NICO Expedition: Mapping the Sea Bottom
Researchers aboard the Pelagia research vessel have been collecting invaluable data on marine biomes in the Caribbean since January as part of the “Netherlands Initiative Changing Oceans (NICO)” expedition organized by the Royal Netherlands Institute of Sea Research (NIOZ) and NWO-Science (ENW). The research vessel has visited the Southern Caribbean where work focused on the mesophotic reefs (> 30m deep) of Bonaire and Curaçao. Bathymetric maps from the 1970s did not offer enough detail for the research team to locate the reefs and cyanobacterial mats, which were the focus of their studies. Detailed bathymetric data was absent for other Dutch Caribbean islands, including the Saba Bank and bathymetric maps will therefore be created. Dr. Henk de Haas, an acoustic researcher and data scientist of the research institute NIOZ who is onboard the Pelagia, has provided insight about how he goes about creating these maps that are so crucial to the success of among others research.
Bathymetry is the measurement of depth of water in oceans, seas, or lakes. Bathymetric maps provide a visual representation of the topography of the sea floor including the shape and elevation of underwater features like seamounts or ocean trenches. These maps are crucial to ocean research as they enable scientists to locate the specific ecosystems which are being investigated. For example, coral reefs are not found in areas with strong water currents and would therefore not be located in underwater valleys where water flow is powerful.
In the past, sea depth was measured using a type of sonar called an echo sounder. A sound pulse would be sent out by a transmitter located on the hull of the ship. The longer it took for sound to travel to the sea floor and back to the receiver on the ship, the deeper the ocean floor. The problem with this technique is that just one measurement can be taken at a time, making the mapping of the sea floor very labor intensive and not very accurate. “We would very often have to sail lines back and forth to make a map of a piece of seabed”, explains Dr. de Haas.
Nowadays, acoustic scientists use a modern multi-beam version of the echo sounder. The model installed underneath the Pelagia transmits 288 sound beams per pulse simultaneously in a fan shape instead of just one vertical pulse. The multibeam echo sounder generates fast and accurate bathymetric measurements and allows the creation of detailed topography maps. In fact, because many of the bundles take two measurements simultaneously, Dr. de Haas notes that the system on board the Pelagia allows for at maximum 432 depth measurements to be produced per sound pulse. The size of the area mapped is dependent on water depth - the width of the bundle is about five times the water depth, meaning that the bundle will be narrower in shallow waters. The multibeam echo sounder can also help determine the nature of the seabed and whether there is soft or hard sediment. If the sediment is hard, such as sand, the sonar signal will come back stronger. As explained by Dr. de Haas, “a map of the strength of the reflected sound signal is actually a map of the sediment on the seabed”.
So every night, in cooperation with the crew member at the bridge Dr. de Haas stays up collecting data from the Pelagia’s multibeam echo sounder while everyone else is sound asleep. The next morning, after checking the data, he is able to create the bathymetric maps that the research team needs to locate mesophotic reefs and cyanobacterial mats around Bonaire and Curaçao. So far, the maps have resulted in some exciting discoveries, notably the presence of deep channels around Curaçao. This was a real surprise as such deep channels are not always common for small islands. As Dr. de Haas concludes, “there is still much to explore here”.
You can track the journey of RV Pelagia here: https://www.marinefacilitiesplanning.com/programme/map
This news-tem was published by DCNA in BioNews 13-2018.