| dc.description.abstract | The carbon dioxide in the atmosphere is causing global warming and other associated climate change events. The world countries have pledged to cut down the carbon dioxide emissions to counter act these climate change effects. However, there are no clear indication that the carbon dioxide emissions are reducing. As the exchange of carbon between the atmosphere, oceans and land is relatively fast, it is important to better understand the global carbon cycle operating between the major reservoirs of carbon.
The carbon dioxide entering the seawater is converted into organic matter by photoautotrophs. Organic matter thus produced supports all the heterotrophic forms of aquatic life through trophic food web interactions. Dissolved organic matter (DOM) accounts to about 700 gigatons of carbon in the oceans, making it the largest reservoir of reduced carbon. The DOM that is immediately consumed by bacteria is termed as labile DOM (L-DOM), producing recalcitrant DOM (R-DOM) that is resistant to most of the microbial degradation. This transformation of L-DOM into R-DOM mediated by bacteria is termed as carbon sequestration.
The thesis work focused on DOM characterization studies, to better understand carbon sequestration mediated by microbial carbon pump in three different environmental settings. In oligotrophic Eastern Mediterranean coast, a mesocosm experiment was conducted with high and low mesozooplankton perturbation combined with daily dosage of nutrients and a gradient labile carbon additions. The chemical composition of DOM did not change significantly between the treatments. It is likely that the nutrient limitations resulted in slower response of the ecosystem functions and did not reflect in carbon sequestration on a short timescale.
During in situ sampling cruise in the Rhodes Gyre, which is a permanent upwelling zone in the oligotrophic Eastern Mediterranean Sea, the primary production was relatively higher compared to the adjacent oligotrophic waters. The availability of L-DOM and inorganic nutrients in the euphotic waters supported carbon sequestration, that occurred relatively fast. Outside of the gyre, a downwelling system was prevalent, exporting the L-DOM up to 1,000 m depth before being sequestered. The reduced sequestration of L-DOM in the euphotic zone may be due reduced bacterial activity from phosphate limitation. Towards, center of the gyre, with nutrient flux from the deep upwelling waters, the chlorophyll a concentration was high. A higher L-DOM relative abundance was seen in the center of the gyre between 500 and 1,000 m. With global warming and increasing temperatures, the vertical stratification in the water column will be more prominent preventing export of carbon and nutrients affecting the biogeochemical cycling of carbon and other elements.
In the Trondheimsfjord, of the North Atlantic coastal water, the seasonal in situ studies revealed that similar to the Rhodes Gyre, the carbon sequestration occurred mainly in the euphotic waters during all seasons. In the late spring season, the L-DOM had higher relative abundance compared to R-DOM likely due to the export of particulate matter into the deep waters. The bacterial production in the aphotic waters were limited due to the reduced availability of L-DOM as most of it was already degraded to R-DOM in the surface waters itself. | en_US |
