How do you know if a coral reef is growing or shrinking?

Dr Ines Lange, marine biologist and postdoctoral research fellow at the University of Exeter, a project partner in the Bertarelli Programme in Marine Science, took part in an expedition to study the coral reefs of the British Indian Ocean Territory.  Here’s her report from the very wet and windy Indian Ocean:

Prof. Chris Perry and myself from the University of Exeter are studying the carbonate budgets of coral reefs around the islands of the Chagos Archipelago. The “Reef Budget” method we use was developed by Chris and calculates how much carbonate is produced by corals and calcifying algae, and how much is eroded by grazing sea urchins and fish, as well as by internal bioeroders such as boring worms and microorganisms. The results provide a metric of reef “health” in terms of whether it is growing or eroding.

The sites in Salomon and Peros Banhos we visited so far show a dramatically reduced coral cover due to the severe bleaching event in 2016, causing carbonate production rates to drop to about a third of the values in 2015. On the upside, there are many Porites and also some Acropora colonies that apparently survived the bleaching, and large numbers of small recruits of different genera. Especially in the understory of the reef structure, we find many live encrusting corals. Also, the substrate is quite clean of macroalgae, thanks to the high abundance of grazing herbivorous fish. Calcareous algae covering the dead coral substrate continue to produce substantial amounts of carbonate which help “glue” the existing structure together and offer a great substrate for further coral recruitment. We therefore hope we can see a fast recovery of the once glorious reefs over the next years.

To investigate local bioerosion rates in the reefs we had a “fun” day sawing 1,000s of blocks from dead Porites skeleton (well, it certainly felt like that, on my last count it was actually 28).

We will deploy the substrates at Peros Banhos, where they will be settled by encrusting and bioeroding organisms. As you can imagine the work days are long, but the company is great and the sunsets quite impressive. Yesterday, a bird tried to land on our heads. Today, I watched eagle rays dancing. New adventures every day…