IN-SITU SEAGRASS PHOTOSYNTHESIS MEASURED USING A SUBMERSIBLE, PULSE-AMPLITUDE MODULATED FLUOROMETER

Assessments of photosynthetic activity in marine plants can now be mad e in situ using a newly developed, submersible, pulse-amplitude modula ted (PAM) fluorometer: Diving-PAM. PAM fluorometry provides a measure of chlorophyll a fluorescence using rapid-light curves in which the el ectron-transport rate can be determined for plants exposed to ambient light conditions. This technique was used to compare the photosyntheti c responses of seagrasses near Rottnest Island, Western Australia. Sev eral fluorescence parameters were measured as a function of time of da y and water depth; electron-transport rate (ETR), quantum yield, photo chemical quenching and non-photochemical quenching and Photosystem II (PSII) photochemical efficiency (F-v:F-m ratio) were measured. Results indicate that recent light-history plays a crucial role in seagrass p hotosynthetic responses. Maximum ETR of Posidonia australis, Amphiboli s antarctica and Halophila ovalis is influenced by the irradiance duri ng the diurnal cycle, with low rates at dawn and dusk (<10 mu mol elec tron m(-2) s(-1)), highest rates in late morning (40 to 60 mu mol elec tron m(-2) s(-1)) and a mid-day depression. Maximum ETR and PSII photo chemical efficiency varied widely between seagrass species and were no t correlated. A comparison of photochemical to non-photochemical quenc hing indicated that seagrasses in shallow water receiving high light h ave a high capacity for non-photochemical quenching (e.g. light protec tion) compared to seagrasses in deep water. These results indicate tha t in situ measurements of photosynthesis will provide new insights int o the mechanisms and adaptive responses of marine plants.