The influence of season and Typhoon Morakot on the distribution of diatoms in surface sediments on the inner shelf of the East China Sea

Diatom fossils in sediment samples are useful environmental indicators often considered in paleoenvironmental reconstructions. The aims of this study were to determine how the distribution of diatoms in surface sediments on the inner shelf of the East China Sea varied by season and because of a typhoon, by examining samples collected in three surveys, one in December 2008 and two in August 2009, before and after a typhoon. We identified 64 species and varieties of diatoms from 27 genera in the samples. The diatom abundances were considerably higher in August 2009 than in December 2008; the absolute diatom abundances ranged from 319 to 3380 valves/g in December 2008, but ranged from 79 to 14,956 valves/g in early August 2009 before Typhoon Morakot, and from 830 to 14,584 valves/g in mid-August 2009 after Typhoon Morakot. The higher nutrient concentration in summer, relative to winter, was clearly the factor controlling diatom abundance within the study area. The composition of the diatom species in the surface sediment between December 2008 and August 2009 reflected growth during spring and summer on the Min-Zhe Coast and the influence of runoff from the Oujiang River and the Taiwan Warm Current. We identified five diatom assemblages that represented the different oceanographic conditions in the study area, the spatial distributions of which were closely related to the environmental conditions. Assemblages WII and SII represented, and reflected changes in, the Taiwan Warm Current. Assemblages WI, SI, and SIII represented coastal and intertidal groups, coastal and intertidal groups mixed with fresh water species, and coastal and planktonic groups, respectively. The distribution of diatoms in surface sediments in the study area were modified considerably by Typhoon Morakot, which suggests that the diatoms' seasonal cycle is completely disrupted because of typhoons that mainly occur in August and September.