Spatial variability in C-N-P concentrations during the fragmentation of an intermittent stream in a small temperate oceanic agricultural catchment

Intermittent rivers and ephemeral streams (IRES) are watercourses that cease to flow at some point during the year (Datry et al., 2017). IRES are found in all climates and biomes and their occurrence is likely to increase with climate change and increasing demand for freshwater. (Messager et al., 2021). Research on IRES has increased in recent years, although they are not as well instrumented as perennial rivers. Knowledge of the biogeochemical dynamics of IRES has increased over the years and is mainly concentrated in the Mediterranean region. (von Schiller et al., 2017). The Brittany region in France, with low-order streams in wet oceanic catchments characterised by regular intermittency and surface waters rich in nutrients due to intensive agriculture, offers research opportunities to understand C-N-P dynamics in these water systems. In this work, we analyse the spatial variability of C-N-P concentrations in the intermittent stream network of a temperate oceanic catchment (Kervidy-Naizin catchment, 7 km², a French Critical Zone Observatory OZCAR IR) during the different phases of intermittency. The stream is characterised by a dry period of a few weeks to a few months between July and October, within a catchment with a shallow water table on crystalline rocks (schist) and an intensive livestock farming with some mixed cropping systems. (Fovet et al., 2018). We hypothesise that the spatial variability of C-N-P concentrations increases during the stream fragmentation, as the formation of isolated pools would lead to different physico-chemical conditions due to variable sunlight exposure, temperature, and nutrient availability (Bonada et al., 2020). To study this, we are realizing a high spatial resolution sampling along the stream network of the Kervidy-Naizin stream during the spring-summer of 2023. Forty sites are sampled and analysed for major anions, NO3-, DOC, TP, PO43- as well as water quality parameters (conductivity, redox potential, temperature and pH) and periphyton biomasses during five field campaigns, spanning from flow recession to the rewetting phase after the summer drought period. Results from the current study are analysed using the coefficient of variation (CV) to assess the relative variability of C-N-P concentrations across the catchment. Spatial statistics will also be used to measure the spatial dependence and identify distribution patterns of C-N-P concentrations within the catchment. After the first campaign carried out in March 2023, we observed a higher relative variability of PO43-, CV of 0.78, a moderate variability of DOC, CV of 0.22 and a low variability of NO3-, CV of 0.13. DOC showed higher concentrations downstream (1.9 - 8.3 mg/L), in contrast to NO3- where the higher concentrations are upstream (41.6 - 69.6 mg/L). With the four remaining field campaigns, we aim to understand nutrient dynamics and improve our understanding of the ecological functioning of IRES in an oceanic temperate context.