Phytoplankton photosynthesis-irradiance parameters in the near-shore UK coastal waters of the North Sea: temporal variation and environmental control

Chlorophyll-specific photosynthesis-irradiance (P-E) parameters of natural phytoplankton in the near-shore UK coastal waters of the North Sea were det ermined during six 2 wk surveys carried out from 1993 to 1995. The initial slope of the P-E curve (alpha (B)) varied from 0.02 to 2.44 mg C mg(-1) chl a d(-1) (pmol photons m(-2) s(-1))(-1) between winter 1993 and October 199 4; the light-saturated rate of photosynthesis (P-max)(B) ranged from 8 mg C mg(-1) chl a d(-1) during winter surveys to 332 mg C mg(-1) chl a d(-1) du ring October 1994. Values of alpha (B) and P-max(B) determined during Octob er 1994 were significantly higher than in other survey periods. Although ph ytoplankton cell biomass was significantly higher during the June 1995 surv ey (due to high abundance of Phaeocystis spp. at some sites) than in Octobe r or winter surveys, the October peak in P-E parameters coincided with a pe riod in which dinoflagellates accounted for a high proportion of phytoplank ton carbon biomass. Multiple linear regression analysis revealed that alpha (B) could be predicted from the total photosynthetically available radiati on (PAR) incident at the surface during the daylight period, whilst P-max(B ) could be predicted from a linear combination of total incident PAR and se a-surface temperature. Temporal variations of alpha (B) and P-max(B) did no t result in significant temporal variation of the light-saturation onset pa rameter (E-k) and the overall mean value of E-k was 176 +/- 6 pmol photons m(-2) s(-1). The high turbidity of nearshore surface waters of the western North Sea appears to restrict penetration of irradiance to the extent that phytoplankton are not exposed to PAR levels at which photoadaptation of the ir photosynthetic apparatus is induced.