CLIMATIC REGULATION OF PHAEOCYSTIS ALGAL BLOOMS IN
COASTAL WATERS OF THE WESTERN ENGLISH CHANNEL
Duncan A. Purdie, Arantza Iriarte and Claire Holeton
School of Ocean and Earth Science, University of Southampton, Southampton
Oceanography Centre, European Way, Southampton SO14 3ZH
The marine algae Phaeocystis sp. is a common member of the spring phytoplankton
assemblage in temperate coastal waters and in regions receiving high nutrient inputs can
form exceptional blooms. In some years, extensive blooms of this organism have
occurred in spring along the UK English Channel coast, resulting in the formation of
surface organic scums and foams. Environmental conditions influencing the interannual
variation in the intensity of spring Phaeocystis blooms in the western English Channel
have been investigated using data from the L4 sampling position situated 10 nautical
miles southwest of Plymouth. Phytoplankton samples have been collected on an almost
weekly basis from this position since 1993 and enumerated by scientists from the
Plymouth Marine Laboratory. Phaeocystis abundance shows large interannual variations
during spring at this site being almost absent in some year (e.g. 2000) and exceptionally
reaching densities of over 7000 cells per ml in 2001. Interannual variations in
Phaeocystis cell numbers at L4 have been compared with estimates of nutrient fluxes
from the River Tamar and multiple linear regression analysis has revealed a strong
correlation between winter nitrate flux and peak cell abundance. Years in which low
Phaeocystis cells numbers were recorded can be correlated with either exceptionally low
spring river flow rates or unusually high surface irradiance levels in late winter, leading
to early diatom blooms that reduce the available nutrients for Phaeocystis growth. The
influence of the North Atlantic Oscillation (NAO) winter index on spring Phaeocystis
and diatom populations in the western English Channel will also be demonstrated.
email: dap1@soc.soton.ac.uk