Monitoring strategies of stream phosphorus under contrasting climate-driven flow regimes

Climate and hydrology are relevant control factors determining the timing and amount of nutrient losses from land to downstream aquatic systems, in particular of phosphorus (P) from agricultural lands. The main objective of the study was to evaluate the differences in P export patterns and the performance of alternative monitoring strategies in streams under contrasting climate-driven flow regimes. We compared a set of paired streams draining lowland microcatchments under temperate climate and stable discharge conditions (Denmark) and under sub-tropical climate and flashy conditions (Uruguay). We applied two alternative nutrient sampling programs (high-frequency composite sampling and low-frequency instantaneous-grab sampling) and estimated the contribution derived from point and diffuse sources fitting a source apportionment model. We expected to detect a pattern of higher total and particulate phosphorus export from diffuse sources in streams in Uruguay streams, mostly as a consequence of higher variability in flow regime (higher flashiness). Contrarily, we found a higher contribution of dissolved P in flashy streams. We did not find a notably poorer performance of the low-frequency sampling program to estimate P exports in flashy streams compared to the less variable streams. We also found signs of interaction between climate/hydrology and land use intensity, in particular in the presence of point sources of P, leading to a bias towards underestimation of P in hydrologically stable streams and overestimation of P in flashy streams. Based on our findings, we suggest that the evaluation and use of more accurate monitoring methods, such as automatized flow-proportional water samplers and automatized bankside analyzers, should be prioritized whenever logistically possible. However, it seems particularly relevant in currently flashy systems and also in systems where climate change predictions suggest an increase in stream flashiness.