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Photoadaptation to intermittent light across the St. Lawrence Estuary freshwater-saltwater transition zone
Vincent, W.F.; Bertrand, N.; Frenette, J.-J. (1994). Photoadaptation to intermittent light across the St. Lawrence Estuary freshwater-saltwater transition zone. Mar. Ecol. Prog. Ser. 110(2-3): 283-292
In: Marine Ecology Progress Series. Inter-Research: Oldendorf/Luhe. ISSN 0171-8630, more
Peer reviewed article  

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Keywords
    Acclimatization; Adaptations; Biomass; Chlorophylls; Estuaries; Exposure tolerance; Irradiance; Light effects; Nannoplankton; Phytoplankton; Rivers; Salinity data; Seston; Spatial variations; Temporal variations; Turbidity; Vertical profiles; Water temperature; ANW, Canada, Quebec, St. Lawrence Estuary [Marine Regions]; Brackish water; Fresh water

Authors  Top 
  • Vincent, W.F.
  • Bertrand, N.
  • Frenette, J.-J.

Abstract
    We evaluated 2 competing hypotheses for the photoadaptive characteristics of phytoplankton distributed across the turbid freshwater-saltwater transition zone (TZ) of the St. Lawrence River (Canada): that the communities were photosynthetically adapted to a low mean water column irradiance, or that they were adapted to intermittent exposure to near-surface irradiance conditions. Two cruises were undertaken in spring-early summer, a period that corresponded to major seasonal changes in the optical environment of the St. Lawrence River. There was a large increase in chlorophyll a (chl a) concentration, maximum photosynthetic rates (PBmax), and the light saturation parameter (Ik) between the 2 cruises. During this period the nanoplankton (cells in the size range 2 to 20 mu m) rose from 33 to 69%, of total chl a. There were no major shifts in photosynthetic characteristics across the transition from freshwater to turbid saltwater conditions, but rather the cells maintained high values of PBmax and Ik, with low alpha (the light limitation parameter) and little inhibitory response to high photon fluence rates. These observations support the hypothesis that the phytoplankton community in this and perhaps other turbid environments are photoadapted to 'intermittent sun' conditions, rather than the 'shade environment' experienced on average through the water column.

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