Harmful algal bloom killed fish and corals, but transplanted corals fight back
Baseline survey of transplantation site in July 2015- Dr Phanor Montoya-Maya and Reef Rescuers trainee Tess Moriarty
On Friday 23rd October I was making the last arrangements for a conference trip to South Africa. At 8.00 pm, I was finalising my presentations for the 9th WIOMSA symposium. As the Technical & Scientific Officer of Nature Seychelles Reef Rescuers Project, the largest coral reef restoration project of its kind attempted thus far in the world, I was given the task to share the results of our project with scientists, managers and other marine stakeholders from the Western Indian Ocean.
I got a call from the Nature Seychelles’ chief scientist for the project, Dr Sarah Frias-Torres saying there were many fishes being washed up on Cousin Island Special Reserve, where our transplantation site is located. Cousin Island's Science Officer Cheryl Sanchez and Chief Warden Sam Hope had told Sarah that the sea water around Cousin was dark green and that they had counted more than 400 dead fish so far. Dr. Nirmal Shah the CEO of Nature Seychelles confirmed that a harmful algal bloom was taking place and directed us to immediately survey our sites and undertake any other research necessary. Dr. Shah shared a scientific paper that showed coral bleaching caused by harmful algal blooms in Oman and said we should keep an eye out for similar impacts. We arranged a dive for first thing Saturday morning to check any effects on fish in and around the coral reef transplantation site.
Lots of dead fishes were seen floating on the surface water around Cousin Island and surrounding islands.-001
Harmful algal blooms (HAB) are known to cause serious negative impacts to coral reef communities. Last year we transplanted more than 24 000 nursery-grown coral colonies in more than 5,000 m2 of degraded reef in the Cousin island Special Reserve. When we compared a natural healthy site, an untouched degraded site and our transplantation sites the results show that our restoration exercise has resulted in a 600% increase in coral cover, two-fold increase in coral settlement and recruitment, 300% increase in fish species, and a 5 fold increase in fish numbers in just two years. These were the results I was going to present at the WIOMSA Symposium.
Once at the dive site, Cheryl and Sam conducted visual surveys of the transplanted, degraded and healthy reef sites (our three long-term monitoring sites) to document their fish and other communities. Despite a few dead fish on the bottom the sites looked ok. Fish were still seen in good numbers and no corals were bleached. There is some evidence that HABs in other parts of the world have killed lots of coral reef fish and caused bleaching mortality of branching coral such as Acropora and Pocillopora corals. Thus, when we heard that the underwater observations did not reveal any conspicuous changes in the communities at our sites we were relieved. However, we knew more surveys were needed to ascertain the true impact of the HAB.
Many recently dead corals were seen at the degraded and healthy monitoring sites.
At the WIOMSA Symposium in South Africa reef managers and scientists were very pleased to see that we were restoring ecosystems services of our transplanted site but concerned whether the transplantation of our nursery-grown corals enhances bleaching resistance and reef resilience, particularly now that we were facing the third global bleaching event due to a severe 2015 El Nino.
The corals that we transplanted were fragments from donor colonies that survived the bleaching event of 1998, which we believed should increase the bleaching resistance of our constructed reef. Our observations during the bleaching season in Seychelles - February to May - suggest that our transplants bleach less than naturally-grown corals.
The bleaching we have seen thus far was temperature related. A strong El Nino causes ocean temperatures to stay in the high 20°s for long periods of time. If the temperatures stay warm for too long the corals can die from starvation or diseases. Coral bleaching caused by HABs is not temperature related but a consequence of a drastic reduction in Oxygen and what is known as photosynthetic active radiation (PAR) during the HAB event.
No signs of recently dead corals were seen at the Reef Rescuers transplanted site
Ten days after the onset of the HAB, we dived again to conduct an assessment of our healthy, degraded and transplanted sites, as we have been doing since 2012 before coral transplantation started. The first dive at the degraded site, we saw that fish were in low numbers and recently dead corals. The recently dead corals had signs of sudden death with portion of the branches clean white and a thin layer of algae already visible. This was different from our first observations the day after the first signs of the HAB.
On the second dive at the transplanted site our observations were mixed: reef fishes were also low in numbers but there were no recently dead corals or bleaching found. We then dived at the healthy control site where coral cover is highest and there is a more complex coral colony and saw effects of the HAB on coral cover. Recently dead corals were very conspicuous. Many large and small colonies were dead. Although fish appeared in higher numbers than at the two other sites, they were still lower than what we have recorded in the past two years.
Analysis of our survey data revealed that the algal bloom caused extensive coral bleaching. We also found that our coral transplants responded better to the stressful conditions caused by the HAB. No dead colonies were observed at the transplanted site. This finding is remarkable. We don't rule out bleaching at the transplanted site but for some unknown reason they appear to recover faster and better than corals at other sites.
Ocean water temperature at 5m around Cousin Island before and after the HAB event.
We also looked at temperature profiles before, during and after the HAB event in an attempt to rule out above normal sea temperatures that could have caused the bleaching. The data showed that water temperature had a steady decrease from October 19th to 31st followed by a sudden increase by November 4th. Worryingly, we also found that water temperatures in October and November 2015 have been consistently higher than the last five year average. Although more studies are required, water temperature at the healthy site was consistently higher than the transplanted site during the HAB event, which might explain the differential response in bleaching mortality between the two sites.
It is very likely that the reduced number of fishes and recent coral mortality were the result of the HAB event in the Seychelles. With the reduced number of herbivore fishes, which control algae on the reef, and the mortality of corals, the reef has weakened and might not be able to respond positively to the expected warm ocean temperatures that the 2015 El Nino event will bring. On the contrary, our observations show an extraordinary response of our "engineered" site. This is a very promising result that adds support to Nature Seychelles’ novel theory that transplanting bleaching resistant colonies enhances the resilience potential of coral reefs in the face of climate change.
By Dr Phanor H Montoya-Maya
Scientific Officer, Nature Seychelles Reef Rescuers Project
