Lagrangian studies
by Jessica Luo, on
We finished our cross-current and along-current transects on Thursday, July 25 and headed back to Nice harbour for the evening to pick up some more scientists and get ready for the Lagrangian studies. Fabien Lombard from LOV, who had come on board for the previous two nights to observe and count Pelagia noctiluca, switched with Martin Lilley (LOV) who joined us for the remainder of the cruise. Two other scientists from Italy, Simone Marini and Mireno Borghini, who work with Annalisa Griffa, also brought drifters that will be used for the Lagrangian studies.
The goal of the Lagrangian portion of the VISUFRONT cruise is to track a water mass over 36h hours using drifters and continuously sample this water mass with ISIIS. The drifters, designed to passively drift with the current, transmit their GPS location every ~ 15 minutes, so we can see their past trajectory and estimate their future location. We aim to sample in transects perpendicular to the drifter tracks. This is particularly interesting because in our other transects (cross-current and to a lesser degree, the along-current ones) we are crossing over different water masses, so we have both a spatial and temporal pattern to tease apart. With the Lagrangian studies, we theoretically are sampling the same "space" (in terms of water mass) over time, so we can isolate the temporal signal in the ISIIS data. In my opinion, this is probably the most novel component of the VISUFRONT cruise; we had never sampled in a Lagrangian fashion with ISIIS and been able to truly isolate the temporal signal in the fine-scale movements of plankton. With these experiments, we are able to ask questions about the diel vertical migration patterns of organisms within multiple trophic levels and size ranges.
However, as fitting with the theme with the rest of the cruise, things did not go entirely according to plan. We lost a day of sampling at the beginning of the Lagrangian portion of the cruise because we had to go to BOUSSOLE to rescue a glider that had lost its connection with the mainland. The drifters were supposed to go in at nighttime according to our original sampling plan, so going to BOUSSOLE and back would result in an entire day lost. (We ended up doing some plankton tows so it wasn't entirely lost, though.)
Upon returning to the coast and putting the drifters into the water, we could not initially obtain accurate GPS positions for the drifters – the positions we were getting by SMS were completely useless. After quite a bit of troubleshooting, we figured out that we could obtain the drifter positions using geoborders' website, so that put us back on track to do the Lagrangian transects.
We sampled with the drifters for nearly 48 hours before pulling them out of the water due to weather concerns. Reports of a bad storm had been coming for the past couple days, and it is supposed to hit later today, on the 28th. So, instead of sampling until tomorrow (the 29th), we are cutting our Lagrangian studies even shorter. However, 48 hours of near continuous Lagrangian sampling is pretty good, so we are pretty happy with what we got. Thus, this morning, we pulled the drifters out, dropped the Italian scientists off in Nice, and are heading out again to brave the winds, swell and rain.
