Photoinhibition in common atlantic macroalgae measured on site in Gran Canaria

The photosynthetic quantum yield was analysed in four common atlantic macro algae, the Rhodophytes Gelidium arbuscula and Halopithys incurvus and the P haeophytes Halopteris scoparia and Lobophora variegata in Gran Canaria, Can ary Islands at their growth site. The fluorescence parameters were measured using a portable pulse amplitude modulated (PAM) fluorometer (PAM 2000) in strument and a diving PAM under water without removing the thalli from thei r growth sites. Solar radiation was monitored continuously above and under water during the whole experimental period using two three-channel dosimete rs (European light dosimeter network; ELDONET) (Real Time Computer, Mohrend orf, Germany). These instruments measure solar radiation in three wavelengt h ranges, ultraviolet (UV)-A. UV-B and photosynthetic active radiation (PAR ). In all four algae the effective photosynthetic quantum yield decreased s ignificantly from the optimal values measured after dark adaptation due to exposure to 15 min solar radiation, but at least partially recovered subseq uently in the shade within several hours. Increasing the exposure period to 30 min intensified the photoinhibition. In some algae no recovery was obse rved after this treatment and in others no significant recovery could be de tected. Exposure to unfiltered solar radiation caused a significantly highe r photoinhibition than PAR-only radiation or PAR plus UV-A. A substantial i nhibition was found in all algae at their growth sites in the water column when the sun was at high angles, as measured with the diving PAM.