Intercambios de co2 atmósfera-agua y sedimentación biogénica en embalses mediterráneos estratificados con distinto estado trófico

Resumen

We are aware that inland waters play an important role in the global C cycle by emitting and burying C. However streams, lakes and reservoirs are generally overlooked in bottom-up approached C budgets. In the present PhD Thesis, we analyzed the main processes involved in the carbon processing in Mediterranean reservoirs, a gap in the current knowledge of the global carbon cycling. During the summer of 2009 and 2010, we monitored Guadalcacín and Bornos reservoirs, in South Spain, to estimate C gross sedimentation and CO2 emissions with high sampling frequency. The stratified period of the reservoirs was selected to simplify processes in the water column. The study reservoirs show different trophic status, which allowed for an analysis of the carbon cycling from different carbon-productivity perspectives. Moreover, we took advantage from the inter-annual differences in rainfall, which led to significant changes in trophic status of the reservoirs. Gross sedimentation of particulate organic carbon (SPOC) was measured from sediment traps sampled with biweekly periodicity. This sampling revealed a predominantly inorganic matrix in the settling material, reflecting the dominating role of inorganic allochthonous material input. Both reservoirs showed relatively high percentages of remineralization (14 to 53 % d-1), although gross sedimentation was an important removing particulate organic carbon (POC) to deep layers. We estimated that from 13 to 8.4 % d-1 were transferred to the hypolimninon during the stratified period. Water incomings played a major role in the regulation of POC fluxes. Water inputs significantly controlled SPOC by suppling new material or diluting particulate pool in the water body. On the other hand, high-frequency measurements at hourly resolution of surface partial pressure of CO2 (pCO2) were simultaneously carried out in both reservoirs. Temporal scales of variability for the pCO2 in surface waters were examined through the temporal decomposition of 5-month time series. The results evidenced that pCO2 had significant variability at daily, biweekly and seasonal scales. Each of these scales explained one third of the total pCO2 variation registered. The wind-induced mixing events strongly operated at biweekly scale, becoming the main environmental factor controlling the pCO2 dynamics at this scale. This result highlighted the importance of taken into consideration the small scales of time variability in the analyses of the carbon fluxes. Our measurements showed that both reservoirs were oversaturated, emitting CO2 to atmosphere during the stratification period. Our study points to the warm-temperate latitude reservoirs as hotspots for C emission to the atmosphere at global scale. During the study period, we estimated that the studied reservoirs emitted more than double the amount of the C laterally exported from the catchment. Moreover, a significant positive relationship was found between FCO2 and SPOC, differently from other previous studies, suggesting a new way of regulation of FCO2 by external inputs. Interestingly, SPOC and the estimations of POC burial were higher than FCO2, classifying the studied reservoirs as net carbon sink systems.