecosystem model, which simulates fisheries exploitation and higher tropic level dynamics (e.g. food web) were online and two-way coupled. Simulating the ecosystem from one end to the other with a holistic approach, the coupled model provided a more realistic representation of the ecosystem. It served as a tool for the analysis of fishing impacts on marine biogeochemical dynamics. Coupled model resolved the inefficiencies of biogeochemical model, which was because of being “closed” by zooplankton. Results pointed out 56% decrease in the mesozooplankton biomass due to higher trophic level predation. Simulations estimated an approximately 24% increase in the carbon export compared to the biogeochemical model simulations, which had no fish compartment. This increase was due to the change in the plankton compositions and enhanced outflows to detritus. The changes in the lower trophic level dynamics were statistically more consistent with the empirical data. Moreover, results obtained by applying different fishing intensities indicated that changes in fisheries exploitation levels directly influence the marine nutrient cycles and hence, the carbon export. Depending on the target and the intensity of fisheries, considerable changes in the biogeochemical responses obtained. For example, in the scenario where new potential target mesopelagics harvested in addition to the current fisheries revealed 12% decrease in the carbon export. The same scenario also indicated 11-15% changes in the remineralization flows. As a result of this study, unlike the models that do not represent the fish explicitly, how marine biogeochemical processes are impacted by the activity of fish assemblages and fisheries exploitation was delineated.

Subject Keywords

Fisheries, Biogeochemistry, Ecosystem Modeling, Carbon Export, End-To-End Modeling

URI

https://hdl.handle.net/11511/95102

Collections

Graduate School of Marine Sciences, Thesis