Tracing the dynamics of dissolved organic matter in marine systems exposed to natural and experimental perturbations

In the coming decades, global warming will affect the biogeochemical cycles evolution, particularly the carbon cycle. In this context, it is necessary to gain knowledge on the Earth natural mechanisms to relieve the atmosphere of the greenhouse gases excess. The “biological pump” is one of the main mechanisms employed by the oceans to “sequester” the CO2 accumulated in the atmosphere. Thereby, the organic carbon produced by the biological activity is transferred from surface to deep waters where part of this pool is accumulated in the seafloor. Another mechanism involving the accumulation of carbon in the ocean, called the “microbial carbon pump” (MCP), has been described recently. It is composed by an intricate set of microbial processes that enable the formation of highly recalcitrant dissolved material and therefore facilitate the accumulation of carbon in the deep waters. The oceans store about 660 Pg C in the form of dissolved organic matter (DOM), a quantity comparable to the atmospheric CO2. Understanding the processes that control the dynamics, recycling and exportation of the DOM is crucial to evaluate the oceans capability to gather the excess of atmospheric CO2. On its course down throughout the water column, microorganisms degraded the DOM produced at the surface layers. Concentrations decrease from ~90 µmol C L-1 down to 40-50 µmol C L-1, values homogeneously distributed in the deep oceans throughout the planet. The fact that below 1000 m and deeper the DOM is degraded at lower speed is still unknown, and the processes that can affect this DOM degradation have been studied in this thesis. In this regard, we performed experiments with deep Atlantic Ocean microbial communities. These communities were exposed to DOM of different quality. The results revealed that the presence of humic-like allocthonous compounds favored the generation of new humic-like compounds in situ. Consequently, we proved that the composition of the DOM that reach the deep ocean conditions its ease-to-degrade nature. In this thesis we also evaluated the effect of global change (acidification and eutrophication) on the quality of the DOM. With this purpose in mind, we developed mesocosms experiments in tanks of 200 L in which we enclosed coastal planktonic communities from the NW Mediterranean Sea. The planktonic populations were exposed to different treatments of pH and eutrophication (addition of inorganic nutrients). The results of these experiments demonstrated that low pH levels favored the increase of the planktonic organisms’ growth rates, while the input of nutrients promoted the transformation to complex DOM. Finally, a monthly monitoring sampling of several biogeochemical variables was carried out at the Estartit Oceanographic Station (EOS). One of the principal aims consisted in identify the DOM sources and its inter-annual variability. The results revealed the importance of the winds in transporting oceanic DOM inputs to the system, which contrasted with previous results observed in nearby sampling stations (e.g. Blanes Bay, Bay of Banyuls-sur-mer), where the major DOM contributions were terrestrial inputs.

Aparicio Bernat F. L., 2016. Tracing the dynamics of dissolved organic matter in marine systems exposed to natural and experimental perturbations. PhD thesis, Universitat Politècnica de Catalunya, 184 p. Thesis.

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