Water temperature dynamics in a headwater forest stream: Contrasting climatic, anthropic and geological conditions create thermal mosaic of aquatic habitats

Abstract

The thermal regime of streams is a relevant driver of their ecological functioning. As this regime is presently submitted to numerous alterations (among others, impoundments, and climate change), it seems important to study both their effects and potential recovery from the latter. Thus, we investigated the surface and hyporheic water temperature along a small headwater stream with contrasting environmental contexts: forest landscape, open grassland landscape without riparian vegetation, several artificial run-of-the-river impoundments and one discharge point of a by-pass impoundment. The main objectives were to study the influence of these contrasting contexts on surface and subsurface water temperature at a local scale. Contrasting contexts were supposed to create effects on both surface and hyporheic thermal regimes at a local scale. Differences of thermal regimes between surface and hyporheos were expected, as well as between geological contexts. Sensors located at multiple stations allowed monitoring of stream and hyporheos temperature along the stream, while comparison with adjacent reference stream allowed for surface water thermal regime benchmark. Impoundments and landscapes significantly influenced stream thermal regime at a local scale (impoundments created up to +3.7°C temperature increase in average). Their effect on hyporheos thermal regime was less marked than the ones generated by solar radiation or geological features. Hyporheos thermal regime varies from stream one by temperature dynamics delay (up to 18h) and decrease (up to -7°C between surface and hyporheos temperature in average). These coupled effects create a mosaic of thermal habitats, which could be used for river biodiversity preservation and restoration.

Fig 1. Study streams, watersheds and distribution of sensors on the field along the Soultzbach (West watershed) and the Trautbach (East).

All tributaries in the study area are ephemeral and were not active during field study. Impoundments are labeled by their order of stream appearance (Table 1); they are located upstream A, between A and B, between E and F, between H and I.

Fig 2. Surface water temperature time series during the study.

Green: Soultzbach stream. Blue: Trautbach stream. Letters indicate monitoring stations (see Fig 1).

Fig 3. Thermal amplitude (daily maximum temperature—daily minimum temperature) along Soultzbach (a) and Trautbach (b) streams.

Letters indicate monitoring stations. Red line: maximum thermal amplitude across the study period. Green line: minimum thermal amplitude across the study period. Blue line: mean thermal amplitude across the study period.

Fig 4. Temperature time series in the stream and adjacent hyporheos.

Upper-left quadrant: B station. Upper-right quadrant: C station. Lower-left quadrant: E station. Lower-right quadrant: I station. Blue line: stream temperature. Brown lines: hyporheos temperature at increasing depths, from 10cm-deep (pale brown) to 20cm-, 30cm- and 40cm-deep (dark brown).

Fig 5. Boxplot of mean hourly temperature along the seasons (from June to December), along the stream (from B to I site) and with depth (blue: Surface water temperature; shades of brown: Hyporheos water temperature).