In-situ Photosynthetic Potential of Five Philippine Seagrasses as Measured using the Submersible Pulse Amplitude Modulated (Diving-PAM) Fluorometer: Rapid Light Curves

[[abstract]]The photosynthetic potential of five species of tropical seagrasses were determined in situ using the submersible pulse amplitude modulated (Diving-PAM) fluorometer. The objective of this paper is to compare the in-situ photosynthetic potential (through light response curves) of the five seagrass species commonly found in three intertidal sections (upper, middle, subtidal) at Maribago, Mactan Island, Central Philippines. Rapid light curve (RLC) measurements were done on the following seagrass species: Enhalus acoroides (Linnaeus) Royle, Thalassia hemprichii (Ehrenberg) Ascherson, Cymodocea rotundata Ehrenberg & Hemprich ex Ascherson, Halodule uninervis (Forsskål) Ascherson, and Halophila ovalis (R. Brown) Ascherson. All RLCs showed similar trends; i.e., increasing electron transport rate (ETRs) with increasing photosynthetic photon flux density (PPFDs) up to a saturation point, and finally decreasing ETR values with further increase in PPFD. All RLCs reached ETRmax values at light intensities between 150–250 μmol photons m–2 sec–1. Unexpectedly, the photosynthetic light response of C. rotundata was higher in specimens that were exposed rather than those that were submerged. Specimens of E. acoroides living in the upper intertidal have comparable ETRmax whether exposed during low tide or submerged during high tide, but the ETRmax were reached at different PPFD. The RLC of T. hemprichii was highest in the upper intertidal followed by the middle intertidal and then subtidal with a higher ETRmax when it was submerged rather than exposed. The light response of H. uninervis found in the subtidal and middle intertidal was similar. Ha. ovalis has the lowest photosynthetic light response and its distribution was limited to the middle intertidal and subtidal. The present information serves as initial efforts to understand the photophysiology of the different seagrass species in this part of the region