21. April 2015 we arrived at Salomon Atoll (Chagos) with great expectations. After Malaysia, Thailand and the Andaman Islands we had nearly given up hope of finding truly healthy coral reefs in our livetime, however this hope was significantly raised after our visit to the Maldives. There many reefs were in a remarkable state of recovery, how much more so would they have to be in the largest no-take marine reserve of the planet (or so we thought).
It came as a shock then to see shining white and yellow coral colonies shimmer throught the surface when we entered the reef pass of Solomon Atoll. The first impression was confirmed when we put on our snorkel gear (scuba diving is prohibited for us ordinary sailors) and jumped into the water for the first time. There was a severe bleaching event going on and apparently it had just started - the sight was actually beautiful with the corals still alive, gleaming in all colors of the rainbow.
Healthy reef building shallow water corals have unicellular algae as symbionts. The job of the algae is to produce sugar by photosynthesis, they therefore need clorophyll and associated pigments - which makes them greenish brown - which in turn makes healthy coral appear rather brownish (with few exceptions).
Corals also possess pigments themselves, mostly to regulate the amount and quality of light that reaches the symbionts and to protect them (e.g. from too much UV radiation).
Stressed corals (e.g. because of higher than usual temperatures for prolonged periods of time) expel their symbiont algae - they "bleach". So what we were seeing were white corals (in species that don't have any pigmentation so you see the white limestone skeleton shine through), or bright pink, yellow or blue ones. Actually, a bleached coral reef is a really pretty sight ...
Andrew Bruckner, PhD, the lead scientist of the B.I.O.T. coral reef research group who also were at the area summed up the situation beautifully in his blog. According to him widespread bleaching started around 15. April. The weather during this period was unusually calm and the water temperature with 30°C to 31°C around 2°C above average.
We only find it hard to share his optimism in the last paragraph, where he states that the reefs are likely to bounce back. On our extended snorkel trips we found lots and lots of hopeful young colonies, especially at the "spur and groove" formations of the outer reef. Apparently they were recruits that colonised the reef after the 2010 bleaching event. It was heartbraking to see an estimated 90% of them dead or dying. The Chagos marine reserve probably has about the highest resilience to events like this that can be imagined. No pollution, abundant fish stocks including predators and herbivores, no pressure from fishing industry or tourism. But what chance do the reefs stand against the increasing bleaching frequency? After the severe coral die-off of 1998 it took until 2010 for the next devastating event - and only 5 more years until now ... Particularly with the current El Niño forecast to last until next year this makes it hard to look optimistic into the future.
However, Bernhard Riegl, PhD, shares Andrew Bruckner's optimism - so we very much hope that we are wrong. These guys are real coral experts after all, and know what they are talking about.
Anyway, we soon decided that despairing in this paradise with it's fantastic colonies of seabirds and unspoilt beaches wasn't an option. So instead of keeping on whining we chose to try and do something useful. As sailors we have the great privilege to be able to stay at the location for 4 weeks - a great opportunity to document the development of stressed corals. We chose 4 sites where we took pictures of marked colonies at intervals of about a week.
Site 1 and 2 were isolated coral bommies inside the lagoon, site 3 was a little bommie close to the back reef of Ile Poule and site 4 was on the northward facing outer reef flat close to Ile Diable:
Panoramic images of three of the monitoring sites:
Of 35 marked corals, 1 colony actually showed signs of recovery (so for some positive news scroll to the bottom), 16 didn't show any signs of change during the observation period, and the condition of 18 was clearly deteriorating. They were bleaching further, were dying and being overgrown by algae:
Colonies showing no or only slight signs of change during our observation period:
Our "hope colony", a large and ancient Porites lobata. This colony is probably up to 400 years old and has survived many major bleaching events. The time series images clearly show how pigmentation is increasing. Superficially this could also be due to overgrowing algae, a close examination however showed that the polyps where extended and alive, the additional pigmentation clearly due to returning endosymbionts.