Riverine dissolved organic matter responds to alterations differently in two distinct hydrological regimes from 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 have heavily altered flow regimes. Such flow conditions, also alter the natural dynamics of Dissolved Organic Matter (DOM), with likely implications for carbon cycling due to changed conditions for transformation, transportation and storage of carbon. Here we looked into the effects of flow alteration on the “DOM regime”, i.e. the seasonal variation of DOM concentration and composition, in 20 rivers belonging to two different hydrological classes (i.e., Mediterranean and Atlantic) in Northern Spain. To further investigate which flow regime components influence DOM properties, we linked the turnover of DOM composition to a range of hydrological indices.

We found that Atlantic rivers with a natural flow regime have on average lower DOC concentration than their altered equivalents, but this is not mirrored in Mediterranean rivers. Moreover, we did not observe much difference in annual DOM composition due to flow alterations in either hydrological class. However, the turnover of DOM composition is higher in natural Atlantic rivers compared to the altered ones. We linked this turnover in DOM composition to the effects of upstream-located reservoirs, creating flow regimes with homogenized or even reversed seasonality. Our results suggest that Mediterranean rivers may have higher resistance to flow alterations, at least in the sense of not showing unusual DOM behaviour, while Atlantic rivers affected by flow alterations lose their naturally high annual variability of DOM composition.