Unusual diatoms linked to climatic events in the northeastern English Channel

Abstract

The composition of the micro-phytoplankton was investigated at two fixed stations in the northeastern English Channel from November 1997 to December 2005. In late summer-autumn 2005, proliferations of the centric diatoms Proboscia indica and Rhizosolenia hebetata f. semispina were recorded for the first time in this area. The weather in 2005 was abnormal: a cold winter and early arrival of summer conditions that extended to late autumn. This resulted in an unusually long, sustained calm and warm period. We suggest that the reduction in mixing may have allowed diatoms of more-stratified waters to be competitive in the normally highly mixed northeastern English Channel in summer-autumn 2005. In 2003 and in 2005, two warm-water diatom species were recorded for the first time. A colony of Eucampia cornuta was observed after an exceptional heat wave in September of 2003, and again in September 2005. Simultaneously at both fixed stations, Chaetoceros peruvianus was recorded in the warm October of 2005. The climatic events are associated with the northward spreading of thermophilic diatoms such as E. cornuta and Ch. peruvianus in the European Atlantic waters, and the proliferation of diatoms of stratified environments such as P. indica and R. hebetata f. semispina in the usually highly turbulent waters of the northeastern English Channel.