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

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

Peter J. Shaw*, Duncan A. Purdie

School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, United Kingdom
*Present address: Centre for Environmental Sciences, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom. E-mail:

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 (αB) 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 (PBmax) 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 αB and PBmax 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 αB could be predicted from the total photosynthetically available radiation (PAR) incident at the surface during the daylight period, whilst PBmax could be predicted from a linear combination of total incident PAR and sea-surface temperature. Temporal variations of αB and PBmax did not result in significant temporal variation of the light-saturation onset parameter (Ek) and the overall mean value of Ek 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

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