NICO expedition: eddies

NICO expedition: eddies

Rising ocean temperatures associated with global warming are changing our oceans and are projected to have a considerable impact on sea level, ocean acidification, hurricanes and bleaching of coral reefs over the next decades. The research vessel RV Pelagia has been at sea since December 2017 for a 7-month multidisciplinary scientific journey, the “Netherlands Initiative Changing Oceans” (NICO)expedition, with the aim of obtaining a better understanding of changing seas and oceans (NIOZ, 2017). While sailing from Aruba north to Hispaniola and then east to St Maarten, the fourth leg of the expedition focusedon eddies and their influence on the distribution and occurrence of pelagic megafauna in the Caribbean, more specifically marine mammals and seabirds. Anti-cyclonic eddies typically have surface waters that are 4 ̊C warmer than the surrounding ocean and “form partially isolated environments with distinct physical and chemical conditions” (NIOZ, 2017). Changes in biological activity within them may provide insight as to how global warming will impact these species(NIOZ, 2017).

Eddies are rotating bodies of water that spawn from meandering, unstable currents, creating a swirling motions in the ocean waters (NOAA, 2017). Mesoscale eddies, which are common in the Caribbean Sea, are large eddies with a horizontal scale of approximately 100 kms and last for several months (van der Boog, 2018; Adcroftet al., 2017; SFSU, 2018). Their impact on the ocean environment is substantial. As the center of eddies contain water with properties that differ from their environment, they are important for the heat and salt transport in the ocean. Depending on their direction of rotation, mesoscale eddies can transport water up or down. In the case of upward transport, this will favor upwelling of nutrients from deeper levels to the surface (SFSU, 2018). Such mesoscale eddies have been described as hot spots of intense biological and physical activity (Michaels, 2007) that cansupport and transportwhole plankton communities (NIOZ, 2017; SFSU, 2018) and help supply nutrients to the surface of the ocean as well as coastal zones (Adcroftet al., 2017). The reverse may be true for eddies that favor downwelling, which is why NICO 4 also sampled for nutrients and biological parameters. Overall, the eddies also have an important role in regulating the weather in the region by transporting heat from the tropics to the poles (Adcroftet al., 2017), and may contribute to the intensification of hurricanes in the Caribbean (van der Boog, 2018). Satellites (SFSU, 2018) can be used to track and study eddies at the ocean surface, but for the eddies in the Caribbean there is very little data about their vertical structure and whatunderlying processes govern their development (van der Boog, 2018). 

Before departure of the RV Pelagia from Aruba, the research team used recent satellite data and ocean model forecasts to chart the circulation in the Caribbean Sea and was able to locate a mesoscale eddy that formed off the coast of Venezuela and moved east towards Aruba. The research vessel then sailed to the eddy and navigated through its center while taking measurements (van der Boog, 2018). Four autonomous floats were deployed inside and outside the eddy, which will continue to measure the temperature and salinity of the Caribbean waters. The float data will be combined with satellite observations to keep track of the eddy’s location over the following months and to learn more about the differences in water properties between eddies and the ambient waters (Heinsman, 2018b). 

To assess the occurrence of pelagic megafauna, the research team used visual surveys as well as passive acoustic monitoring for whales(NIOZ, 2017). Previous studies have found that both permanent physiographic features (ocean depth, seafloor slope) and hydrographic characteristics influence the distribution of prey and therefore pelagic megafauna, however the specific influence of eddies on the distributionand occurrence of organisms at a higher trophic level has never been studied (NIOZ, 2017).Unfortunately, visual surveys for whales and dolphins on board the RV Pelagia had little success due to rough seas that made it hard to spot anything in the water. The few marine mammals that were recorded were dolphins that were attracted to the ship and accompanied it for a little while. 

Throughout the journey from Aruba to St. Maarten, a team of bird experts surveyed the seabird population to assess whether the presence of an eddy affects bird density. Steve Geelhoed and Mardik Leopold, both marine ecologists at Wageningen University, observed a very small number of seabirds and wondered where all the birds have gone(Buiter, 2018). Ruud van Halewijn described a rich bird life for the area in the seventies (Buiter, 2018), however the total number of seabirds spotted was very small and included some Brown Boobies, a few Black-capped Petrels and Royal terns. What was even more confusing was the plentiful presence of flying fish, meaning that seabirds have an abundant source of food. Geelhoed and Leopold believe that changes on land rather than at sea are to blame, notably the drastic reduction of seabird breeding habitat on the islands to make way for tourism development, as well as an increased presence of introduced predators such as rats, cats and mongoose that are very fond of bird chicks and bird eggs (Buiter, 2018)."In this case”, explains Leopold, “I think we should look not so much at the oceans but at the dramatic changes on the Caribbean islands. The bigger problems for the seabirds seem to play there. " (Buiter, 2018)

It was not all bad news, though. Geelhoed and Leopold were excited to spot not one but twelve of the very rare and almost extinct black-capped petrel. Only very few remaining colonies for this seabird are known in Haiti and the Dominican Republic, where the population is greatly endangered and hard to study because the petrels breed on steep slopes about 400 to 1200 meters above sea level (Heinsman, 2018). The unexpected discovery shows that in an age of environmental devastation and loss of species, there is still hope.

This news-tem was published by DCNAin BioNews 14-2018.