The effects of pH changes on photosynthesis in three Mediterranean seagrass species were assessed by combining laboratory experiments with field records of pH. The response of photosynthesis to increasing pH was examined under laboratory conditions. Posidonia oceanica and Cymodocea nodosa showed a linear decrease in photosynthetic rates with pH; values 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, maintaining high photosynthetic rates up to pH 8.8 and showing a significant reduction only at pH 9. Daily changes in pH over the seagrass meadows showed a maximum amplitude of ca. 0.5 pH units. However, the maximum daily values of pH were reached towards the end of the daily period of photosynthesis, and hence the estimated reduction in photosynthesis caused by the rising pH was relatively small (13-17%), in very shallow (i.e. 1 m deep or less) P. oceanica and C. nodosa meadows, and even less (about 4%) in deeper areas.
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