Variability of DOC and nitrate responses to storms in a small Mediterranean forested catchment

Abstract

Severe drought periods followed by intense rainfall often leads to major floods in Mediterranean catchments. The resulting hydrology is complex and the response of solutes in the streams is often unpredictable. This study aimed to identify the most relevant factors controlling the hydrological responses to storms of an intermittent Mediterranean stream and to link those factors with dissolved organic carbon (DOC) and nitrate during storm events. Measurements of climate, hydrology, DOC and nitrate concentrations during 26 storm events over three hydrological years were analysed. The contribution of the storm events to the total DOC and nitrate annual export was also calculated. Nitrate was mainly mobilised during high flow, while most of the DOC export occurred during baseflow. Solute concentrations peaked after drought periods and the solute export was maximal during the largest rainfalls (i.e.>100 L m -2 ). One single large storm contributed some 22% of the total annual export of DOC, and about 80% of that of nitrate. Discharge was a good predictor of neither DOC nor nitrate responses, so variables other than discharge were considered. Factor Analysis was used to identify the main factors controlling the biogeochemical responses. Antecedent moisture conditions and the magnitude of the storm event were the most relevant factors and accounted for 63% of the total variance. Solute responses during high flow were highly variable. However, solute concentration changes showed a significant and moderate relationship with the factors controlling the hydrological responses (i.e. ∆ DOC v. the antecedent moisture conditions and ∆ NO 3 -N v. the magnitude of the storm event).

Organic matter decomposition by fungi in a Mediterranean forested stream : contribution of streambed substrata

“…However, physicochemical parameters, such as discharge and DIN, may also determine the time-pattern of the enzymatic activity. The abrupt flows in autumn or early winter in Mediterranean streams (Gasith & Resh 1999) may mobilise most of the nitrate in the catchment of the Fuirosos stream (Bernal et al 2002). After the dry period (summer) the first storms caused the weathering of dissolved and particulate OM accumulated on soil, and N concentration in streamwater was positively correlated with lignocellulosic activities (P and PO).…”

“…Litter fall peaks in autumn. Flooding in autumn coincides with the increase in water nutrient content as a result of soil leachates (Bernal et al 2002) and leaves input during the fall season. We studied a stream reach of 33 m in length and 3 to 5 m wide.…”

High‐frequency monitoring of catchment nutrient exports reveals highly variable storm event responses and dynamic source zone activation

“…Precipitation intensity during storm events was particularly important for predicting maximum concentrations and relative increases in concentrations for both NO 3 -N and DOC. This is likely due to rapid runoff through the near-surface soil horizon during events of high precipitation intensity, thus mobilizing and routing the product of recently decomposed and nitrified organic matter to the river channel [Bernal et al, 2002]. Previous studies have found evidence of nutrient source exhaustion following consecutive storm events [e.g., Outram et al, 2016].…”

Bottom-up effects of streambed drying on consumer performance through changes in resource quality

“…The higher C:N ratios observed in this study are likely to be related to chlorophyll degradation, which have been positively related to a decrease in leucine-aminopeptidase activity during desiccation in seasonal summer drought (Timoner et al 2012). Although in some cases a rapid recovery (within hours) of algal biomass has been reported after flow resumption (Romaní et al 2013), it has also been related to nutrients wash from the dried streambed or watershed after flow resumption (Bernal et al 2002), which did not take place in our experiment.…”

“…Monthly mean temperature ranges from 4 °C (December) to 28 °C (July-August) and precipitation (650 mm year −1 ) mostly occurs in autumn and spring with occasional storms in summer (Bernal et al 2002). Baseline water flow ranges from 5 to 20 L s −1 , although in summer flow usually ceases from July/August to September/October (Sabater et al 2011).…”