Phytoplankton community assemblage in the English Channel: A comparison using chlorophyll a derived from HPLC-CHEMTAX and carbon derived from microscopy cell counts

Abstract

The phytoplankton community assemblage in surface water of the English Channel (Station L4) was measured and compared from March 1999 to October 2002 using two different methods. Pigment-CHEMTAX was used to derive class apportioned chlorophyll a (Chl a) and cell counts obtained using microscopy were used ta derive phytoplankton carbon (phyto-C) estimations. Phyto-C (10-340 μg C L-1) showed a strong linear relationship with total Chl a (0.5-4.8 μg Chl a L-1) when the Chl a:phyto-C ratio was >0.04 (r2 = 0.80; average Chl a:phyto-C = 0.044) but the relationship was weaker when the Chl a:phyto-C ratio was <0.04 (r2 = 0.42; average Chl a:phyto-C = 0.013). Correlation between class biomass estimates for phyto-C and Chl a was strong for diatoms during winter (r2 = 0.80) but poor for other classes during both summer and winter. The Chl a:phyto-C ratio declined as irradiance increased with strongest correlation on clear sky days in 2001 when diatoms dominated (r2 = 0.80). The Chl a:phyto-C versus irradiance relationship agrees with that produced from an empirically based dynamic model of phytoplankton acclimation to light. Results are discussed in relation to differences in the techniques used, the species present, cellular pigment concentrations and surface mixed layer irradiance.