Nearshore dynamics of nutrients and chlorophyll during Mediterranean-type flash-floods

A year-long study in the Bay of Banyuls-sur-Mer (France) assessed the inputs from the local intermittent Baillaury River and the stimulation of nearshore phytoplankton blooms. During flash-floods, dissolved inorganic nitrogen, phosphate and dissolved organic carbon concentrations in the river were neither correlated to the river discharge nor to the season. Silicate was correlated to the river discharge. The particulate organic matter load was diluted in a mineral matrix (3–4%) but had a high nutritional quality at the beginning of the flash-flood. The offshore response to the single major flow event (October 2005) was monitored daily during two weeks at 350 m (8 m water depth) and 1.4 km from the river mouth (long-term monitoring station, 27 m water depth). The flash-flood signal was partly hidden at the shallowest site by swell resuspension of bottom sediments. At the deepest station, three phases were identified: (1) at the beginning of flow, river dissolved inputs dilute conservatively leading to increased dissolved inorganic nitrogen and silicate concentrations and decreased seawater salinity, (2) during the main turbid pulse, phosphate is released and (3) after the water column cleared, diatom photosynthesis occurred at 3 m below the water surface, leading seven days after the flow peak discharge to a high of 2 µg L−1 of chlorophyll a, which vanished three days later. The nutrients and salinity recovered their pre-flow values at that time. This dynamics of nutrients and chlorophyll during a flash-flood event is consistent with long-term monitoring data.