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

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 Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 382: [41][42][43][44][45][46][47] 2009 and Middelboe & Hansen 2007 for shallow macro-algal habitats). Also, seagrasses can increase the pH of the surrounding seawater; Invers et al. (1997) showed a pH increase of up to 0.5 units in seagrass meadows, which negatively affected photosynthetic rates of 3 seagrasses (Posidonia oceanica, Cymodocea nodosa and Zostera noltii). Similarly, Beer et al. (2006) showed that dominant species from a tropical bay could raise the pH to 8.5-9.2 and suggested that certain species (in this case Halophila ovalis) were excluded from growing in mono-specific tidal pools with other species (e.g. Thalassia hemprichii) because they could not photosynthesise at the high pH values generated.One effect of seawater pH that has been studied more than the effects of photosynthesis and growth is calcification (e.g. Yates & Halley 2006). However, most studies of macrophytes considered mainly the negative effects of decreasing pH as brought about by the projected future enrichment in atmospheric and, accordingly, dissolved CO 2 (Smith & Roth 1979, Borowitzka 1981, Gao et al. 1993, de Beer & Larkum 2001. While some higher pH values have also been tested (Smi...

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