Carbon isotope ratio and photosynthetic activity of phytoplankton in the eutrophic Mikawa Bay, Japan

Abstract

Abstract Primary production by phytoplankton in the eutrophic Mikawa Bay, Japan, was studied by simultaneous measurements of natural carbon isotope ratio ( δ 13 C) and short‐term carbon uptake rates ( 13 C tracer study) of size‐fractionated nannoplankton (<10 μm) and net plankton (>10 μm) samples. Short‐term photosynthetic rates, which represent the physiological state of algae, were variable regardless of standing stock sizes. The δ 13 C values of particulate organic carbon (POC) in June and July displayed horizontal variations for both the net plankton fraction (−19.8 to −12.7‰) and the nannoplankton fraction (−22.0 to −12.8‰). For both fractions, low concentrations of POC had more negative δ 13 C values (−22 to −18‰). High δ 13 C values for the net plankton were found when POC concentrations were much higher, due to red tide. This suggests that the increase in algal standing crop for the net plankton fraction resulted from accelerated photosynthetic activity. However the nannoplankton fractions with higher POC values have relatively low δ 13 C values.