Abstract:
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.
