Lacunal gas discharge as a measure of productivity in the seagrasses Zostera capricorni,Cymodocea serrulata and Syringodium isoetifolium

A new method for estimating the rate of photosynthetic fixation of carbon in seagrasses is described. This method, which is sensitive and simple to apply, is based on the close relationship between photosynthetic rate and the volume of gas discharged through the lacunae.During photosynthesis, the gas discharged from the lacunae was composed primarily of oxygen (32.5%) and nitrogen (67.5%). The rate of discharge was proportional to the rate of photosynthesis and hence was a function of light intensity. There was a linear relationship between gases discharged from the lacunae and oxygen released into the water column. Calibration curves were derived relating volume of gases (oxygen and nitrogen) released from the lacunae to total oxygen produced during photosynthesis for three species of seagrasses (Zostera capricorni Aschers., Cymodocea serrulata (R. Br.) Aschers. & Magnus and Syringodium isoetifolium (Aschers.) Dandy). Molar ratios of ¹⁴C fixed to oxygen produced were close to unity (1.008±0.016; n=8) indicating that measurements of lacunal gas released may be used to measure productivity.