Phytoplankton, its pigments, and primary production at a central north sea station in May, July and September 1981

Abstract

In May, July and September 1981 cruises of two weeks were made to the Oyster Ground area, in the central North Sea. Samples were taken near a sub-surface drifter. Neverthless, uneven distribution of the plankton caused an irregular daily variability in algal cell numbers. Patchy distribution of phytoplankton was also revealed by short-term variability in the concentration of chlorophylls and carotenoids, which were measured by reversed-phase high-performance liquid chromatography (HPLC). Chlorophyll a measured in this way was about 30% lower than “chlorophyll a” measured with the conventional fluorimetric acidification method, partly due to the presence of chlorophyll a derivatives fluorimetrically similar to chlorophyll a. Phytoplankton biomass was estimated by relating particulate organic carbon (POC) concentrations to the HPLC-measured chlorophyll a concentrations. The mean contribution of algal carbon to total POC was 25 to 65%, to labile, non-refractory POC 45 to 85%. Estimates of algal biomass on the basis of cell numbers were not reliable because many cells appeared to be overlooked by microscope. It was possible to detect groups of algae missing in the microscopy record and estimate their abundancy by chemotaxonomic analysis on the basis of concentration data of the major algal pigments, including class-specific pigments such as alloxanthin (Cryptophyceae) and hexanoyloxyfucoxanthin (Prymnesiophyceae). Algal classes dominated by representatives in the nanoplankton appeared to be important contributors to the crop during all cruises. The vertical distribution of algal biomass resembled the vertical profile of primary production only in September. In early May much of the crop was present near the bottom, presumably after sinking; in July a dense dinoflagellate maximum layer maintained itself at 25 m near the bottom of the euphotic zone, 5 m below the depth of maximum primary production. Mean primary carbon production, based on 14C uptake measurements, was 1240 mg·m −2·d −1 in May; in July 1057 mg·m −2·d −1; and in September 1013 mg·m −2d −1. Extrapolation of the 14C measurements suggests an annual primary carbon production level of about 250 g·m −2.