Large centric diatoms of the genus Coscinodiscus are found from October onwards. The species on this photo is Coscinodiscus wailesii (Bacillariophyceae). It is the largest diatom of the Marsdiep (and of the North Sea), with a minimum of ⅓ mm and a maximum of ½ mm in diameter. It is an invasive species, originating from the Indian and Pacific Ocean. Since 1977 it appeared for the first time in the English Channel, probably introduced by ballast water or by the import of oysters.