Estimation of phytoplankton photosynthesis by active fluorescence

Photosynthesis can be described by target theory. A t low photon flux densities, photosynthesis is a linear function of irradiance (E), the number of reaction centers (n), their effective absorption capture cross section (o ), and a quantum yield (0 ) . As photosynthesis becomes increasingly light saturated, an increased fraction of reaction centers close. At light saturation the maximum photosynthetic rate is given as the product o f the number of reaction centers (n) and their maximum electron transport rate (1 It). Using active fluoromctry it is possible to measure non-destructively and in real time the fraction of open or closed reaction centers under ambient irradiance conditions in situ, as well as <7 and 0 . rcan be readily calculated from knowledge o f the light saturation parameter, E k (which can be deduced in situ by active fluorescence measurements) and a. We built a pump and probe fluorometer, which is interfaced with a CTD. The instrument measures the fluorescence yield of a weak probe flash preceding (F0) and succeeding (Fnl) a saturating pump flash. Profiles o f these fluorescence yields are used to derive the instantaneous rate o f gross photosynthesis in natural phytoplankton communities without any incubation. Correlations with short­ term simulated in situ radiocarbon measurements are extremely high. The average slope between photosynthesis derived from fluorescence and that measured by radiocarbon is 1.06 and corresponds to the average photosynthetic quotient. The intercept is about 12% of the maximum radiocarbon uptake and corresponds to the average net community respiration. Profiles of photosynthesis and sections showing the variability in its composite parameters reveal a significant effect of nutrient availability on photosynthetic energy conversion efficiency in the ocean, independent of phytoplankton species composition.