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Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow
Stockholm University, Faculty of Science, Department of Botany. University of Dar es Salaam, Tanzania.
Tel Aviv University, Israel.
Stockholm University, Faculty of Science, Department of Botany. University of Dar es Salaam, Tanzania.
2009 (English)In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 382, 41-47 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2009. Vol. 382, 41-47 p.
Keyword [en]
calcareous algae, calcification, Halimeda sp., Hydrolithon sp., Mesophylum sp., photosynthesis
National Category
Botany
Research subject
Plant Physiology
Identifiers
URN: urn:nbn:se:su:diva-27477DOI: 10.3354/meps07973ISI: 000266474900004OAI: oai:DiVA.org:su-27477DiVA: diva2:214422
Available from: 2009-05-05 Created: 2009-05-05 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Seawater pH as a Controlling Factor in Macroalgal Calcification and Photosynthesis
Open this publication in new window or tab >>Seawater pH as a Controlling Factor in Macroalgal Calcification and Photosynthesis
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biological calcification and photosynthesis are important processes with a great influence on both structure and function of oceanic ecosystems. The pH of the seawater has a strong influence on both these processes and therefore the impacts of different pH levels on calcareous macroalgae were investigated in laboratory and field experiments at Chwaka and Fumba Bays in Zanzibar, Tanzania. The levels of pH were manipulated, first in the laboratory by adding HCl, NaOH or by bubbling seawater with CO2 enriched air. Secondly, pH was allowed to fluctuate naturally as a consequence of marine photosynthetic carbon uptake or release through respiration by mussels. The effects on both photosynthesis and calcification were then analyzed on a seagrass (Thalassia hemprichii), and the calcareous red and green algae Mesophyllum sp., Hydrolithon sp., Amphiroa fragilissima and Halimeda renschii, as well as on the mussel Pinna muricata. The laboratory studies revealed a significant decrease in calcification rates in Hydrolithon sp. with decreasing pH, while photosynthesis showed an opposite trend. Also, increased dissolved CO2 lowered pH from 8.1 to 7.8 and caused a ~20% decline in calcification rates. In the field, seagrasses raised pH to ~9, increasing calcification rates significantly in the calcareous algae while photosynthetic rates showed no significant differences with changes in pH expect in Mesophyllum sp., in which rates increased at elevated pH caused by the presence of seagrasses. Conversely, seagrass photosynthesis increased significantly in the presence of mussels. Based on these findings, we conclude that pH is important in shaping biological processes that determines ecological interactions within shallow tidal areas by modifying seawater carbon composition and, thus, influencing calcification and photosynthesis processes.

Place, publisher, year, edition, pages
Stockholm: Department of Botany, Stockholm University, 2009. 47 p.
Keyword
Calcareous macroalgae, CO2, mussels, pH, seagrasses, tropical lagoons
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-27479 (URN)978-91-7155-887-9 (ISBN)
Public defence
2009-06-04, föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2009-05-14 Created: 2009-05-05 Last updated: 2009-05-05Bibliographically approved

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