Hot spots in the Ocean
By Sheena Fennell, School of Natural Sciences, NUI Galway. Come and meet more amazing women scientists at Soapbox Science Galway on Saturday 15th July 2017.
I had the opportunity between 2011 and 2016 to take part in multidisciplinary transatlantic research cruises from Ireland to Newfoundland. Although the R.V. Celtic Explorer had previously crossed the Atlantic at the equator the crossing in 2011 was the first time it crossed the North Atlantic. I went on board to carry out temperature profiles down to a depth of 1800m every 50km. Also during this cruise fisheries acousticians from Canada were on board logging data on the distribution of the Deep Scattering Layer. After this initial trip we started to look at both data sets and found some interesting results. We decided to look more closely at the data and organise the surveys around a number of hypothesis. I have always enjoyed the science between the interaction of oceanography and biology and this was a fantastic opportunity to do research in this area.
My area of research was to look at how the oceanography influences the distribution of the Deep Scattering Layer (DSL). The Deep Scattering Layer is a layer of small fish that is generally found between 200-800m deep and has been found in many of our oceans. A large component of the DSL contains myctophids (lanternfish).
Myctophids from the Deep Scattering Layer. They range from 2-30cm in length. Photo: Brynn Devine,Fisheries and Marine Insitute, Memorial University of Newfoundland.
We had the opportunity to study the distribution across the Atlantic and also to focus in on their distribution in warm core eddies.
What are ocean eddies?
Ocean eddies form in the Western Atlantic because of the meandering nature of the Gulf Stream. They are formed in a process similar to the way an oxbow lake forms in a river and a great animation on how ocean eddies form can be found at http://tornado.sfsu.edu/geosciences/classes/m415_715/Monteverdi/Satellite/Oceanography/eddy.htm
These eddies can be over 100km in diameter and rotate either clockwise or anticlockwise depending on whether the center of the eddy comprises of either warm or cold water. These systems have been previously described as oasis in a desert (Godo et al., 2012) and the eddies we found between 2013-2016 very much fit this description.
How did we sample these systems?
We tracked them using Sea Level anomalies (changes in sea surface height) as we transited across the North Atlantic aboard the RV Celtic Explorer. The eddy that we sampled in 2015 can be seen between 35-45W below.
2 Sea Level Anomaly (m) downloaded from AVISO (Archiving, Validation and Interpretation of Satellites Oceanographic Data) and overlain with sampling stations (black dots) for the 2015 cruise.
Once within the eddy we ran fisheries acoustics, took vertical profiles of salinity (how salty the water was), water temperature, nutrients and water current speeds. We also took samples for picoplankton, zooplankton as well as deploying nets to fish in the Deep Scattering Layer. We also had seabird and cetacean (dolphin and whale) observers on board.
What did we find?
The three eddies we studied over the three years had water temperatures between 12 and 15° C and were bounded on the west by 2°C water. This is a huge difference in temperature over such a small area! These features have a large increase in the concentration of Deep Scattering Layer and seem to provide a perfect feeding ground all the way up the food chain to cetaceans and birds. We also found that annual changes in water temperatures across the Atlantic had an influence on the concentration of the DSL. This is important to understand as our climate changes and how much this will have an influence on the most abundant fishes in the world (Irigoien et al., 2014).
Godo, O.R., Samuelson, A., Macaulay, G.J., Patel, R., Hjollo, S.S., Horne, J., Kaartvedy, S., Johannessen, S.A., 2012. Mesoscale Eddies Are Oases for Higher Trophic Marine Life. PLoS ONE 7(1): e30161. doi:10.1371/journal.pone.0030161.
Irigoien, X., Klevjer, T.A., Rostad, A., Martinez, U., Boyra, G., Acuna, J.L., Bode, A., Echevarria, F., Gonzalez-Gordillo, J.I., Hernandez-Leon, S., Agusti, S., Aksnes, D.L., Duarte, C.M., Kaartvedy, S., 2014. Large mesopelagic fishes biomass and trophic efficiency in the open ocean. Nature communications, 5, 3271