Understanding the Mesophotic Zone

Image: Temperature loggers after two years left at 60 metres and 80 metres depth



I’m Norbert Englebert, a PhD candidate in my final year at UQ working on mesophotic or “twilight zone” coral reef communities. My interest in this largely unexplored and understudied area of coral reef ecology started seven years ago during both of my MSc projects in the Caribbean and on the Great Barrier Reef focussing on physiology and genetics of light-dependent corals that occur over large depth ranges. At the start of my PhD, it became clear to me how little understanding we actually have about the biodiversity and extent of mesophotic reefs, especially in the depth range of 60-100 m, and the role these deep reefs may play for the overall reef ecology.  



Limiting factors for light-dependent coral communities


During the very first Catlin Seaview Survey expedition to the Great Barrier Reef in 2012, we had the opportunity to visit multiple locations along the outer reefs to look at the depth distribution of light-dependent coral communities and answer questions about what types of coral, if any, survive and thrive in these light limited environments. Finding out that a surprisingly diverse array of light-dependent corals exist in these deeper regions, up to depths of 125 m for the Great Barrier Reef, fascinated me and threw up a whole lot more questions. One of the key questions I would like to answer is what are the limiting factors determining these depth distribution limits for light-dependent coral. This question is quite complex as it might be a combination of different factors, such as temperature, light availability, sedimentation and substrate availability, acting in isolation or together to influence coral survival.



Deploying long-term temperature loggers


To determine if temperature regimes on the deep reef are a major factor limiting the depth distribution for corals on the GBR, we decided to try and record the water temperatures to which these deep coral communities get exposed to. Deploying long-term temperature loggers on the reef down to 100m is really challenging as this can only be done by remotely operated vehicles (ROV’s). Myself and Dr Pim Bongaerts spent many weeks developing methods and means to be able to deploy these long term temperature loggers on our main study sites. And during the Catlin Seaview Survey expeditions of 2012 we were able to deploy all temperature logger stands successfully with the ROV.



Revisiting the deep reef


This year’s fortuitous partnership with the Waitt Foundation enabled us to return to several of our study sites on the northern Great Barrier Reef. We revisited our main study sites at Great Detached Reef, Tijou Reef and Yonge Reef during a wonderful two-week expedition. Thanks to great weather in the far northern section, and a small window of opportunity at Yonge Reef (the wind died down for a couple hours), we were able to deploy the ROV at the exposed windward sides of the reefs to retrieve our temperature loggers that have been recording for a 15-month period. Retrieving the loggers was very tricky with all the loggers completely overgrown by sponges and algae. But great team work from the boat crew, ROV pilot, and good preparation of exactly where to find the loggers on these extensive reefs, resulted in successful retrieval of all loggers deployed in 2012. Now the next step will be to analyse the retrieved temperature data and combine it with the coral depth distribution data to see if temperature (e.g. cold oceanic water influxes) might be a limiting factor for coral distribution for our study sites on the Great Barrier Reef.