Large scale northward expansion of warm water species Skeletonema tropicum (Bacillariophyceae) in China seas

S keletonema tropicum is regarded as a species with an affinity to warm waters and it has never been reported in seas where temperatures drop below 11°C in winter. Previous studies in China reported that S. tropicum was restricted to subtropical and warm temperate seas (East and South China Seas), but the species was recently found during August cruises of 2009 and 2010 in Jiaozhou Bay, Yellow Sea, located several hundred kilometers to the north. Here, winter water temperatures often drop below 5°C. Identification of S. tropicum was confirmed under light and scanning electronic microscopes and maximum cell abundance in Jiaozhou Bay was estimated as 1. 73×10⁴ cell/L. This record of S. tropicum in Jiaozhou Bay represents a significant northward expansion in the geographic range of the species. Ship ballast water was identified as a possible carrier of S. tropicum from southern places along Chinese coastline, and in addition, thermal pollution from local power stations and seawater desalination plants may provide suitable conditions for species over-wintering.