Large Tortuga Eddy

By: Jessica Luo

June 19, 2015

My name is Jessica Luo, and I am a 5th year PhD student with Bob Cowen, and this is my 5th oceanographic cruise with this lab (which is pretty low by Cowen lab standards), and the final one of my graduate school career. I have joined Bob, Cedric Guigand, and many others to sample plankton with the In Situ Ichthyoplankton Imaging System (ISIIS, now called DPI-2) off California, the Gulf of Mexico, the Mediterranean coast, and the Straits of Florida, and every time it has been an incredible experience. I love that magical moment when the mixture of stress and adrenaline from fixing broken instruments and trying to get all our sampling done in time finally coalesce, and we achieve that tight synchrony of everyone and everything fitting into place. But until then – all bets are off.

One of our scientific goals during this cruise is to study how a quasi-stationary mesoscale eddy (e.g. a Tortugas eddy) concentrates food (and predators) for larval fish. Most recently, Katie Shulzitski found reef fish entrained in these mesoscale eddies have faster growth rates and thus, higher survivorship [See journal article]. In addition, some large pelagic fishes (e.g. sailfish) will target eddies for spawning in order to increase the odds of survivorship for their young [See journal article]. It is thought these mesoscale eddies are beneficial for larval fish because they are retention zones for their prey, but a direct link has not yet been shown. So we are pairing plankton imaging sampling with net samples in order to: 1) characterize the physical characteristics of the eddy, 2) describe the prey field of the larval fish, and 3) measure the recent growth rates of fish found within the eddy. However, the challenge in this case has been to locate the eddy we want to sample.

Satellite imagery (see cover photo) from April - May 2015 showed signs of a promising eddy located south of the Dry Tortugas. However this particular eddy, which stayed stationary for over a month, started moving eastward in the week prior to our cruise. As these Tortugas eddies move eastward (up the Florida Keys), they are constrained by the seafloor depth of the region and become elongated, which eventually causes them to lose rotational power and degrade. We were worried  this process would happen before we would be able to sample the eddy. (Which was a distinct possibility since the last satellite imagery we were able to obtain was from June 7, 2015, three days before our cruise, and these eddies move at around 9-15 km/day.) However, we went forward, optimistic that at the very least, our shipboard instruments should help us locate the eddy and we would be able to adaptively sample it at the eddy center and edges.

 

To be continued…