Responses of riverine dissolved organic matter to damming in two distinct hydrological regimes in northern Spain

Abstract

Iberian rivers are characterized by flow regimes with high seasonal flow variation. They also host one-fifth of Europe's reservoirs for hydropower generation, irrigation, or water supply needs, and thus many rivers in this region have heavily altered flow regimes. Such flow conditions also alter the natural dynamics of dissolved organic matter (DOM), which likely has implications for carbon cycling due to changed conditions for the transformation, transportation, production, and storage of carbon. Here we looked into the effects of flow alteration on the DOM regime, i.e. the seasonal variation in DOM concentration and composition, in 20 rivers belonging to two different natural (reference) flow regimes (i.e. Mediterranean and Atlantic) in northern Spain. To further investigate which flow regime components influence DOM properties, we linked the observed seasonal shifts in DOM composition to a range of hydrological indices. We found that Atlantic rivers with a natural flow regime tended to have lower annual mean dissolved organic carbon (DOC) concentrations than their altered equivalents; this flow alteration trend is weakly mirrored in Mediterranean rivers. We did not observe much difference in annual average DOM composition due to flow alteration in either Atlantic or Mediterranean rivers. However, the seasonal variation in DOM composition was lower in altered Atlantic rivers compared to natural ones. This flow alteration effect on the DOM regime was not as distinctive in Mediterranean rivers, which showed a higher diversity of DOM regimes across rivers. We linked the lack of seasonal variation in DOM composition in flow-altered rivers mainly to the prevention of transmission of upstream-sourced DOM from the reservoirs. It appears that in our study area, reservoirs mostly act as a temporally homogenizing buffer, averaging out naturally occurring shifts in DOM composition by transiently storing upstream-sourced carbon inputs and subjecting them to bio- and photo-degradation, thus sending relatively invariable amounts of DOM further downstream. This effect of dams on DOM regimes appears robust across both Atlantic and Mediterranean regimes despite some heterogeneity of dam types and purposes, with potentially important consequences for riverine carbon cycling.