MEPS 216:83-94 (2001)  -  doi:10.3354/meps216083

ABSTRACT: Chlorophyll-specific photosynthesis-irradiance (P-E) parameters of natural phytoplankton in the near-shore UK coastal waters of the North Sea were determined during six 2 wk surveys carried out from 1993 to 1995. The initial slope of the P-E curve (α^B) varied from 0.02 to 2.44 mg C mg^-1 chl a d^-1 (µmol photons m^-2 s^-1)^-1 between winter 1993 and October 1994; the light-saturated rate of photosynthesis (PB_max) ranged from 8 mg C mg^-1 chl a d^-1 during winter surveys to 332 mg C mg^-1 chl a d^-1 during October 1994. Values of α^B and PB_max determined during October 1994 were significantly higher than in other survey periods. Although phytoplankton cell biomass was significantly higher during the June 1995 survey (due to high abundance of Phaeocystis spp. at some sites) than in October or winter surveys, the October peak in P-E parameters coincided with a period in which dinoflagellates accounted for a high proportion of phytoplankton carbon biomass. Multiple linear regression analysis revealed that α^B could be predicted from the total photosynthetically available radiation (PAR) incident at the surface during the daylight period, whilst PB_max could be predicted from a linear combination of total incident PAR and sea-surface temperature. Temporal variations of α^B and PB_max did not result in significant temporal variation of the light-saturation onset parameter (E_k) and the overall mean value of E_k was 176 ± 6 µmol photons m^-2 s^-1. The high turbidity of near-shore 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 their photosynthetic apparatus is induced.

KEY WORDS: Primary production · Phytoplankton · Photosynthesis · Coastal · North Sea