Net seaweed photosynthesis measured from changes in natural stable carbon isotope ratios in incubation water

Seaweed photosynthesis has usually been measured based on oxygen, which demanded conversion to carbon units for quantification of organic production using a photosynthetic quotient (PQ). PQ has often been measured based on pH and alkalinity or with isotope additions to water. Here I present an alternative technique based on isotope measurement but without artificial additions. It was used to find the PQ in 10 seaweed species, with values between 0.13 and 2.55, and trend of higher values at higher irradiances.