Measuring photosynthetic rates in seagrasses by pulse amplitude modulated (PAM) fluorometry

Photosynthetic rates of seagrasses have until recently been measured as gas exchange of chamber-enclosed leaves mainly in the laboratory, and in situ measurements under natural conditions are scarce. In this work we explore the possibility of measuring such rates by pulse amplitude modulated (PAM) fluorometry, using a newly developed underwater device. This was done by first comparing photosynthetic O-2 evolution (net photosynthesis corrected for dark respiration) with rates of electron transport (ETR) derived from fluorescence measurements of the effective quantum yield of photosystem II multiplied with the estimated photon flux of photosynthetic active radiation absorbed by this photosystem In the field, ETRs were then measured both as rapid light curves (RLCs) and by in situ point measurements under ambient light during the day. Photosynthetic O-2 evolution showed a linear relationship with ETR within a range of irradiances for the Mediterranean seagrass Cymodocea nodosa, while the tropical Halophila stipulacea and a temperate intertidal population of Zostera marina exhibited decreasing O-2 evolution rates relative to ETRs at high irradiances. These differences are Likely due to photorespiration, which is absent in C. nodosa. The molar ratio between photosynthetic O-2 evolution and ETR within the range of their linear relationship was found to be 0.3 for C. nodosa, which is close to the theoretical stoichiometric ratio of 0.25, but was higher and lower for Z. marina and H. stipulacea, respectively. Point measurements of ETR in the field showed good agreements with rates derived from RLCs for H, stipulacea and Z. marina, but values varied greatly between replicate measurements for C. nodosa at high irradiances. It is speculated that this variation was partly due to light-flecks caused by waves in the shallow water where these measurements were done. In all, this work shows that PAM fluorometry can efficiently yield photosynthetic rates for seagrasses in the laboratory, without the typical lag experienced by O-2 electrodes, as well as in situ under natural conditions which are not disturbed by enclosures.