MEPS 382:41-47 (2009)  -  doi:10.3354/meps07973

Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow

I. Sware Semesi1,2,3,*, Sven Beer1,4, Mats Björk1,3

1Institute of Marine Sciences, University of Dar es Salaam, PO Box 668, Zanzibar, Tanzania
2Department of Aquatic Resources and Conservation, University of Dar es Salaam, Box 35064, Dar es Salaam, Tanzania
3Botany Department, Stockholm University, 10691 Stockholm, Sweden
4Department of Plant Sciences, Tel Aviv University, Tel Aviv 69978, Israel

ABSTRACT: Diel fluctuations in seawater pH can be >1 pH unit (7.9 to >8.9) in the seagrass meadows of Chwaka Bay (Zanzibar, Tanzania). The high daily pH values are generated by the photosynthetic activity of the bay’s submerged seagrasses and macroalgae, and maintained by the relatively low, tide-dominated, water exchange rate. Since pH in principle can affect rates of both calcification and photosynthesis, we investigated whether diel variations in pH caused by photosynthesis could affect rates of calcification and photosynthesis of the calcareous red (Hydrolithon sp. and Mesophyllum sp.) and green (Halimeda renschii) algae growing within these meadows. This was done by measuring rates of calcification and relative photosynthetic electron transport (rETR) of the algae in situ in open-bottom incubation cylinders either in the natural presence of the rooted seagrasses or after the leaves had been removed. The results showed that seagrass photosynthesis increased the seawater pH within the cylinders from 8.3–8.4 to 8.6–8.9 after 2.5 h (largely in conformity with that of the surrounding seawater), which, in turn, enhanced the rates of calcification 5.8-fold for Hydrolithon sp. and 1.6-fold for the other 2 species. The rETRs of all algae largely followed the irradiance throughout the day and were (in Mesophyllum sp.) significantly higher in the presence of seagrasses despite the higher pH values generated by the latter. We conclude that algal calcification within seagrass meadows such as those of Chwaka Bay is considerably enhanced by the photosynthetic activity of the seagrasses, which in turn increases the seawater pH.


KEY WORDS: Calcareous algae · Calcification · Halimeda sp. · Hydrolithon sp. · Mesophyllum sp. · Photosynthesis · Halimeda renschii


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Cite this article as: Semesi IS, Beer S, Björk M, (2009) Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow. Mar Ecol Prog Ser 382:41-47

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