Can corals recover at D’Arros Island?
There is evidence that the ocean might flush the coral reefs of D’Arros Island in the Seychelles with cold water. This could provide vital relief from rising sea temperatures and potentially render corals resilient to coral bleaching events. Phil is deploying oceanographic moorings to track temperature, current flow and particles in the water (which may contain coral larvae). He wants to understand whether the ocean’s internal waves are indeed bringing cooler water and resupplying bleached surface reefs with new coral larvae from deeper (mesophotic) reefs. The results of his research will help prioritise the most critical reefs to protect and build a framework to monitor them.
Phil Hosegood
I’d love to say that I grew up with an innate fascination for ocean physics, but I didn’t. I’ve always loved to be in and, even better, under water, but it was only after my first university degree that I started to become fascinated by what makes the oceans move and how these currents affect both the climate and the behaviour of marine animals and organisms. An MSc in marine science from the University of Plymouth in 1998 got my foot in the door to study for a PhD in the Netherlands and then on to Seattle for a post-doc...
Coral bleaching and species zonation in the internal wave surf zone
Our general aim is to relate internal wave breaking on submarine slopes to coral reef survival during bleaching events and to the zonation of benthic species with depth due to the prevailing hydrodynamic regime.
Previous results have demonstrated that short period temperature fluctuations are responsible for some coral reefs avoiding bleaching but the attribution of these changes in temperature to internal waves is currently lacking. From a physical oceanographic perspective, internal waves are well understood but no-one has yet made the clear link with coral reef survival rates. We have an unprecedented opportunity to address this due to the previous observations of temperature and coral reef health at D’Arros so it is critical that we act in a timely manner to provide clear evidence of the role played by internal waves on coral reef health so that their effects can be accounted for in conservation plans.
Coral reefs constitute the bed rock of tropical ecosystems by offering a refuge for smaller species and by consequence, productive foraging grounds for predators. Our evolving climate is presenting significant challenges for these critical communities, demanding a more sophisticated understanding of the mechanisms that influence both the health of the corals and the species that depend on them for their survival over relevant timescales.
Internal waves are similar to surface waves but, rather than propagating along the interface between the air and the sea, they are supported by vertical density stratification in the ocean interior whereby the density of seawater increases with depth. Typically, this is due to the effect of temperature such that, due to the effect of the sun, warm water is found at the surface and becomes progressively cooler with depth. The overall importance of internal waves to coral ecosystems arises from the upwards displacement of cold water from depth over the slope, relieving the thermal stress experienced by corals towards the surface where water temperatures are highest.
Coral communities are widely understood to suffer widespread bleaching during warming events (e.g. during 1998 in Seychelles) when the zooxanthellae are expelled from the coral tissue. The increase in sea temperature typically occurs on interannual timescales and is driven by planetary scale processes, such as El-Nino. Over recent years, it has become apparent that higher frequency temperature fluctuations, on timescales of hours to minutes, diminish the severity of coral bleaching by reducing the thermal stress experienced by the coral community. It has been suggested that spatial heterogeneity in bleaching on scales of <1 km is governed by high frequency temperature variability. A 19C decrease in daily temperature range was proposed to reduce the odds of bleaching by a factor of 33 but no suggestion was made of the mechanism that leads to the decreases in temperature. To date, the only explicit evidence presented for the role played by internal waves in relieving thermal stress was in the Andaman Sea where large amplitude (>100 m) waves were proposed to explain significant regional variations in coral bleaching. It remains to be established, however, whether internal waves with more common, smaller amplitudes (<20 m) have significant effects throughout tropical regions.
In the Seychelles, we will monitor and analyse the internal wave field around D’Arros and St. Joseph, where we have an unprecedented opportunity to highlight the importance of internal waves to coral health due to prior knowledge of both the coral health and presence of internal waves. Previous data collected in 2012 demonstrate that the slope north of D’Arros is subject to periodic, short-lived reductions in temperature, whilst the coral community remained healthy in the face of warming. During the same time period, a site at the same depth to the south of St. Joseph exhibited significantly less temperature variability and the corals showed significant signs of bleaching. At this initial stage, we hypothesise that internal waves are generated at Eagle Island, 30 km to the north. The waves radiate away from the source region, in this case propagating southwards towards D’Arros. As the waves reach the sloping sides of D’Arros, they steepen and break just as surface waves do on reaching a beach. Cold water from depth is forced up the submarine slopes, flushing coral higher up the slope with cooler water than would otherwise be experienced at a given depth. This cool water provides the coral with temporary relief from the thermal stress, which we hypothesise, is sufficient to reduce the occurrence and/or severity of bleaching.
- Monitor the internal wave field to identify timescales of variability and depth ranges of influence
- Survey the mesophotic reef to identify zonation in benthic species arising from internal wave activity
- Correlate the presence of internal waves to reef health and potential survival