Measuring rates of photosynthesis of two tropical seagrasses by pulse amplitude modulated (PAM) fluorometry

Pulse amplitude modulated (PAM) fluorometers can potentially be used for measurements of photosynthesis rates. In this work, we attempt to assess the validity of such measurements for two common tropical seagrass species using a newly marketed portable device. This was done by comparing calculated photosynthetic electron transport rates (ETRs), based on fluorescence parameters, with rates of photosynthetic O-2 evolution. While a linear relationship was found for Halophila ovalis over a wide range of photon irradiances, Halodule wrightii showed a curvilinear response with apparently lower rates of O-2 evolution at high irradiance. The apparent average molar ratio of O-2 evolution to ETR was 0.28 for H. stipulacea and 0.57 within the linear correlation range for H. wrightii; the deviation of the latter value from the theoretical maximal ratio of 0.25 mol O-2 evolved per mol electrons transported through the photosystems is discussed. Results from in situ fluorescence measurements of these two seagrasses in a shallow intertidal habitat at high natural irradiances show that photosynthesis of H. wrightii was significantly reduced at mid-day. The results demonstrate that PAM fluorometry can be used to measure photosynthetic performances in seagrasses. However, in order to quantify ETRs it may be necessary to determine the fraction of incident light absorbed by thicker-leaved species more exactly than was done in these initial trials. (C)2000 Elsevier Science B.V. All rights reserved.