Photosynthesis of Ulva lactuca at different tidal levels during submergence and emergence

The relative photosynthetic rates of fronds of Ulva lactuca Linn. (Ulvales, Chlorophyceae) sampled from upper, middle and lower littoral zones, were measured when submerged and at different levels of desiccation under laboratory conditions. The upper and middle littoral fronds showed increased rates of photosynthesis under exposure to air but the lower littoral ones showed maximal rates under submergence and registered lower rates under exposure. The rates of the upper littoral fronds were significantly higher than those of lower ones, under desiccated conditions. The upper ones had higher moisture content than the middle and lower littoral fronds during submergence but they were fast to lose their moisture under desiccation. The moisture levels at which the photosynthetic rates began to fall were 45%, 72% and 100%, in the upper, middle and lower littoral fronds, respectively. It is inferred from this study that the upper littoral fronds of the species, which are subjected to longer periods of exposure and consequently more desiccation during emergence in nature, are better adapted to emerged conditions than the lower ones. The results further suggest that the three types of fronds could be locally adapted ecads or ecotypes, the differences being the outcome of their responses to the physical conditions operating along the intertidal belt.