Estimation of annual primary production in the Kuroshio waters northeast of Taiwan using a photosynthesis-irradiance model

Photosynthesis-irradiance (PB–Ē) relationships for Kuroshio waters at different depths in different seasons were established using repeated measurements of primary productivity at a station northeast of Taiwan. The PB–Ē approach is different from the conventional PB–E approach in that a much longer incubation time is required to measure primary productivity. By plotting the daytime averaged, chlorophyll specific primary productivity (PB) against the daytime averaged irradiance (Ē), the optimal rate of photosynthesis (PBopt) in the mixed layer was found to be 5.87 mgC [mgChl a]-1 h-1 in summer and 2.88 mgC [mgChl a]-1 h-1 in winter. The initial slope of the PB–Ē curve, however, remained around 0.01 mgC [mgChl a]-1 h-1 [μEinstein m-2 s-1]-1 throughout the year. Below the mixed layer, the PBopt decreased substantially to less than half of that within the mixed layer. A similar decrease was also observed in the initial slope. The euphotic zone integrated primary production as calculated from these PB–Ē parameters was close to that measured in situ, with relative error less than 6%. Based on the measured PB–Ē parameters, it was estimated here that the annual primary production was 104 gC m-2 yr-1 (285 mgC m-2 d-1). The established PB–Ē relationships may serve as a better regional algorithm in the assessment of primary production for Kuroshio waters.