Impacts and effects of a historical high and ENSO linked freshwater inflow in the tropical estuary Nha Phu, southeast Vietnam

The frequency of strong and extreme precipitation events and high river discharges is foreseen to increase in southeast Asia in the future. The aims of the study were to evaluate physical and biological impacts of a historical (one-in-36 years) (1980-2015) extreme freshwater inflow in November 2010 into the small tropical Nha Phu estuary. The extreme event impacts were assessed relative to a dataset of samplings (n=11) through 1.5 year of phytoplankton biomass (chlorophyll a), nutrients, CTD, primary production and suspended particulate matter prior to the extreme event. Results show, that the event significantly decreased salinity, increased chl-a 2-3 times, suspended particulate matter and beam attenuation coefficients by 7 times, and the colored dissolved organic matter absorption coefficient by 3 times. The extreme inflow initiated a phytoplankton bloom fueled by catchment area derived nutrients. The bloom established a considerable carbon production and transport of carbon to the coastal and shelf areas. There was likewise a significant export of suspended particulate matter out of the estuary. Residence time decreased from 4.2 to 3.8 days during the extreme event, and pre-inflow water column conditions were re-established within 4-5 days, shown in a previous study. Annual precipitation in the area was linked to the Pacific ENSO-cycle (El Niño Southern Oscillation) with 2010 as a very strong La Niña year. The entire event accounted for 68% of annual precipitation, 39% of annual discharge, and 53% of annual sediment transport in the Dinh River, which discharges into the Nha Phu estuary.