NICO Expedition: Mesophotic reefs, Cyanobacteria mats and Submarine Groundwater Discharge around the ABC-islands

After setting sail from Texel, The Netherlands in December 2017, the research vessel Pelagia travelled to Aruba, Bonaire and Curaçao where a multi-disciplinary team of researchers explored the island’s mesophotic reefs as part of the NICO (Netherlands Initiative Changing Oceans) expedition. Researchers from the Royal Netherlands Institute for Sea Research (NIOZ), Wageningen University & Research, the University of Amsterdam, Federal Institute for Geosciences and Natural Resources and the Delft University of Technology set out to map the islands’ cyanobacterial mats and uncover the cause of their proliferation, specifically whether anthropogenic nutrients stimulate their growth. A secondary aim was to investigate where and at what rate onshore groundwaters enter the offshore environment, and how this groundwater discharge is affecting the islands’ mesophotic reefs (> 30m deep). With this knowledge, seepage areas of nutrient rich water/pollution sources can be mapped and managed.

Mesophotic reefs are still largely unexplored because of their remoteness or inaccessibility but are believed to be of great ecological value, providing offspring to shallow reef communities that are more prone to climate change and pollution. After searching for a week, the research team was able to find a real mesophotic reef (40 to 100m depth) on the eastern tip of Curaçao, at Awa Blancu. Sadly this area has been nominated to be sold and developed into a large tourist area. No mesophotic reefs were found around Bonaire. Many of the researchers on the team were saddened that the deep reefs they remember diving in their youth appear to have disappeared.

Cyanobacterial mats which “grow like dark filamentous mats on the sand” were located with underwater camera’s off the coast of Kralendijk, Bonaire and Curaçao, at a depth of 55-75 meters. Unfortunately due to the lack of time and problems with equipment no data on mesophotic reefs and deep water cyanobacteria mats was collected around Aruba. “I have been doing research here for a long time, but blue-green algae bloom is really something of the last years" notes Dr. Erik Meesters, coral researcher at Wageningen Marine Research and one of the principal scientists of this research expedition (Buiter, 2018). Cyanobacteria are important primary producers and suppliers of nitrogen within coral reefs (Charpy et al., 2012). In healthy reefs, almost all dissolved nutrients are absorbed by coral polyps and macroalgae. However, disruption of the reef’s delicate system by an abnormally large inflow of nutrients can lead to the proliferation of cyanobacteria mats, which has “serious direct and indirect effects on numerous reef organisms and ecological processes. Some mats overgrow and smother benthic organisms, including scleractinian corals and fleshy algae” (Ford et al., 2018). Cyanobacteria have certain physiological properties that are potentially harmful to coral reefs. They can fix atmospheric nitrogen, bringing additional nutrients into the system, and can produce toxins that are harmful to the animals that consume them.

De Bakker et al. (2017) carried out a study of the reefs at Karpata, Bonaire, and reefs in Curaçao and found that the benthic coral reef community had shifted from a dominance of hard coral and crustose coralline to a dominance of algae and subsequently cyanobacterial mats[1] (de Bakker et al 2017). This new trend was observed all the way down to 40m, although it was less pronounced at depth. De Bakker et al. (2017) suggest that local (eutrophication) and regional (elevated temperatures) stress likely initiated this shift, and that the sudden and sharp rise of benthic cyanobacterial mats is worrisome as the mats reduce the ability of corals to recover from disturbances such as storms and bleaching events.

Erik Meesters, however, does not believe that the deep mats they found are currently a threat to Bonaire’s and Curaçao’s reefs as “they only occur at depth where coral does not catch enough light anymore. Above that, the coral polyps win the competition for the nutrients”. He does however believe that the mats could become a significant issue if nutrient levels in the water increase further, causing the mats to spread over a reef that is already under a lot of pressure from tourism and warming ocean temperatures. This highlights the importance of setting up adequate waste water treatment on both islands.

To investigate the composition and functioning of cyanobacterial mats, Dr. Petra Visser from the University of Amsterdam, with the help of divers, collected samples from the deep mats and took measurements on their photosynthesis and nitrogen fixation. The deep profiles showed that there is a halocline and thermocline between 35 and 60 meters, which may support the favorable conditions for the growth of the deep mats (>40m depth). Due to density differences over the halocline, organic matter sinking from above may settle on the halocline and subsequently settles on the bottom where the halocline intersects with the bottom. This supply of organic matter to the deep cyanobacterial mats probably supports the growth of deep cyanobacterial mats on sandy slopes in front of Kralendijk and elsewhere along urbanized coasts.

The coastline of both Bonaire and Curaçao is primarily made up of limestone formations, and both islands have no rivers. This means that rainwater from land primarily flows into the sea directly during and after rain showers and indirectly via the subsurface as groundwater flow and not via river discharge. Groundwater on both islands is known to be contaminated by inadequate treated waste water which results in elevated nutrient concentrations. While there is no long-term data on nutrient concentrations of reefs on both islands, some indication of eutrophication was found on Bonaire’s reefs in 2014 (Slijkerman et al., 2014); elevated nutrient concentrations were found at Karpata in 2012 and 2013 (De Bakker et al 2017). The hypothesis here is that submarine groundwater discharge (SGD) contributes the eutrophication of waters and to the proliferation of cyanobacterial mats. “These harmful algal blooms are stimulated by environmental factors like light, salinity and nutrient levels such as nitrogen and phosphorus. Nitrogen and phosphorus are found in high concentrations in human waste and wastewater” (Florida Atlantic University, 2018). Based on the distribution of shallow and deep cyanobacterial mats along Curaçao and Bonaire, there is a strong indication that urbanization (assumed to coincide with eutrophication among others) and wave energy (wave height) along the leeward coast plays an important role in the distribution of mats (Brocke et al. 2015). Although correlation between urbanization and occurrence of mats is obvious, the causal agent has not been exactly identified yet.

In order to determine where and at what rate onshore groundwater enters the offshore environment, the research team used photographic and acoustic mapping to map out the bottom topography on both the leeward and windward side of the islands. A small boat was used to get close to shore and find the areas where groundwater flows into ocean. Finding “the relatively small amount of groundwater exiting in the vast ocean was a big challenge”. To detect the seepage of nutrients from groundwater, water samples of the seawater near the seabed were collected, and the salinity, temperature and radon content measured. Water samples were also taken at various depths with the help of an underwater robot (small lander of NIOZ), which collects water close to the bottom for inorganic nutrients and organic matter analyses. In addition CTD profiles (temperature, conductivity and depth measurements) were taken, while sampling contemporaneously water from the bottom to the surface. The team from Delft University of Technology and Wageningen University used the same robot to collect water samples to map the possible groundwater flows around the islands.

The research expedition to Aruba, Bonaire and Curaçao is part of a the NICO expedition (Netherlands Initiative Changing Oceans), a scientific cruise initiated by the Netherlands Organisation for Scientific Research (NWO) and the NIOZ to investigate changing oceans. Over 100 researchers from 20 organizations are participating in the expedition which aims to examine the characteristics of the ocean and the seas. The multidisciplinary character is unique to the expedition. The 7-month expedition sails from the Caribbean Sea, via the Mississippi delta and Ireland back to Texel in the Netherlands and lasts until the end of July 2018. During the trip, geologists, marine biologists, ecologists and oceanographers sail along who have picked up research questions from the Ocean note approved by the Council of Ministers last year. An overview of the projects in the Dutch Caribbbean waters can be found in BioNews-11.

You can track the journey of RV Pelagia here: https://www.marinefacilitiesplanning.com/programme/map

www.nico-expeditie.nl

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