Ecological studies on the phytoplankton of Boknafjorden, western Norway. II. Environmental control of photosynthesis

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Abstract
Both predicted (incubator) and measured (in situ) 14C-assimilation rates were studied from February to November 1981 at three stations in Boknafjorden, a deep silled fjord of western Norway. Samples were taken from different light depths within the euphotic zone. A high degree of conformity was found between the two approaches. Daily values of carbon assimilation integrated over the euphotic zone varied between 0.05 and 1.4 g C m−2. Yearly primary production varied between stations from 82 to 112 g C m−2 (120–148 g C m−2 when based on average light conditions). The light-saturation curve parameters αB and PBmax ranged from 0.0056 to 0.0537 mg C mg Chla−1 h−1 μE−1 m2 and from 0.7 to 8.5 mg C mg Chla−1 h−2 (in situ assimilation numbers ranged from 0.9 to 9.3 mg C mg Chla−1 h−1) respectively, which compare well with those published from the northwestern side of the Atlantic. The overall importance of light in controlling photosynthesis throughout the year was revealed by the light utilization index ψ, estimated to be 0.43 mg C mg Chla−1 E−1 m2. The maximum quantum yield was encountered on August 17, with 0.089 mol CE−1. Chla/C ratios above and below 0.010 were found to be typical for shade- and light-adapted cells respectively. Assimilation numbers and growth rates were linearly related only when considering light-adapted cells. Consistent with the findings of this study, the applicability of IK, αB and PBmax as indicators of light-shade adaptation properties should be questioned. Maximum growth rates were encountered during an autumn bloom of the dinoflagellate Gyrodinium aureolum (1.0 doublings day−1), while 0.7–0.8 doublings day−1 were found for a winter bloom (water temperature of 2°C) of the diatom Skeletonema costatum. No unambiguous temperature effect on assimilation number and growth of phytoplankton could be recognized in Boknafjorden. A tendency towards increased assimilation numbers coinciding with increased water column stability was revealed. The highest PBmax values were often encountered at almost undetectable nutrient concentrations. At least during summer this could be attributed to recycling of nutrients by macro- and/or microzooplankton, responsible for a greater part of the primary production now being grazed down. This study supports the convention that the depth of the euphotic zone may extend considerably below the 1% light depth.