Mass mortality of Desmophyllum diathus in the Comau Fjord

D. dianthus can create dense banks with up to 1500 individuals/m² on vertical and overhanging rock portions, providing habitat for many invertebrates and fishes that benefit from the protection or substratum provided by these structures. It is a cosmopolite species found down to 2500 m depth and is one of the most common and widely distributed deep-sea corals. Studies showed that the skeletal structure and composition can provide information on the climate and oceanography of the Holocene.

Other studies have shown that D. dianthus tolerates relative low pH using an internal pH up-regulation. Studies on age and growth indicate that individuals live up to 100-200 years. Several genetic studies tried to resolve genetic differences at population level, but more studies are necessary in the future.

Due to the technical difficulties that are generally related with deep water coral research, advances are slow and only fragmentary knowledge on their ecology, physiology, reproduction and mortality is available.

Observed mass mortalities of cold-water corals in Chilean Patagonia

The history of cold-water corals in shallow water is not very old (around 12.000 years) and the channels and fjords are known for their dynamic habitats with fluctuations in important environmental parameters. Therefore the coral communities thriving in these habitats may be suspected to have comparably eury-oecious characteristics and/or high capacities of recovery as it is known from the cold-water coral Lophelia pertusa which inhabits shallow waters of Norwegian fjords.

In May 2012, divers from the Huinay Scientific Field Station (see Fig. 1) discovered during a routine monitoring dive that all D. dianthus at a study site close to the Field Station were dead (see Fig. 1, marked as X-Huinay). At the site a recruitment plate array is located that is regularly monitored since 2009 (more information see below) and two racks with transplanted specimens of D. dianthus. All D. dianthus on these experiments had died as well. The extension of the affected area is unknown yet but it extends at least down to 30 m and covers a shore line of at least several hundreds of meters.

Factors causing coral mortality

Cases of mass mortality in tropical shallow coral reefs are well documented. Also for deeper cold-water coral reefs mass mortality has been the subject of studies and publications. For tropical reefs elevated temperatures and mechanic destruction (by storms or through direct human impact) has been identified as the most important mortality factors.

For cold-water corals mechanical damage, mainly through bottom trawling, is the main threat. However, there exist a number of other reasons that have been documented to have caused mass mortality as well: Ocean acidification is predicted to become the most important cause for cold-water coral mortality, especially in deeper habitats. The fjords and channels in Chilean Patagonia are known to have comparably low pH in comparison to other marine areas. Nevertheless, it is expected that the mortality through climatically driven pH shifts would have a slow, gradual and wide-spread effect on the corals, which does not fit with the sudden and patchy death of the corals e.g. in the Comau Fjord (restricted to the site X-Huinay, see Fig. 1).

A former study revealed high tolerance of D. dianthus to low pH in the Comau Fjord, at least partly because of its internal regulation mechanisms.

If the observed death of the corals would be caused by a long-term pH shift this would reflect in the absence of recovery of the coral population.

Diseases are known to cause mass mortalities in tropical Scleractinia and have recently also been documented in cold-water gorgonian corals but generally cause slower and less patchy mortality than observed in the Comau Fjord. Due to the very limited number of places where cold-water corals exist close to the surface and due to their comparably recent discovery, there is no information available that deals with mass mortality of cold-water corals in shallow depths.

The knowledge from mass mortality of shallow water tropical corals cannot easily be adapted to the shallow cold-water corals in Chilean fjords and channels since the conditions and dynamics differ a lot. The same can be said for information deriving from mass mortalities of deep-water corals. Mechanic destruction can be excluded as cause for the death of the scleractinian populations in the Comau Fjord where the corals died mechanically undamaged. Sediment stress is also known to negatively affect corals. Mediterranean cold-water corals were shown to be sensitive on low oxygen contents. Species affected by the mass mortalities in Chilean Patagonia The two observed mass mortalities in the Northern Patagonian Province affected only one of the three present coral species, the cosmopolite scleractinian coral D. dianthus, but specimens of the endemic species Tethocyathus endesa and Caryophyllia huinayensis appear to be not affected or at least to a much lesser extent. Both latter species were observed at the mortality sites in high numbers after the die-off of D. dianthus. In the Copihue Channel Errina Antarctica (see section: Mass mortality of Errina antarctica in the Madre de Dios Archipelago), a stylasterid species which within the cnidarians is taxonomically comparably distant to the scleractinians, was affected. At this latter site, no scleractinian corals were observed. Possible causes of coral mass mortalities in Chilean Patagonia It cannot be excluded that the mass mortalities have similar causes. Nevertheless, due to the fact that not all scleractinian corals at one site were affected and due to the fact that in Canal Copihue stylasterid corals were affected while at the other two sites scleractinian corals were killed, it is even more likely that each mass mortality has individual and independent causes. In the Comau Fjord H₂S rich water deriving from submarine volcanic vents known from the site X-Huinay may have been responsible for the mass mortality. Just before the observed die-off, the activity of the vents increased significantly in May 2012. This was indicated by increased patches of chemotrophic bacteria (Fig. 2) associated to the vents, flocks of chemotrophic bacteria floating in the water and sulfuric smell even on the surface at this site. Here future studies of the Huinay Scientific Field Station (HSFS) aim not only to test this hypothesis but also figure out experimentally why only two of the three scleractinian species were affected.

Fig. 2: Chemotrophic bacteria patches at the dive site X-Huinay, were the mass mortality of Desmophyllum dianthus was observed in 2012.

Consequences of coral mass mortalities to associated fauna

Beside the question of the cause of the mass mortality, these cases offer a unique and immediate opportunity to test the consequences of a mass mortality of the structure-building species for the associated benthic fauna. In the literature it is documented how damages to the corals in tropical reefs generally lead to a decrease in species diversity and less stable ecological conditions. Also for deep water coral reefs it is documented that the destruction of the reef structure leads to decreased biomass and decreased species diversity. But again, there is no literature available that indicates changes in community structure for disturbed cold-water coral banks in the euphotic zone. Future studies of the HFSF aim to address this lack of knowledge by comparing disturbed coral banks with undisturbed adjacent coral banks and, where possible, by comparing community data from before the disturbance with community data after the death of the structure forming coral species.

Recovery of cold-water corals

Another research aim is to look at the patterns, speed and conditions of recovery. Most publications dealing with the recovery of deep-water coral reef communities indicate slow recovery if any at all. Nevertheless, most examples refer to massive mechanical destructions of reefs or mounds that have been growing for hundreds of years and are located in deeper waters with generally low growth rates.

Here, the sites allow comparing mortalities with different time lapse after disturbance and between different affected taxa within cnidarians. For the Comau Fjord site we even have the unique opportunity to continue a monitoring time series that has started three years before the disturbance.

Due to the proximity to the research station we can document succession in comparably short intervals and with optimum methods including the use of high resolution still photography with divers and Remotely Operated Vehicle (ROV) and the deployment and monitoring of transects and recruitment plates. Genetic studies will reveal the origin of coral recruits and thus will help to reveal the potential and limitations of recovery of coral banks after mass mortality. 

Recruitment of Desmophyllum dianthus

For a description of the in-situ growth of the cold-water coral species D. dianthus longterm photo stations (see Fig. 3) were installed in 2009.

Coral recruits were photographed and the size measured regularly. An image analysis is used to reveal the in situ growth. Following a mass mortality of D. dianthus (8.5 km of distance in X-Huinay) fixed photo stations were set up in 2014.

Fig. 3: Fixed photo station at X-Huinay in the Comau Fjord and marked new Desmophyllum dianthus recruit.

The photos are also used to document the changes in benthic community structure. Since some years D. dianthus recruits grow on a water pipe at the Jetty (former aquarium water inlet). After the above-mentioned mass mortality of D. dianthus in X-Huinay, first recruits were discovered in X-Huinay North (See Fig. 3). The recruits were measured and marked in 2015 and recently in order to describe juvenile growth at a second site.

Effect of substrate inclination and light on the growth of hard corals

In order to describe the succession of benthic communities, and especially the recruitment of D. dianthus and growth rates depending on the inclination of the substrate three concrete tubes (1.5 m in diameter) were installed at the sites Jetty in 2009/2010 and Lilihuapi in 2014 (see Fig. 1). Replicated recruitment tiles (see Fig. 4) were attached inside and outside in 15 positions.

Preliminary results show that D. dianthus colonized the succession tiles (see Fig. 5). Additionally, the information is used to describe the succession of the associated community.

Fig. 4: Recruitment tiles one year after deployment (2010, left) and after seven years (2016, right).

Fig. 5: Desmophyllum diathus recruit (arrow) growing on a recruitment tile.

Observed mass mortalities outside of the Comau Fjord

In 2005, a benthic inventory study was conducted in the Guaitecas Archipelago in the Northern Patagonian Zone (South of the Comau Fjord). At two sites in a distance of 23 kilometers diameter only dead specimens of D. dianthus were found. Some of skeletons had sizes that normally correspond to ages of more than 50 years. The status of erosion and overgrowth of all observed dead corals was more or less similar which suggests a simultaneous death. Most skeletons were still hanging on the vertical wall and the status of erosion and the type and age of the epibionts indicated a die-off time not less than 3 years and not more than 7 years before the discovery. Only at one site 2 comparably small specimens of D. dianthus were found alive.