In late 2013, the Lady Elliot Island Eco Resort teamed up with Southern Cross University to investigate the island’s aquifer, which we believe plays an important role in supporting the island’s vegetation, and supporting the teeming marine life in the waters around the island. We know that trees such as the island’s iconic pisonia are able to survive extended dry spells by putting down roots that tap into the aquifer. Research carried out in recent years has led to the realisation that a coral cay’s aquifer is a large reservoir of nutrients (nitrogen and phosphorus), regularly recharged by seabird poo, and that the aquifer drip-feeds the nutrients to the surrounding marine waters. This may well explain the long standing mystery of why islands like Lady Elliot Island support host such incredible marine biodiversity, when the oceans around the island are known to be nutrient poor.
We wanted to better understand the Lady Elliot Island aquifer, and by early 2014 we had managed to drill six boreholes around the island, finding the aquifer at a depth of 3 to 4 m. Coral is hard, so the drilling work took a lot of effort and ingenuity by the resort’s maintenance people, with especially big applause for Laurie Marsden and Kendall Steers.
What have we found so far? Well, this past summer has seen the island experience one of the longest dry spells ever recorded by the weather station, which was established in 1939, but incredibly we still found a thin layer of fresh water on the surface of the aquifer in the middle of the island. And laboratory analyses of borehole samples confirmed that the aquifer indeed is a reservoir for nutrients – it holds something like 30 tonnes of nitrogen in the form of nitrate – hardly surprising when you remember that the island was strip-mined for guano in the late 1880s. We estimate that seabird poo provides the aquifer with some 10 to 20 tonnes a year of nitrogen, and of course what goes into the aquifer has to come out. The way this happens is fascinating. Coral is very porous and very permeable, and the regular rise and fall of the tide around the island act to pump the nutrients out of the aquifer. Seawater enters the island at high tide, mixes with the aquifer, and when the seawater drains out of the island at low tide it brings nutrients with it. This is known as tidal pumping, and is one of several mechanisms that help to drip feed the nutrients to the waters surrounding the island.
Where to now? Well, we’re planning a lot of follow-on work, including a geophysical survey of the island to map out the groundwater resource, studies using naturally occurring groundwater tracers (e.g. radon) to better understand the drip-feed processes, nitrogen isotope studies to quantify the nutrient cycle, and we’d also like to develop a computer model of the aquifer that will help us to understand how it responds to tides and to recharge by rainfall. How much fun is this going to be?
Steve Carter, Dirk Erler, Peter Gash, Jess Miller, Laurie Marsden, and Kendall Steers.