Investigating stream water temperature in southern Belgium : environmental drivers and potential impact on a thermal sensitive species (Salmo trutta fario)

Stream water temperature (WT) is considered as a key ecological factor. Oxygen solubility, organic matter, decomposition rates,… are influenced by WT. Moreover, WT directly influences aquatic species by interacting with the metabolism, growth and survival of species.

In Wallonia (Southern Belgium, 16,000 km²), a water level monitoring network of about 140 stations also measures continuous WT taken at intervals of 10 minutes. Continuous monitoring allows detecting extreme thermal events generally harmful to aquatic organisms and time-limited because of high WT variability.

The objectives of this study were :
- To study WT evolution between 2012 and 2018, which are still underestimated due to the lack of regular and continuous monitoring over long periods;
- To demonstrate the importance of having continuous temperature data for understanding and anticipating thermal damage to the aquatic ecosystem with the example of the brown trout (Salmo trutta fario L.);
- To study the influence of land cover, topographical, hydromorphological and seasonal parameters on WT.

The results showed a typical yearly sinusoidal evolution of the WT between 2012 and 2018. However, between years, remarkable thermal differences were observed reflecting the meteorological assessment established for the study period. Moreover, our data allowed us to map areas and target periods when temperatures too hot for brown trout are recorded. In addition to this essential information for the management of aquatic environments, the study of environmental factors has shown that shade has a strong impact on river temperature variability. This result provides an objective basis for conservatory management of riparian forest cover.