Newly recorded Karlodinium veneficum dinoflagellate blooms in stratified water of the East China Sea

Karlodinium veneficum is a cosmopolitan species, but has been poorly recorded in the East China Sea (ECS) due to its small size and difficulty in identification. The bloom dynamics of this species is not well understood globally. In this study, we examined its morphological characteristics that suggest the K. veneficum is the co-occurring bloom causative species of large scale Prorocentrum donghaiense blooms in spring 2011. The epicone of K. veneficum recorded in the investigated area is conical or rounded, and the hypocone is hemispherically rounded. The ventral pore is located at the left side of the apical groove. Nucleus is positioned centrally within the hypocone. Four large irregular chloroplasts are equally distributed in the epicone and hypocone. The mean length of cultured cells was 13.6±1.2 μm (range 11.0–15.8 μm) and the mean width was 10.0±1.1 μm (range 8.0–12 μm) (n=50). Cell abundance of K. veneficum population was low, in the region 1000–1600 cells L−1, along a transect in the East China Sea on April 19, 2011, when the water column was not distinctly stratified. Cell densities reached 3×107 cells L−1 along the same transect on May 13 2011 when the bloom occurred in the 10 m layer surface and the water column was distinctly stratified. Cell abundances therefore appear closely related to water column stratification.