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

Pulse amplitude modulated (PAM) fluorometers can potentially be used for me asurements of photosynthesis rates. In this work, we attempt to assess the validity of such measurements for two common tropical seagrass species usin g a newly marketed portable device. This was done by comparing calculated p hotosynthetic electron transport rates (ETRs), based on fluorescence parame ters, with rates of photosynthetic O-2 evolution. While a linear relationsh ip 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 c orrelation range for H. wrightii; the deviation of the latter value from th e theoretical maximal ratio of 0.25 mol O-2 evolved per mol electrons trans ported through the photosystems is discussed. Results from in situ fluoresc ence measurements of these two seagrasses in a shallow intertidal habitat a t high natural irradiances show that photosynthesis of H. wrightii was sign ificantly 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 don e in these initial trials. (C)2000 Elsevier Science B.V. All rights reserve d.