In November 2013, an artificial inlet was opened in the barrier island that separates the waters of the Setiu estuary-lagoon system of Terengganu, Malaysia, from the South China Sea. The inlet was opened to stabilize salinity and improve tidal flushing so that fish-farming could be expanded but, as a result, another inlet that was open in 2009 closed naturally by November 2015. These geomorphic changes caused salinity in the Setiu estuary to undergo significant increase from November 2013 to November 2015.

To investigate the foraminiferal response to these salinity changes, 24 of 56 foraminiferal sampling stations occupied in 2009 were resampled in 2016. Salinity increased throughout the estuary from 2009 (mean 6.6, range 0–28.7) to 2016 (mean 33.3, range 23.3–34.8) and foraminiferal live communities and dead assemblages changed. Diversity of dead assemblages increased significantly (mean number of species/sample in 2009 = 10.9 versus 2016 = 25.8) as the result of transport of specimens of nearshore, neritic species into the estuary through the new inlet. Diversity of live populations increased slightly (2008 = 4.6 versus 2016 = 6.2). Dead assemblages and live populations dominated by agglutinated foraminifera in 2009 were co-dominated by calcareous hyaline foraminifera in 2016. Live calcareous porcelaneous foraminifera were rare in 2009 and 2016, but dead miliolids were more common and more widespread in 2016. Stations in higher energy environments immediately adjacent to the two different inlets of 2009 and 2016 were characterized by few live or dead foraminiferal tests. Changes in foraminiferal diversity, abundance, and distribution between the two sampling times of early June 2009 and late May 2016 were significant and took place within the 30-month interval between November 2013 (when the artificial inlet was opened) and May 2016 when samples were collected, but they most likely occurred over a shorter interval of time.

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