The following report is by PhD candidate Casey Bowden, a researcher at the Reef Function Hub, James Cook University.  Casey is the 2023 Ian Potter Foundation Doctoral Fellow. She will return to LIRS later this year for a final field trip. 

With a changing climate creating disruptions in global ecosystems, it is important to understand how animals live and behave in their ecosystems to inform us of the potential threats of climate change to these organisms. On coral reefs, we have a good understanding of how marine organisms utilise the physical reef structure. However, the whole habitat of coral reefs also includes the water. Water plays many important roles in this habitat. It transports nutrients and sediments, can aid in the recruitment of new organisms, and can even influence how much energy organisms require to live within it. However, there is still a lot to learn about how individual organisms on reefs are influenced by the highly dynamic water flow of marine systems, which is becoming more important to understand given the changing ecosystems.

 

Damselfishes Dascyllus aruanus utilising reef structure and water flow on a coral reef. Photo: Casey Bowden.

 

Understanding how organisms such as fishes rely on and interact with water flows at the scale of their body size will provide insights into how they may behave on future coral reefs with the loss of corals for flow refuging, changes in sediment loads, increases in storms etc. My project aimed to create a new method of recording water flow that is affordable and accessible to make this information more readily accessible and available to inform us how aquatic organisms are impacted by water flow. Using my method, I aimed to record water flow on reefs at a scale that is relevant to an individual reef fish’s behaviour.

A size comparison of the new water measuring device against a traditional Marrotte HS Current meter. Photo: Casey Bowden.

 

These simple and affordable water flow measuring devices utilise a small submersible camera (GoPro) and a ping pong ball. This device lets us measure the speed and direction of water flow at small scales on reefs. The LIRRF Fellowship allowed me to deploy these devices in the field on two field trips to collect important data on the dynamic differences of water flow at the scale of two different groups of reef fishes. Firstly, I deployed my devices to better understand how water flow changes around and above small coral bommies where the damselfish Dascyllus aruanus resided. Specifically, I measured water flow in a 1-meter square on the benthos, 0.5 meter above the benthos and then 1 meter above the benthos; effectively recording a 1m cube of water flow dynamics. This water flow data will provide insights into why small fishes may prefer certain locations around reef structures and up in the water column. I will use this information, paired with behavioural recordings of Dascyllus aruanus, to explore how the behaviour of this damselfish changes with different strengths of and variability in water flow. Secondly, I have deployed these devices in locations where a larger fish species chose to rest on the reef. This data shows how water flow on the reef impacts larger fishes. This data will aid us to model larger-bodied fish distributions and their behavioural choices given the water flow.

 

New water measuring devices deployed in a 1 metre square, 0.5 metres above the benthos. Photo: Casey Bowden.

 

Whilst data processing is ongoing for the interactions of these key fish species with water flow, these new devices have already provided useful data on how dynamic and variable water flow is at fish scales. The complex nature of reef structures strongly shapes water dynamics with 180-degree differences in flow at sites just 35 cm apart. This new method will help us to improve our understanding of this important aspect of marine ecology by providing an accessible means of recording water flow. These data are key to understanding an organism’s behaviours in a complex and ever changing medium.

The research conducted on LIRS was made possible thanks to funding from The Ian Potter Doctoral Foundation. The Ian Potter Doctoral fellowship has been beneficial for this research project, and the progression of my PhD thesis and scientific career. I am deeply grateful for the support provided by this fellowship and for the opportunity to conduct this research at Lizard Island Research Station.