Effects of diurnal variations in phytoplankton photosynthesis obtained from natural fluorescence
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Effects of diurnal variation in phytoplankton photosynthesis on estimating daily primary production (DPP) were examined using field data from Sagami Bay, Japan. DPP at 5 m depth was calculated from the continuous data of chlorophyll a (Chl a) and light intensity monitored by a natural fluorescence sensor with and without considering time-dependent changes in the photosynthesis–irradiance (P–E) relationship. Chl a could be estimated from natural fluorescence examining the variations in the quantum yield of fluorescence (φf) and Chl a-specific light absorption coefficient (a*ph), and relating them to Chl a. The P–E relationship was determined by water sampling three times daily. A distinct diurnal pattern was observed for the maximum photosynthetic rate (P*max), being maximal at noon, while periodicity of the maximum light utilization coefficient (α*) was less obvious. The actual DPP was calculated by interpolating the P–E parameters from those obtained at dawn, noon, and dusk. For comparison, DPP was calculated by fixing the P–E parameters as the constants measured at dawn, noon or dusk for a day. The difference from the actual DPP was small when the P–E parameters measured at dawn (3% on average) and noon (5%) were used as the constants for a day. The difference was largest when the values at dusk were used (−43%). The medium values of P*max at dawn, its low values at dusk, and the fact that a major part of the DPP was produced around noon were responsible for these results. The present study demonstrates that measurement of the P–E parameters at dawn or noon can give a good estimation of DPP from natural fluorescence.
KeywordsPhytoplankton Particulate Organic Carbon Nonphotochemical Quenching Phytoplankton Assemblage Photosynthetically Available Radiation
We are grateful to the scientists and crew members of the R/V Tansei-Maru cruises for their cooperation at sea, and to Z. Suzuki and S. Saitoh for data on the light absorption spectrum of phytoplankton. We are also grateful to A. Hinuma, C. Sukigara, Y. Mino, and T. Saino for their generous cooperation with the mass spectrometry. Financial research support was provided by Creative Basic Research Funds (#12NP0201) and the Project on Ocean Productivity Profiling System, Core Research for Evolutionary Science and Technology. T. Y. acknowledges financial support from Asian Natural Environmental Science Center of the University of Tokyo.
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