Influence of non-photosynthetic pigments on light absorption and quantum yield of photosynthesis in the western equatorial Pacific and the Subarctic North Pacific

Abstract

Regional variations in the contribution of non-photosynthetic pigments (ānp*) to the total light absorption of phytoplankton (āph*) and its influence on the maximum quantum yield of photosynthesis (φm) were investigated. In the western equatorial Pacific, the surface ānp* : āph* ratio was higher in the western warm pool than that in the upwelling region. This difference appears to be attributable to severe nitrate depletion and higher percentage of prokaryotes, which can accumulate very high concentrations of zeaxanthin in the western warm pool. In the subarctic North Pacific, the ānp* : āph* ratio was expected to be higher in the Alaskan Gyre where the thermocline is sharper and iron limitation may possibly be more severe than in the Western Subarctic Gyre. However, the ratio was actually higher in the Western Subarctic Gyre, contradictory to our expectations. This east-west variation appears to be attributable to changes in the taxonomic composition; cyanobacteria were more abundant in the Western Subarctic Gyre. The values of ānp* : āph* and its vertical variations were relatively small in the subarctic North Pacific compared to those in the western equatorial Pacific. These inter-regional variations appear to be attributable to the lower solar radiation intensity, smaller percentage of cyanobacteria, and relatively strong vertical mixing in the subarctic North Pacific. The spatial variations in ānp* : āph* significantly influence φm. In comparison with φm based on the total light absorption (φm ph), the values corrected for the contribution of non-photosynthetic pigments (φm ps) showed an increase in both the western equatorial Pacific and the subarctic North Pacific.