By: Jessica Luo
June 19, 2015
...Continued from Part 1
Readers of this blog already know what is happening next: our shipboard acoustic current doppler profiler (ADCP) was miscalibrated and gave us erroneous readings for the whole first three days of the cruise. On Day 1, though, we had no idea this was the case, and we steamed southwest from Miami, passing the Florida Keys one by one and never seeing the key directional change in the currents that would tell us that we had reached the eddy. We eventually decided to just go ahead and put DPI-2 (formerly ISIIS) in the water to see what the DPI-2 physical data would tell us.
DPI-2 simultaneously records plankton images as well as data from a suite of environmental sensors, so we are able to correlate any organism to the particular depth and latitude/longitude, as well as the temperature, salinity, fluorometry, oxygen, and irradiance of the water in which it was imaged. The physical data that are collected can also help us define the boundaries of water masses or the structure of any fine-scale features that we sampled across (e.g. fronts, thin layers).
Normally, we take all the data, imagery and physical, back to the lab to analyze, but this is not terribly useful for us in the field when conditions are changing rapidly and we have situations like our current one when our other shipboard instruments are malfunctioning. Fortunately, in the last couple years, because of analytical advances in our lab, we are now able to map the physical environment in real time.
The physical signature of a cyclonic eddy in the Northern Hemisphere is a “cold core,” which means that colder, deeper, more nutrient rich waters are brought up closer to the surface. The plot below shows the temperature, salinity, fluorometry, and oxygen signal by depth and distance along the transect (in nautical miles) of a smaller, spin-off eddy that we sampled last year, but similar patterns should be evident in larger, meso-scale eddies. The characteristic “dome” in the physical data marks the center of the eddy, and the areas of higher biological activity are expected to occur at the eddy edges.
I am showing you the physical data from last year’s sub-mesoscale eddy because it better illustrates what a cyclonic eddy should look like than any single transect we surveyed this year. The key feature to note is the “dome,” present in all four panels, which show the cooler, saltier, more productive, and less oxygenated waters coming up from depth to the surface waters. The top of the dome marks the center of the eddy.
Since this year’s eddy was so large, and we couldn’t define it from the ADCP data or satellites (it was too cloudy), we had to sample it with DPI-2 in pieces. Furthermore, when mesoscale eddies are being sheared and broken down, they start to lose a lot of the key characteristics that make it easy to define its structure in real time. But this period in the eddy’s life-cycle is also particularly interesting in terms of the biology: the larval fish found inside it presumably have already had quite a while to stay in the eddy, find favorable food environments to grow fast and big, and the predators would have had time to figure out where all the fish were. The age-old dance between those trying to eat and those trying to avoid being eaten would be playing out in real time as we crisscrossed the eddy, towing imaging systems and nets along the way.
As we get back to land and the lab, we will slowly be able to piece together the multiple lines of data – the ADCP that eventually got re-calibrated, the DPI-2 physical data, and satellite imagery – to determine what water masses we actually sampled (because we did get distinct water masses!) and where the eddy actually was located. The story of the larval fish in its quest to find food, avoid being eaten, and find its way back home continues to be revealed, and we biologists are enjoying the ride every step of the way.