EFFECTS OF PH ON SEAGRASS PHOTOSYNTHESIS - A LABORATORY AND FIELD ASSESSMENT

The effects of pH changes on photosynthesis in three Mediterranean sea grass species were assessed by combining laboratory experiments with f ield records of pH. The response of photosynthesis to increasing pH wa s examined under laboratory conditions. Posidonia oceanica and Cymodoc ea nodosa showed a linear decrease in photosynthetic rates with pH; va lues at pH 8.8 were 25-80% of those obtained at pH 8.2. Zostera noltii was much less sensitive to pH increase than the other two species, ma intaining high photosynthetic rates up to pH 8.8 and showing a signifi cant reduction only at pH 9. Daily changes in pH over the seagrass mea dows showed a maximum amplitude of ca. 0.5 pH units. However, the maxi mum daily values of pH were reached towards the end of the daily perio d of photosynthesis, and hence the estimated reduction in photosynthes is caused by the rising pH was relatively small (13-17%), in very shal low(i.e. 1 m deep or less) P. oceanica and C. nodosa meadows, and even less (about 4%) in deeper areas. (C) 1997 Elsevier Science B.V.