Non-additive effects of multiple global-change drivers on aquatic ecosystems of both hemispheres

Laburpena

Aquatic ecosystems worldwide are being exposed to an unprecedented number of alterations caused by the impacts of global climate change. Over the last decades an increasing amount of attention and effort has been given to examining the effects of these perturbations, however, existing studies reveal few consistent response patterns that allow us predicting and understanding how ecosystems will behave in the upcoming future under these ongoing alterations. The rationale behind these contrasting responses underlie in that most of these studies have been focused in study of the single impacts of these drivers, therefore neglecting the complexity of the natural environment. Thus, the purpose of this doctoral dissertation is study the interactive effects of three of the main global change drivers (ultraviolet radiation [UVR], nutrient inputs and temperature) that are currently impacting the ecosystems. To achieve this objective, it was carried out a combination of long-term and remote-sensing data (i.e. atmospheric aerosols, surface solar irradiances and particulate organic carbon) together with a serie of laboratory and in situ experiments. The experimental studies performed considered both different combinations of global change drivers, different temporal scales (from hours to weeks), different levels of biological organization (from populations to ecosystems) as different ecosystems from both hemispheres (i.e., high-mountain lakes, coastal and open sea areas). The results obtained from these experiments showed a differential sensitivity of phytoplankton populations to UVR under increases in nutrient inputs and temperatures. Thus, under a future environmental scenario Bacillariophyceae (i.e. Chaetoceros gracilis) and Haptophyceae (i.e. Isochrysis galbana) were benefited, increasing their photosynthetic activity and decreasing the respiratory processes, whereas that in Chlorophyceae (i.e. Dunaliella salina) and Dinophyceae (i.e. Alexandrium tamarense) the harmful UVR effects observed under current environmental conditions was accentuated. However, when these results to a population level were extrapolated to community level through the seasonal phytoplanktonic succession (from pre-bloom to bloom), it was found that the natural environmental variability increased even more the harmful effects of a future global change scenario of increased nutrient inputs and temperature under UVR. This higher inhibitory effect on the primary productivity of marine ecosystems during the most productive stage of the succession matched with lower in situ temperatures and solar irradiances as well as with a higher predominance of large bloom-forming diatoms, such as Odontella aurita. Nevertheless, and despite the key role that phytoplankton plays in the functioning of ecosystems as well as in the global carbon (C) cycle, another important link inside food webs are the decomposers, thus bacteria. In fact, the trophic state of ecosystems is widely dependent of the existing metabolic balance between production by photoautotrophs and the respiration by photoautotrophs and heterotrophs. Considering this fact, a future global change scenario of rising frequency of nutrient inputs and high UVR levels prompted shifts in the structure of planktonic communities from a complete dominance of photoautotrophs (i.e. Monoraphidium sp.) to a dominance by mixotrophic nanoflagellates (MNFs, Chromulina nevadensis). Moreover, it was also shown that the predominance of one or other group in the planktonic community was linked to the nutrient inputs timing into the system through dust deposition. A rising frequency of nutrient inputs benefited MNFs communities whereas that low frequency of nutrient inputs benefited photoautotrophs communities. Contrarily, bacterial community did not show any clear response pattern. This predominance of MNFs under a future scenario produced the strengthening of the commensalistic phytoplankton-bacteria interaction due to a greater dual control on the latter through the release of photosynthetic organic carbon to satisfy their carbon demands, and an increased predatory pressure by MNFs. Therefore, as multiple global change drivers can greatly impact the interaction within food webs between producers and decomposers, the last step was study how a future global change scenario with increased atmospheric aerosol-dust deposition and / or riverine inputs under high UVR could impact the metabolic balance of the highest level of biological organization, the ecosystems, and consequently how it could influence the C-cycle. From results obtained in two contrasting ecosystems of both hemispheres, one in the South Atlantic Ocean (SAO) and other in the Southwestern (SW) Mediterranean Sea, it was evidenced that the combination of both drivers (nutrients and UVR) had a dual impact motivated by the ecosystem productivity. In this sense, in unproductive ecosystems such as oligotrophic areas (SW Mediterranean Sea waters), the metabolic balance can be autotrophic or heterotrophic, that is, metabolically diverse. Notwithstanding, the interaction between the two global change drivers assayed prompted shifts from a strong heterotrophy towards autotrophy due to an enhanced activity of the biological pump compared to the respiratory processes. Conversely, in highly productive ecosystems such as coastal areas of the SAO, the joint action of both drivers did not alter the metabolic balance of these ecosystems as in the case of unproductive areas mentioned above, however, it severely reduced (~18%) the C-sink capacity of these systems. As a whole, the results presented in this PhD thesis show the importance not only of considering multi-drivers approaches and complex biotic interactions in global change studies, but also unravel the fact that these studies must consider both different levels of biological organization, different temporal scales adapted to the response time of organisms and exposure to global change drivers, and different ecosystems of both hemispheres. Taken together, we will obtain a more comprehensive and integrative understanding of the potential impacts that the global change context in which we are immersed will have on the ecosystems functioning in the future.