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Searching for groundwater to protect coral reefs
The world’s seas are changing. The Netherlands Initiative Changing Oceans (NICO) has set up an expedition to enable over 20 organisations to conduct research on the opportunities and risks posed by these changes. They are spending seven months on the research ship Pelagia (owned by the Royal Netherlands Institute for Sea Research) carrying out research in five ocean provinces. Victor Bense from the Hydrology and quantitative water management group is one of those taking part. He is searching for groundwater to protect coral reefs. He, Vincent Post (Bundesanstalt für Geowissenschaften und Rohstoffe) and Boris van Breukelen (TU Delft) posted the following report on the NICO website:
“The third stage of the NICO expedition takes us from Curacao round Bonaire. We are using the trip to get a better understanding of what happens to the nutrients that enter the sea from land. Organisms and micro-organisms need nutrients, particularly nitrogen and phosphate. But if the natural balance of nutrients in the seawater is disturbed, algae or cyanobacteria can flourish, harming the coral reef around the islands. Algae grow too fast, and soon overgrow the corals. Over the next few days, we will examine the effect of nutrients on the marine ecosystem and try to discover which processes determine the prevalence of these nutrients. For this reason, we have a multidisciplinary team of scientists on board: hydrologists, marine biologists and microbiologists.
Groundwater seeps
Our first task was to find out how the nutrients enter the sea from land. The ABC islands (as they are known) are largely formed from fossil coral reefs. Much of the limestone they are made from is fractured and partly dissolved. Rainwater is able to infiltrate this damaged rock, and flow towards the sea. This flux is known as submarine groundwater gischarge. We suspect that a substantial proportion of rainwater enters the sea via groundwater flow and not via rivers at the surface. There are very few rivers on the islands.
High concentrations of nutrients
Much of the groundwater we measured on land on Curacao is polluted with very high concentrations of nutrients, probably due to the use of cesspools and a lack of water purification. In other areas where water flows seawards via sewers or rivers, it is easier for hydrologists to confirm the amounts. But on Bonaire and Curacao, most of the nutrients are carried in groundwater, the flow of which is harder to detect and quantify. How can you measure the volume of polluted groundwater on its way to the sea when you can’t see what is under the ground? This is the key question we are trying to answer from the Pelagia. We are measuring the salinity, temperature and radon levels of the seawater close to the seabed. We would expect to find anomalies in these levels at the spots where the groundwater flows into the sea.
Searching for outgoing water
The first measurements we took during the past few days show that the sea around the islands is stratified. Seawater up to a depth of 60 metres is slightly less saline than water below this depth. This is probably the effect of major sea currents bringing in less saline water from the South American continent. We are trying to get closer to the coast in a small boat (because the Pelagia cannot access these spots) to pinpoint the places where groundwater enters the sea. Subtle disturbances in the electric conductivity, temperature and more specialised measurements taken there are encouraging. Further analysis and more data will give us more certainty. It is proving a huge challenge to find this relatively minor flow of outgoing groundwater in the vast expanse of the ocean.”