EFFECTS OF ENVIRONMENTAL-FACTORS ON NET PHOTOSYNTHESIS AND GROWTH OF INTERTIDAL SPECIES OF THE GENUS GELIDIUM (GELIDIACEAE, RHODOPHYTA) IN NORTHERN SPAIN

The effect of temperature, air exposure, salinity and light have been tested for three species of Gelidium on the basis of photosynthetic an d growth responses in order to assess the zonation patterns observed o n the Atlantic coasts of northern Spain. The vertical distribution of three Gelidium species is described: G. sesquipedale reaches its limit in the low intertidal zone, whereas both G. latifolium and G. pulchel lum extend from the mid- to the lower intertidal zone. These dif feren ces in location on the shore gradient are in accordance with the physi ological responses to the factors tested. Gelidium sesquipedale has th e lowest saturated net photosynthesis (P-max) and saturating (P-k) Pho tosynthetic Photon Flux Density (PPFD) and is the less tolerant to cha nges in temperature and to dessication. Exposure to air causing water losses above 50% (wet weight) produced negative net photosynthesis val ues in G. sesquipedale. Gelidium latifolium showed broader tolerance r anges to the factors mentioned above, although exposure to long period s of dessication caused significant reductions in the photosynthetic r esponse. Gelidium pulchellum, which has the highest vertical position in the shore, was the most tolerant to temperature and dessication. Th e response was similar when growth, instead of net photosynthesis, was tested against various combinations of temperature and salinity. Thes e differences in net photosynthesis and growth are consistent with the observed zonation pattern. Although other causes can be invoked, it i s concluded that tolerance ranges are decisive in determining the vert ical distribution of these three species.