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Significance of NW Mediterranean flash flood events on the functionning of the nearshore marine ecosystem


Guizien Katell

A study was carried out in the Bay of Banyuls-sur-Mer from October 2004 to October 2005 to assess the inputs of the local intermittent Baillaury River and its impact of the marine pelagic ecosystem. Over the period, the river runoffs were monitored during two flash floods at two seasons (spring and fall).During floods, dissolved concentrations for both inorganic nutrients and DOC were neither correlated to the river discharge nor to the season. However, they were clearly higher than during low water except for silicates which showed a dilution-like pattern in the river.
The flood POM transfer is always highly diluted in a sedimentary matrix but rather fresh at the very beginning of the flood. The sea response to the major flood (October 2005) was monitored daily during 2 weeks near the river mouth (8 m depth) and further offshore, at the Service d'Observation du Laboratoire Arago (SOLA) long term monitoring station (27 m depth).
The flood signal was partly hidden at the shallowest sampling site by resuspension caused by moderate swells. At the SOLA station, three phases clearly showed : (1) at the beginning of flood, river inputs dilute by advective-diffusive processes leading to an increase in DIN and silicates concentrations consistent with a lowered salinity, (2) during the main turbid pulse, phosphates are regenerated over the entire water column and (3) after water column cleared, diatom photosynthesis occured at 3 m below free surface leading 7 days after the flood peak to a subsequent peak of 2 10-6 g/l of chlorophyll a. At that time, the nutrients and salinity had recovered their pre-flood values.
This increase in Chlorophyll a biomass vanished 3 days after. Looking to the eight years weekly monitoring of nutrients, salinity and chlorophyll a concentration at the SOLA station, riverine low salinity inputs are often observed and correlated to DIN and silicates concentrations increases, like in our high frequency study.Over the period, fourteen chlorophyll a flash peaks are observed. Five of them can be directly related to a low salinity event.
Applying the SOMLIT weekly sampling period to our high frequency study suggest that the flashiness of flood events makes their signature difficult to observe in both salinity and Chlorophyll a weekly time series.
However, this high frequency study together with the long term monitoring at SOLA station clearly demonstrates the role of flash river floods in stimulating the nearshore primary production, contributing to sustain the nearshore ecosystem productivity.