VARIATIONS IN CHLOROPHYLL FLUORESCENCE YIELDS IN PHYTOPLANKTON IN THEWORLD OCEANS

The ocean is optically thin and lends itself to large-scale measuremen ts of in vivo chlorophyll fluorescence. In the open ocean, however, ph ytoplankton chlorophyll concentrations average only 0.2 mu g L(-1), an d hence high sensitivity is required for precise measurements of the f luorescence yields. Over the past decade, we have developed two approa ches to achieve the required sensitivity; these are the pump- and prob e-technique and a fast repetition rate (FRR) method. Both methods have been adapted for in situ studies and are used to rapidly measure the maximum change in the quantum yield (Delta 0(max)) of photosystem II ( PSII), as well as the effective absorption cross-section of PSII (sigm a(PSII)) Sections of variable fluorescence across the Pacific and Atla ntic Oceans reveal the influence of geophysical processes in controlli ng the quantum yields of phytoplankton photosynthesis. Areas of upwell ing, such as off the coast of north-western Africa, have F-v/F-m value s of 0.65, which are close to the maximum achievable values in nutrien t-replete cultures. Throughout most of the nutrient-deficient central ocean basins, this quantum efficiency is reduced by more than 50%. In high-nutrient, low-chlorophyll regions of the eastern Equatorial Pacif ic, the deliberate, large-scale addition of nanomolar iron directly to the ocean leads to a rapid increase in quantum efficiency of the natu ral phytoplankton community, thereby revealing that in these regions p hytoplankton photosynthetic energy conversion efficiency is iron limit ed. Diel patterns of variation in the upper ocean display midday, inte nsity-dependent reductions in both sigma(PSII) and Delta 0(max). We in terpret the former as indicative of non-photochemical quenching in the antenna, while the latter is a consequence of both rapidly reversible and slowly reversible damage to reaction centres. From knowledge of t he incident spectral irradiance, Delta 0(max), sigma(PSII) and photoch emical quenching, the absolute photosynthetic electron transport rate can be derived in real-time. Using unattended, moored continuous measu rements of in vivo fluorescence parameters, the derived in situ electr on transport rates can be related to satellite observations of the glo bal ocean with basin-scale, seasonal estimates of phytoplankton carbon fixation. Thus, unlike any other photosynthetic parameter, chlorophyl l fluorescence can be used to bridge the scales of biophysical respons es to ecosystem dynamics.