Field Report 2015-12-11: Flow sensors go Farther & Deeper

Baruch Figueroa-Zavala (from CEA) recently installed a new Cave Pearl flow sensor just down the coast from Akumal bay:

The latest reef deployment of the flow loggers

The latest reef deployment, 14m depth. (photo courtesy Baruch)

This marks our deepest ocean sensor deployment to date, and two backup bungees were added to ensure this unit does not get torn from it’s anchor like the last one we put outside the sheltered environment of the bay. The turbulence shadow from the reef will likely have some effect on the flow, and it will be interesting to see if this logger accumulates the same amount of bio-growth as the shallow water units…

Marco and I snorkeled out to retrieve both of the B-generation units in Akumal Bay, which were still running well. Both were heavily encrusted despite the thorough cleaning they had in August, and to my eye it looks like there might be even more gowning on them now than they had last time. I am wondering if the acid bath somehow roughens the surface, allowing more critters to get a foothold (?)  Of course it could just as easily be a result of some seasonal nutrient flux, so I leave it up to the biologists to comment. One thing that surprises me is that we are only now seeing the first evidence of a marine animal burrowing into the instruments, and they chose to attack the epoxy rather than the PVC. A subtle reminder that poly vinyl chloride is not the most benign substance in the world.

These units have been under water since the second flow meter deployment.  With twenty months of continuous operation under their belt, they are still producing solid results despite some fairly dodgy Dupont jumper cables that I would never use in my current builds. This makes me feel pretty good about all the laborious hand-sanding I did on those early housings:


Akumal Bay (North) Tilt angle (°) (from raw accelerometer readings)

It looks like a nice two-week tidal signal was coming through until the big rains took over the flow pattern. The temp sensor on-board shows how all that precipitation lowered the mid column water temperature in a pattern that is now beginning to look very familiar:


Unit B4: Akumal Bay (North) DS18b20 Temp (°C)

I really have to build myself a CTD to find out if the water temperature also tracks salinity, and if so, I wonder how that affects all the critters out there on the reef?

A few days later, after a good cleaning and a fresh set of batteries, we tried go back and  reinstall the flow sensors, only to find police waving everyone away from Akumal at the highway. For several days a group of protesters from the pueblo blocked roadway access in a vigorous dispute over access to the beach, so we had to leave the loggers with CEA staff for later deployment. Yet another reminder of  how the combined pressures of tourism and development completely dominate the regional dynamics, and I hope that the situation can be resolved in a way that preserves the bay for future generations.

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