Photosynthetic rates of seagrasses have until recently been measured a s 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 rneasunng such rates by pulse amplitude modulated (PAM) fluorometry, using a newly developed underwater device. This was done by first comparing photosynthetic O2 evolution (net photosynthesis corrected for dark respiration) with rates of electron transport (ETR) derived from fluorescen.ce measurements of the effective quantum yield of photosystem I1 multiplied with the estlnlated 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 O2 volution 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 O2 evolution rates relative to ETRs at high lrradiances. These differences are likely due to photorespiration, w h~c h is absent in C. nodosa. The molar ratio between photosynthetic O2 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 2. manna and H. stjpulacea, respectively. Point measurements of ETR in the field showed good agreements wlth rates derived from RLCs for H. stipulacea and Z. marina, but values varied greatly between replicate measurements for C. nodosa a t high irradiances. It is speculated that this variation was partly due to lightflecks 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 O2 electrodes, a s well a s in situ under natural conditions which are not disturbed by enclosures.

The aim of the present study was to investigate lvhether trop~cal intertidal seagrasses were better adapted to tolerate des~ccatlon than subt~dally growing seagrasses To do t h~s , the photosynthetic performance of 8 seagrass species, gro1nng from the upper intertidal to the shallow s u b t~d a l m Zanzibar East Africa was studied during the event of air exposure and the subsequent rehydration Photosynthet~c eff~ciencies were measured by pulse amplitude modulated (PAM) fluorometry as effective electron quantum y~e l d s of photosystem I1 (Y) since it had recently been shown that t h~s measure parallels rates of O2 evolutlon tor several specles under a d e f~n e d ~r r a d~a n c e Contrary to our expectabons it was found that the shallow intertidal specles wele in genelal more sensitive to desiccation than the deeper species Thls was expressed both as a faster decllne In Y at decreasing water contents and as an l n a b~l~t y to regain full photosynthetic rates d u r~n g reh\ dratlon follow~ng even mild d e s~c c a t~o n as compared with the deeper-glowing speclcs One exception was the subtldally growlng S y n n g o d~u m ~soetlfoiium, w h~c h was very sensitive to deslccatlon The 2 specles whlch grow highest up in the Intert~d a l zone Haioph~ia ovalis and Halodule wnghtu may not desiccate much in s~t u during low tide because the leaves lie flat on the molst sand and for the latter specles, overlap one another so as to m n l m s e water loss Thus ~t seems that d e s~c c a t~o n tolerance is not a trait whlch determnes the vertical zonatlon of trop~cal seagrasses Rather, it IS hypothesised that the ablllty to t o l e~a t e h~g h ~~r a d~a n c e s , as well as to benefit from h~g h nutnent lnputs f~o r n the shore, allows the shallow specles to occupy the uppermost lntert~dal zone KEY WORDS Desiccat~on . PAM fluorometry Photosynthes~s Seagrass . Tropical

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