Basin-scale variability of phytoplankton bio-optical characteristics in relation to bloom state and community structure in the Northeast Atlantic

Phytoplankton physiological data collected throughout the Iceland Basin and Rockall Trough during the North Atlantic spring bloom from May to June 2001 are presented. Physiological parameters including the maximum photochemical quantum efficiency (Fv/Fm) and the functional absorption cross section of photosystem II (σPSII) were measured using fast repetition rate fluorometry. Information on the taxonomic and pigment characteristics of the phytoplankton populations was also collected, with pigment data being used to reconstruct absorption spectra. Significant changes in the physiological properties of PSII were found to be associated with the progression of the spring bloom from diatom to flagellate domination. Changes in both community composition and physiology were in turn correlated with environmental parameters. Lower Fv/Fm, higher σPSII and corresponding decreases in cell size were associated with the observed decrease of nutrients that accompanied increasing stratification. Differences in σPSII were primarily associated with the changing pigment composition of the phytoplankton populations, with the largest changes appearing to be governed by the amount of absorption by photosynthetic carotenoids. The physiological state of PSII was thus found to be an indicator of bloom status and community structure in this productive temperate region principally as a result of taxon specific variability.