Seagrass meadows have long been known to be highly productive habitats, and as a result producing oodles of oxygen in the midday sun. Anyone who’s ever snorkelled over a seagrass meadow on a sunny day will have seen seagrass leaves furiously bubbling away. This photosynthetic productivity can result in an increase in the pH of the water column (becoming less acidic). This is primarily because CO2 and, thus, its form when dissolved in seawater, carbonic acid, are withdrawn from the water as a substrate for photosynthesis. This results in the production of the bubbling O2. But what are the consequences of such a pH change?
Recent research by the Universities of Dar es Salaam, Tel Aviv and Stockholm published in the Marine Ecology Progress Series (volume 382) and conducted in tropical seagrass meadows of East Africa have investigated the impact of such pH changes. Semasi et al. revealed that this change in pH can cause localised increases in the rates of calcification and growth of calcareous algae such as Hydrolithon sp., Mesophyllum sp., and Halimeda sp., hence seagrass buffers high acidity (low pH).
As has been debated by ClimateShifts previously, there is increasing evidence that oceans have become more acidic since the start of the industrial era. Recent predictions suggest that oceans could become much more acidic over the next 100 years as a result of increasing CO2 emissions. Current predictions suggest that this will result in (amongst other things) declining reef calcification rates.
Although this study by Semesi et al. shows the effects of seagrass upon algae, the questions on the lips of many reef conservationists will be whether such findings are cross transferable to the calcification of corals. These studies in Zanzibar were small scale, carried out in seagrass mesocosms, and currently only reflect small scale patterns. Whether seagrass productivity can result in larger spatial scale changes that could buffer pH changes on nearby reefs remains to be seen. Maybe the World should be looking at seagrass meadows with greater attention?