Maintenance Operations, November 8th - 12th

In the week of November 8th - 12th, a team from the Atlantic Oceanographic and Meteorological Laboratory (AOML) traveled from Miami to La Parguera, PR, to work on the CREWS station located near the Media Luna reef. The main purpose of this trip was to perform our annual swap-out of all meteorological and oceanographic instruments on the station, but we had several additional jobs to do, as will be described below. The AOML team consisted of LTjg Rachel Kotkowski (NOAA Corps) and Mike Jankulak (University of Miami). We were joined in the field by Wess Merten (University of Puerto Rico), who in our absence makes monthly trips to clean the station and connect its "groundtruth" CT. Wess has been writing detailed entries in this blog and has also been instrumental in diagnosing the station's ills from afar (one notable example of this from earlier this year is described below).

We started out (on Tuesday the 9th) with a brief survey of the station's underwater supports to make absolutely certain that it was safe to climb. We observed nothing unexpected. This first photo on the left, taken on Tuesday, shows the lower hounds, lashings and base plate attachment.

A second photo seen at the right, taken Thursday (when visibility was unusually clear for this location), shows the "trailer hitch ball" beneath the pylon, the pivot point for the entire structure. Clicking on any of these photos should lead to the (larger) original versions of each picture.

After a quick survey to document conditions and take some photos, we set to work swapping out the underwater instruments. These operations differed from routine swapouts in two ways. For one, there was no "deep" light sensor to remove since this instrument had been removed in April after its failure and flooding caused the station to go offline for about 12 days. You can follow the progress of this event in previous postings to this blog, including the station's initial power failure, the first survey of the station after going offline, the removal of the suspect instrument, and the station's subsequent recovery. So we were not required to remove a "deep" light sensor before installing the fresh instrument.

Secondly, during the course of last year's maintenance visit it became clear that the station's "shallow" light sensor was not properly supporting the instrument and holding it steady and upright. This is documented in the blog post from that visit, which also includes a photo of the offending mounting bracket. Thus we decided in this visit to replace this sensor mount with the solid-platform type mount that we use at our newer stations. The "deep" sensor's mount had undergone a similar upgrade several years previously. The new shallow light sensor and mount are shown in a picture at left.

Our first day's underwater work concluded with the replacement of both CTDs, shallow and deep. After that, I climbed the pylon to remove all aerial instruments and electronics for replacement and reprogramming on land, except for the surface light sensor, which was left in place due to lack of time. The station was therefore powered down on Tuesday, November 9th at 20:26 UTC (4:26 PM local time), and remained offline until everything was reconnected on Thursday, November 11th, at 15:51 UTC (11:51 AM local time).

Wednesday's work began with the removal of the surface light sensor; at this station, this is the most stubborn of the three instrument masts and the one which requires the most effort to remove. Another very important goal of this trip was to replace the station's navigational light. The nav light is a crucial safety feature of the station, because it warns mariners to keep their distance when conditions are dark, and its flash pattern is listed on navigational charts. We had been hearing many second-hand reports that the nav light was no longer flashing. Wess attempted to verify these reports first-hand by visiting the station at sundown but he in fact was able to document that the nav light was still working at least partially. [My best guess at this point in time is that the light was slowing failing, either because its batteries could no longer hold a full charge or because its solar panels were blocked or failing, and that it probably wasn't powerful enough to continue flashing throughout an entire night. In fact, when we arrived in La Parguera on Monday night we could not find the familiar flashing beacon when we looked for it from the hotel's seafront, and the light post-removal did not continue to flash steadily for a full day on the workbench as it should have.] In any case, the nav light had been targeted for replacement on this trip simply because of its age (over five years since deployment of this pylon), and on this Wednesday morning I removed the light at the top of the pylon and installed its replacement. The final picture in this blog entry will show the new nav light, which I was able to observe flashing from the hotel seafront on Wednesday night, Thursday night, and very early Friday morning, so we are confident that the replacement light is fully functional.

Once Wednesday's aerial work was complete, we turned our attention to one more side-project for this trip: the installation of two standalone temperature loggers (see picture at right). This equipment was provided to us by UM/RSMAS student Xiaofang Zhu, who is particularly interested in shallow-water temperatures at both the La Parguera and Little Cayman CREWS stations. Xiaofang plans to use the data from these two temperature loggers, supplemented by data from our two CTDs on the station. She had given us a list of considerations for positioning the instruments on the pylon, which would naturally have to be adapted in the field to conditions as we found them, such as the depth of the Shallow CTD and the positions of the upper "hounds" (which are the four attachment points for the eight chains-and-spectra which support the pylon). We were able to place the two temperature loggers, one above the other, between the Shallow CTD and the water's surface. However, there remains some question about whether the Shallow CTD is correctly reporting its instrument depth (since it seems like it was much shallower than expected). The upper tip of the upper temperature logger was observed to be breaking the surface on Thursday morning, on a very calm day when the tide was coming in (and only a few hours before high tide), so there is some concern that the instrument's lower end (where the temperature sensor is in fact located) might also be above the water's surface at low tide. Still, Xiaofang has decided for the present to leave the instruments as they are, and she plans to work with Wess to collect more objective measurements of instrument depth and distances between instruments. Her software, cables and tools have been left in Wess' care and he may be able to work with Xiaofang to download her data after a month or two.

This concluded our work for Wednesday, and the rest of the day was spent in the lab, replacing the aerial instruments on their masts, reprogramming the data logger and other electronics, and running the new program on the workbench with all available instruments connected.

Thursday morning was the calmest day yet, and visibility was extremely good for this location, which generally has very murky conditions. We were surprised and pleased to find that we could see the ocean bottom from the boat. Rachel took the opportunity to snorkel around the area and take some pictures of nearby corals (see photo at left). Many of her photos showed the telltale signs of a bad bleaching season in the area, which Wess tells us was unusually bad this year although not as bad as 2005.

Thursday's work was all about reinstalling everything that had been removed on Tuesday and Wednesday: the surface light sensor, the integrated "weather transmitter," the anemometer and electronic compass, and the control package of electronics which we call the "brain." This was accomplished in about two hours, and then the station was powered on and a radio link was used to verify from the boat that all instruments were correctly reporting. After that, it was just a matter of tying down all loose cables (both above and below the surface) and plugging up the external openings of the "brain" chamber's conduits. A photo at the right shows the deep CTD and light sensor (note the similar platform-type mount for the light sensor, which is now several years old) after their cables had been tied down.

A final photo (at left) will serve to illustrate two points. One, note the newly-replaced navigational light (centrally located, just above the bird). This new light has its solar panels on top, and since it is located directly beneath the (square, white) satellite transmission antenna, it may not receive a lot of direct sunlight throughout the day. Still, it is believed that it will receive more than enough light to keep it fully powered, and the light is "smart" enough to adjust its intensity downward if it determines that it is too low on power to keep flashing at full intensity until morning. Also, its location beneath the satellite transmission antenna (which has a protruding cable) probably shields it from acting as a convenient bird roost or having its solar panel dirtied and blocked with bird guano.

The second thing to observe in this photo is the bird itself. Believed to be some kind of booby, this bird and others like it are an extremely common sight at this station. We have been told that up to four boobies have been observed at one time sitting on this station. One booby was sitting on the station lid when I first started climbing the pylon on Tuesday, and bravely stood its ground until I reached the top. While I was working, multiple boobies circled the station and at times seemed as though they might attempt a landing, even with me there. It is certainly true that of all of the CREWS stations, this station's instruments are most covered in guano when they are retrieved. There are a few implications of this bird activity for instrument and station operations.

Some of the station surfaces are more suited as resting spots than others. The station lid, the transmitter antenna, the Vaisala "weather transmitter" (or WXT, which provides data for winds, temperature, humidity, barometric pressure and precipitation) and the surface light sensor each in their own way provides a comfortable resting spot. Let us consider each in turn.

There is no adverse effect to the use of the station lid as a bird roost, other than a mild "yuck" factor for the pylon climber. It seems possible that the presence of a bird on the satellite transmission antenna could lead to garbled or blocked transmissions, but in fact this has not been a problem at this station. This CREWS station, termed LPPR1 by AOML and known as LPRP4 by the National Data Buoy Center (NDBC), has the best transmission success rate of all of our stations. With regards to the WXT, we have actually had ongoing problems with wind measurements by this instrument for many years, and I believe the main problem to be bird-related. There may also be a strong impact on the reliability of precipitation data, although the WXT's temperature, humidity and barometric pressure data are not believed to be impacted. The good news is that all of our newest WXT instruments have come with bird-deterrent spikes pre-installed, and following our work in this week this station features a WXT bird-deterrent for the first time in its history. So I am very hopeful that the WXT wind data will be more accurate in the coming year.

Let me briefly discount bird influences for the anemometer (which pivots enough in higher winds that it is probably not a good perch, although the presence of bird guano suggests it may be hosting bird visits in unusually calm circumstances) and the navigational light (which, as stated above, does not offer much headroom for these rather large birds). Then we are left with the surface light sensor. Unfortunately, this is still a concern. Last September, in advance of this visit, I contacted Biospherical, which is the manufacturer of all of our light sensors. My question was whether they sold any add-on bird deterrent mechanism for their surface light sensors (like Vaisala does for the WXTs). In fact they do not sell such a thing, so it will be left to us at AOML to come up with something on our own. Because it is certainly the case that the surface light sensor provides a welcoming perch for these birds, and it has been observed many times to be acting as a bird roost. We also see this effect in the data, when the average light levels reported by the surface sensor may fall lower than the underwater sensor light levels for a period of some hours in the middle of the day. This is probably the result of the sensor being blocked by birds, and this is a challenge that we should address in the the coming year so that we can test a new bird deterrent in next year's visit. Obviously the main challenge will be to bar the birds from landing on the flat sensor surface without in any way affecting the levels of light that reach the sensor.

In any case, this brings to a close a very long description of an extremely successful field trip. We offer our thanks to everyone concerned (UPR, UM, NOAA) for being willing to work through Thursday's government holiday. And as always, our appreciation goes to Wess Merten, Francisco Pagan, Milton Carlo, and everyone else at UPR's Isla Magüeyes for taking care of the station all year and providing support with boat ops, diving support, tanks and lab space when we visit. Muchas gracias!