Intermittent rivers and ephemeral streams (IRES) are watercourses that stop flowing at some point during the year. IRES are found in all climates and biomes and their occurrence is predicted to increase with climate change and increasing demand for freshwater. Knowledge of biogeochemical cycles in IRES is mainly based on research in the Mediterranean region. The region of Brittany in western France, characterised by a temperate oceanic climate and intensive agriculture, offers research opportunities to understand C-N-P dynamics in temperate IRES with high nutrient loadings.

In this work, we analyse the spatial variability of C-N-P concentrations in the intermittent stream network of the Kervidy-Naizin catchment (7 km²) during the different phases of intermittency. We hypothesise that the spatial variability of C-N-P concentrations increases during the stream fragmentation, as the formation of isolated pools leads to different physico-chemical conditions due to variable solar radiation, temperature, and nutrient availability. To investigate this, we conducted repeated synoptic sampling campaigns at a high spatial resolution (every 100 to 200 m) along the stream network during the spring-summer of 2023. We sampled forty sites and analysed, among others, DOC, DIN and TP and physico-chemical parameters (conductivity, redox potential, temperature and pH) during four field campaigns spanning from stream recession to the rewetting phase after the summer dry period.

The results showed an increasing spatial variability of concentrations with stream fragmentation, with spatial coefficients of variation increasing from 27% to 49% for DOC, from 15% to 64% for DIN and from 44% to 74% for TP. During the stream fragmentation, mean DOC concentrations increased from 2.43 to 4.76 mg.L^-1, mean DIN concentrations decreased from 15.1 to 8.47 mg.L^-1 and mean TP concentrations increased from 0.023 to 0.071 mg.L^-1. Spatial patterns of concentrations observed during the flowing phase tended to be disrupted by the stream fragmentation, with isolated pools exhibiting extremely high or low concentration values. We interpret these changes in spatial patterns as a consequence of redox processes and nutrient assimilation.