Hydrological extremes modulate nutrient dynamics in mediterranean climate streams across different spatial scales

Hydrology is a key factor in the terrestrial landscape that connects upland and riparian ecosystems to streams. Hydrological connectivity through the catchment and along the stream network influences not only the timing and the magnitude of terrestrial inputs to stream ecosystems, but also regulates stream metabolism and biogeochemical cycling. In mediterranean climate regions (med-regions), there is a marked seasonal alternation of dry and wet periods, and streams experience a wide range of hydrological conditions from floods to droughts. Moreover, nutrient dynamics in med-streams are highly variable within the year and between years compared to temperate streams. In this review, we explore how seasonal changes in hydrological connectivity and hydrological extremes affect nutrient cycling and transport at different spatial scales in med-catchments. We show that during the dry period, the stream network contracts more severely in med-catchments than in temperate catchments. During the contraction phase, upland-stream disconnection decreases nutrient supply from terrestrial ecosystems, while stream fragmentation increases the spatial variation of stream nutrient concentrations. Moreover, the precipitation regime typical of med-regions (with irregular distribution of precipitation and episodic large rainfall events) decreases the ability of terrestrial and aquatic biota to retain nutrients and favours nutrient export to downstream ecosystems. Altogether, this hydrological setting confers to med-streams a characteristic temporal pattern in stream nutrient dynamics that is analogous to a particular biogeochemical heartbeat with higher amplitude, variation and unpredictability than that observed in temperate streams.