Being used to it: impacts of seasonal extreme climate events on C and N cycling in Mediterranean catchments

Abstract:

Mediterranean catchments naturally experience marked seasonal changes in hydrological connectivity that span from droughts to floods. This hydrological regime strongly influences the cycling and transport of carbon (C) and nitrogen (N) throughout the catchment as well as to downstream ecosystems. In this talk, we will present results obtained from both empirical and modelling studies performed during the last decade in headwater Mediterranean catchments to illustrate the characteristic biogeochemical patterns of those ecosystems periodically subjected to extreme climate events. For most of the year, hillslopes are disconnected from the fluvial network and microbial activity in hillslope soils is water limited. During such periods, riparian aquifers can make the most of stream discharge, and C and N export can be tightly linked to riparian biogeochemistry. At the other end, large storm events lead to pulses of microbial activity, and thus to increases in nutrient availability within the catchment. Yet, terrestrial biota can take limited advantage of this opportunity because nutrients are quickly flushed out towards the stream when floods reconnect the different landscape units. This alternation between droughts and floods drive the temporal pattern of C and N inputs to stream ecosystems as well as their capacity to retain and transform nutrients. In-stream processes can regulate N exports during low flow conditions; while transitorily diminish their assimilation capacity during storm flows. We have learned that the alternation of droughts and floods amplifies N losses and ultimately limits the N retention capacity of Mediterranean catchments compared to more humid systems. In urban streams, the observed patterns of water and nutrient export can be over-imposed by inputs from waste water treatment plants or by the flashiness of urban runoff, which can distort and even amplify the already extreme patterns of nutrient export from Mediterranean catchments. Together, these studies highlight that hillslope-riparian-stream linkages are key for understanding patterns and drivers of C and N export in catchments subjected to extreme climatic events. This integrative view of catchment biogeochemistry is crucial to foreseen how future drying and flooding could affect C and N export in other regions of the world.