Resuspension of intertidal cohesive sediment during the passage of extreme typhoons 

Abstract

Typhoon is an extreme tropical storm, severely disturbing the coastal environments with strong winds and heavy precipitations. At the shallow intertidal flat, in particular, the typhoon can vigorously stir up the benthic sediment layers. To understand the effect of typhoons on the resuspension of intertidal cohesive sediment and phytoplankton, two in-situ moorings were installed at mud (M1) and mixed (M2) flats, the Muui Island. During the mooring period, four sediment erodibility experiments were conducted using Gust erosion microcosm system (GEMS). Two successive typhoons (Maysak and Haishen) generated strong wind stress up to 0.5 Pa, resulting in high suspended sediment concentration (SSC) (M1: 614 mg/l; M2: 434 mg/l), which is three times higher than that under fair weather. The Chl-a, as a proxy of phytoplankton, maintained low (< 6 mg/l) during the combined period of typhoon and neap tide. However, it drastically increased to 19 mg/l at the early stage of the spring tide. The response of SSC at the mud flat was different from that at the mixed flat because of the sediment composition. The GEMS experiments showed that the sediment erodibility of M2 (Erosion rate (E): 89.98 mg/m2 s; erosion threshold (tce): 0.0520 Pa) was higher than M1 (E: 4.08 mg/m2 s; tce: 0.2433 Pa). After the typhoon, the mud flat showed an abrupt increase and rapid recovery of SSC, whereas the mixed flat showed a gradual increase of SSC, and it maintained high SSC for relatively longer than the mud flat. The outcomes from this study suggest that the successive typhoons flushed out surface sediments, resulting in the tidal flat's ecologically “reset” condition. The low solar radiation and continuous high SSC prohibited phytoplankton from photosynthesizing, resulting in low Chl-a during the typhoons.