SEASONAL-VARIATION OF PHOTOSYNTHETIC PERFORMANCE AND LIGHT ATTENUATION IN ULVA CANOPIES FROM PALMONES RIVER ESTUARY

The primary production of Ulva populations relies or! their photosynth etic performance which is dependent on the light availability under na tural conditions. This study concerns the light attenuation characteri stics in Ulva canopies and the seasonal photosynthetic performance of two different species (Ulva rotundata Blid., Ulva curvata (Kutz.) De T oni) blooming in the Palmones river estuary. Light within canopies dif fered from that reaching the surface. Light availability was reduced t hrough the water column (at high tide) and Ulva canopies. In addition, light was spectrally filtered. As a result, the photosynthetically us able radiation (PUR) was further attenuated through Ulva canopies, inc reasing the photosynthetically active radiation/PUR ratio. The muddy s ediment deposited on and between the Ulva thalli also drastically rest ricted the light availability. Thick Ulva mats are frequently found co vering the intertidal mudflats, and therefore, thalli within these mat s may be subjected to steep light gradients. As a consequence, individ ual Ulva growth rates cannot be ex trapolated to estimate the primary production of Ulva canopies. Interspecific differences were observed f or light-saturated photosynthetic rates (P-max) and light compensation points (L-CP), with Ulva curvata generally displaying higher values t han did U. rotundata. For both species, maxima were recorded in winter for P-max, quantum yield, chlorophyll content, and absorptance, where as minima were found in summer. Dark respiration (R-d) was not seasona lly affected, and a maximum L-CP was found in summer. To extrapolate t hese data to field situations, the temperature dependence of photosynt hesis should be considered. The ao values were 2.44 for R-d and 1.79 f or P-max, whereas the photosynthesis rate at subsaturating light level s was unaffected. The Q(10) values showed an enhanced respiratory rate in summer and a minimum in winter, whereas the seasonal differences o n P-max were damped.