Using Gliders to Study a Phytoplankton Bloom in the Ross Sea, Antarctica

Vernon Asper

Department of Marine Science University of Southern Mississippi Stennis Space Center, MS, USA

Walker Smith

Virginia Institute of Marine Science College of William and Mary Gloucester Point, VA, USA

Craig Lee, Jason Gobat Applied Physics Laboratory University of Washington

Seattle, WA, USA

Karen Heywood, Bastien Queste School of Environmental Sciences

University of East Anglia Norwich, United Kingdom

Michael Dinniman

Center for Coastal Physical Oceanography Old Dominion University

Norfolk, VA, USA

Abstract-Over the last several decades, numerous approaches have been used to observe the rapid development of the annual phytoplankton bloom in the Ross Sea, including ship-based sampling, moored instrumentation, satellite images, and computer modeling efforts. In the Austral Spring of 2010, our group deployed a pair of iRobot Seagliders equipped with fluorometers, oxygen sensors and CTDs in order to obtain data on this phenomenon over the entire duration of the bloom. Data from these deployments will be used, along with samples from the recovery cruise and satellite data, to model and better understand the dynamics of this phytoplankton bloom.

Keywords-component; gliders; Antarctica; phytoplankton biomass

I. INTRODUCTION The Ross Sea continental shelf is an extremely

productive region in which much of the photosynthesis occurs within a seasonal polynya. This polynya generally becomes ice-free during mid summer [Arrigo and van Dijken, 2003; Barber and Massom, 2007], allowing increased light into the surface waters. The melting ice also decreases the surface salinity, increasing the density stratification and further enhancing phytoplankton growth. The blooms occurring in austral spring (November–December) are often dominated by the colonial haptophyte Phaeocystis antarctica [El-Sayed et al., 1983; Smith and Gordon, 1997; Arrigo et al., 1999]. Its growth is initiated in early November, and its biomass reaches its maximum in mid- to late December [Smith and Gordon, 1997; Smith et al., 2000; Tremblay and Smith, 2007]. These blooms disappear even more rapidly than they are generated, with chlorophyll concentrations often decreasing by an order of

magnitude over three weeks or less [Smith et al., 2000, 2006; Smith and Asper, 2001).

In contrast to P. antarctica, diatoms accumulate in summer [e.g., Smith and Nelson, 1985; DiTullio and Smith, 1996], and also are more likely to be observed near ice edges [Arrigo et al., 1999; Garrison et al., 2003]. The spatial and temporal partitionings of these groups may be related to differential responses to available irradiance (which in turn is a function of ice cover and vertical mixing depth) and/or micronutrient concentrations. It has also been speculated that increased stratification induced by anthropogenic atmospheric forcing will alter the phytoplankton assemblage to one that can optimize growth in high light environments [i.e., diatoms; Arrigo et al., 1999]. A tremendous amount of interannual variability in the biomass and distribution of phytoplankton biomass and net community production has been observed [Arrigo and van Dijken, 2003, Smith et al., 2006; Peloquin and Smith, 2007, Smith and Comiso 2008].

In order to understand the distributions of these organisms at any particular period and location, it is necessary to be able to access this remote region and to make detailed measurements of appropriate temporal and spatial scales. Before the arrival of scientific icebreakers, this access was sporadic at best but even with these vessels, it is impossible to study the early stages of the blooms because of the ice conditions. In order to overcome this limitation, we elected to employ autonomous gliders to make these measurements as the polynya was forming and the blooms were in their early stages. Described below are the details of this experiment, its results, and some suggestions for future projects.

National Science Foundation ANT 0838980

II. METHODS

A. The gliders Although there are several excellent autonomous gliders

on the market, we chose the iRobot Seaglider because it was the only one to offer the endurance we needed for this work. With a standard lithium battery, the Seaglider is capable of missions lasting as long as 9 months and covering several thousand kilometers. While the cold temperatures and shallow depths were expected to reduce this capacity in our application, battery power was not expected to be an issue for the 10 week planned duration. Like all autonomous gliders, the iRobot Seaglider functions by varying its buoyancy and shifting an internal weight (battery pack). When heavier than water and with the weight forward, the vehicle glides forward and down when positively buoyant and with the weight back, the glider flies up. The operator has the option of controlling the “thrust” of the glider by manipulating the magnitude of the buoyancy changes and the position of the internal weight, allowing more aggressive maneuvering when currents are present. Unlike some gliders, the Seaglider employs no external moving parts and, instead, uses the position of the battery, which is heaviest on its lower surface, to roll the vehicle. For example, to roll left during a climb, the battery is rotated to the left, causing the glider to roll in that direction and creating more turning force through lift in the wings. This mechanism, along with the optimized, streamlined shape, make the Seaglider one of the most efficient designs available.

The standard Seaglider sensors were used for this project, including the on board Seabird CTD, Aanderaa 4330 dissolved oxygen sensor, and a Wetlabs BBFL2-VMT optical sensor which measures optical backscatter at 650nm as well as both chlorophyll and CDOM fluorescence. These sensors are installed so that they produce minimal drag and yet are exposed to the undisturbed oncoming flow of water. Data are logged at different intervals depending on the depth with higher rates in the upper water column. In the upper 250m, for example, all sensors were logged every 5 seconds while between 250 and 500m, the CT sensor was sampled every 10 seconds, the oxygen sensor was sampled every 20 seconds and the optical sensors were not sampled at all. This scheme was employed to provide dense data sampling in the upper, euphotic zone and reduced sampling at depths where biological effects were greatly reduced.

All data were transmitted via Iridium satellite to a server at the UW APL (University of Washington Applied Physics Laboratory) where they were posted to a web site that the team in the field could access. The glider was also piloted from the APL, including transmission of revised waypoint files, fine tuning the attitude and buoyancy settings, changing thrust in response to observed trajectory offsets, modifications to the diver terminal depth settings, and numerous other, controllable parameters.

Although three gliders were prepared and transported to McMurdo Station in Antarctica, the plan was to deploy two

and use the third as a spare. The NSF contractor (Raytheon Polar Services) handled all of the shipment logistics from Port Hueneme to McMurdo, making this process relatively simple for the science team. Concerns about safety of the lithium battery packs required special consideration both in the transit to the ice and for helicopter transfer to the launch locations.

B. Vehicle preparations Before launching the gliders into this formidable

environment, we wanted to do whatever was possible to increase the likelihood of success. In addition to the normal factors that can potentially result in a failed glider mission, we addressed three additional concerns that are specific to the polar environment:

1) Temperature Water temperatures in the Ross Sea can occasionally

reach 0°C but are mostly around -1.8°C with air temperatures far below that. This results in several potential problems including the viscosity of the oil in the buoyancy engine, battery capacity, current draw of the pitch and roll actuator motors, and the overall function of the electronics. Each of these were addressed by placing the gliders in a cold room (-9.48°C) during which all systems were exercised and proven to be functional.

2) Navigation and the magnetic compass

Figure 1. The glider was transported to a remote location onthe sea ice for compass calibration.

While underwater, the glider navigates by dead reckoning with GPS fixes obtained at every surfacing. This dead reckoning utilizes a Sparton SP3003D magnetic compass module for directional information and speed through the water is calculated using ahydrodynamic model that is based on the pitch, density differential of the vehicle, and observed vertical speed. Given this dependence on the magnetic compass, the steep inclination (nearly vertical) and declination (~140°) of the magnetic field, we took steps to ensure that the vehicle was able to function properly under these conditions. To calibrate the compass, we rigidly attached a ComNav Vector G2 satellite compass to the vehicle in place of its vertical fin (Fig. 1). This sensor determines its true heading using GPS satellites so it is immune to the vagaries of the magnetic field. With this compass installed, we took the gliders out onto the sea ice, several miles from shore, where we were confident that magnetic interference from the igneous rocks at McMurdo Station would be minimal. The calibration process, which is part of the vehicle’s software and was modified specifically for this deployment, required that the glider be aimed at several points of the compass and pitched and rolled 30° in each direction. During this process, the true heading from the satellite compass and the magnetic heading from the vehicle’s onboard magnetic compass were logged continuously, resulting in a calibration table that the vehicle could use for navigation. Once the process was complete, calibration coefficients were loaded into the vehicle’s memory and accuracy checked by again orienting the vehicle in multiple directions and establishing multiple pitch and roll

attitudes. At each orientation, we confirmed that the vehicle was able to calculate an accurate true heading based on the measured magnetic field and the applied calibration. To convert magnetic to true, the glider used declination values from the standard geomagnetic model onboard its GPS receiver.

3) Ice cover Working with autonomous vehicles in polar regions

requires careful planning to avoid hazards associated with both sea ice and icebergs. Because the gliders require access to the surface for both navigation and communication, it is

Figure 2. Location map showing the launch locations and trajectories of both gliders.

Figure 3. Glider 502 was launched using a VideoRay ROV in the McMurdo Sound polynya where a thin layer of grease ice did not pose a

problem.

imperative that under-ice periods be minimized. Also, while the gliders do travel very slowly (~0.2m/sec) and collisions with ice are not expected to cause damage, it is possible for the vehicle to become entrapped or even crushed by heavy planned our deployments to minimize under-ice transits and to spend as much time in the polynyas as possible. When under-ice operation was unavoidable, for example at launch at the ice-edge or when transiting pack ice, the glider relied on ice avoidance algorithms developed for APL-UW Seaglider deployments in Davis Strait.

C. Deployment plan A standard launch procedure includes an initial, shallow

(~100m) dive with rapid return to the surface to ensure that all systems are functional. Consequently, we diligently searched for a suitable large opening in the ice which was accessible by helicopters which are not permitted to fly over open water due to safety and contractual constraints. Ideally, we hoped to deploy the gliders on the northeast side of Ross Island, near Cape Crozier (Fig. 2). A reconnaissance flight to this area on November 15, however, revealed very little open water near the solid, shore-fast ice and generally poor sea ice conditions near what open water was present.

Alternatives were considered including lowering the vehicles from the cliff formed by the Ross Ice shelf roughly 50m above the Ross Sea, launching in the McMurdo Sound polyna and making a long transit around Cape Bird and Beafort Island into the Ross Sea, or possibly cancelling the launch altogether.

D. Deployments Following the reconnaissance flight and numerous

discussions with scientists and pilots who had overflown the area, we elected to deploy the first glider (SN 502) in the McMurdo Sound Polynya, roughly 50km from the McMurdo Station. This site offered excellent, solid, shore-fast ice from which to launch, and the aircraft pilots reported a large area of open water adjacent to the sea ice. Upon arrival at this site, we discovered that the “open” water was actually

Figure 4. Glider 503 was launched into the Ross Sea through a breathing hole that was created and maintained by minke whales.

Figure 5. For each dive, a complete set of engineering data was transmitted, allowing the pilot to fine tune all parameters.

Figure 7. . Temperature plots from dive 20 illustrate glider 503’s excursion under the Ross Ice Shelf and its contact with water at sub-

freezing temperatures.

covered with a very thin layer of “grease” ice. We elected to deploy the glider in spite of this minimal ice cover but used a small ROV (VideoRay Pro-4) to tow the glider as far as possible from the ice edge to minimize the likelihood that it would be drawn under the ice by prevailing currents (Fig. 3). The glider left the surface at around midnight on 11/22/2010 and was not able to reach the surface again for nearly 36 hours.

The second glider was deployed at Cape Crozier on the SW edge of the Ross Sea polynya just before midnight on 11/29/2010. The site was selected because of its proximity

to the polynya and because a minke whale breathing hole had been spotted by one of the helicopter pilots when flying over this area. We used this opening to launch the glider (Fig. 4) in spite of misgivings regarding the relatively strong southerly current that promised to drag the glider under the shore-fast ice. This glider failed to gain access to satellite contact for 36 hours and, when it did, its location was to the east and immediately adjacent to the Ross Ice shelf

III. RESULTS

A. Glider trajectories During the missions, each glider provided complete

engineering data (Fig. 5) so that the pilot at UW/APL could monitor its progress and fine tune all of its operational parameters. These data included the voltage levels of both propulsion and science battery packs so that we could ensure that ongoing demands would not exceed capacity (Fig. 6). As noted in this figure, the batteries performed well but some of the behavior remains under consideration because the voltage levels cannot be entirely explained by the loads placed on them. Because of the large area of pack ice above Beaufort Island (Fig. 2) and the fast ice bridge south of it, we elected to keep glider 502 in the McMurdo polynya for more than a week with the expectation that conditions would improve. On 12/5/2011, the ice area was beginning to thin so we aimed glider 502 north and then, on 12/10/2011, gave it a waypoint in the Ross Sea polynya, causing it to transit under/beneath the pack ice. As expected, contact was lost periodically during the 40km under ice transit but on 14 December, it emerged in the polynya. From there, we assigned it the task of making east-west transects of the polynya study area, making a total of 701 dives and covering 1342km by the time it was recovered on 1/20/2011. As seen in Fig. 1, these transects were not entirely linear due to some rather impressive tidal currents encountered at the eastern terminus of the transect.

When glider 503 emerged on 11/30/2010, it reported that it had completed 23 dives to depths up to 415m but the closest it was able to get to the surface was about 40m depth. Most of the dives ended at about 90m when “no vertical velocity” was detected, indicating that it had been swept under the Ross Ice Shelf. Also, during several of these dives, temperatures as low as -1.96 C (Fig. 7) were measured which is below the surface freezing point of seawater (-1.90 C) at the measured salinity, and thus is evidence of water that had been in contact with ice well below the ocean surface due to the depression of the freezing point of seawater with pressure (~ 7.53e-4 deg C/decibar). We have no way of knowing how far under this floating glacier the glider was carried but we were impressed that it was finally able to overcome the currents and emerge from this situation. Once clear of the ice, we assigned glider 503 the task of making an hourglass shaped survey over a focal point at 76° 30'S, 176°E. This glider covered 1,671 km in 923 dives during the 63 days it was in the water.

Figure 6. Science batteries performed as expected, showing gradual decline, but the propulsion batteries exhibited some behavior that is under

analysis.

B. Data In spite of the severe conditions, the gliders functioned

well, and, with the exception of the optical sensor pack on 503 which failed after 55 dives, produced excellent data. A detailed discussion of the findings is beyond the scope of this paper and will be presented elsewhere, along with the results of models that will help interpret the data. However, a few highlights are readily apparent. First and as expected, there was a large amount of spatial variation in the amount of chlorophyll fluorescence in the water. As shown in Fig. 8, the water on the western side of Beaufort Island (early in the transect) was almost devoid of any signal while that in the Ross Sea polynya (last part of the transect) exhibited large signals. Second, temperatures in the surface waters warmed dramatically during the duration of the study, creating considerable stratification in the polynya (Fig. 9). This, in turn, stabilizes the water column and would be expected to enhance primary productivity. Finally, the salinity signal in the water column exhibits surprising spatial variations all through the water column. In contrast to the temperature

signal, this is more likely to be due to advective processes but the final interpretation will be based on model results as well as a thorough examination of these and other data.

IV. FUTURE PLANS This project demonstrated the utility of using

autonomous gliders to study biogeochemical processes in environments as extreme as the Ross Sea. It is clear that navigation by magnetic compass with GPS fixes can be sufficient for the vehicles to complete assigned tracks. As expected, however, logistic considerations for deployment and recovery will be expected to limit glider operations. This includes the challenge of transporting the equipment and team to the ice edge, arranging for a vessel to retrieve the glider at the end of the mission, safety issues associated with all operations near this dangerously cold water, and the impacts of weather and ice conditions on all activities. As glider technology continues to improve, however, we can expect to see enhancements in autonomous navigation, perhaps associated with long baseline acoustics, that will

Figure 8. Results from Glider 502 illustrate the stark contrast in chlorophyll fluoresence between the waters to the east (a) and west (b) of Beaufort Island.

Figure 9 Data from glider 503 show the development of a warm, somewhat less saline layer at the surface as the season progressed from 12/6 (a) to 1/28 (b)

allow longer transects beneath the ice. We are pleased to be among the first to deploy gliders at these latitudes and we look forward to seeing many others emulate this success so that a fleet of gliders will eventually patrol these forbidding regions.

ACKNOWLEDGMENT We would like to thank the staff of Raytheon Polar

Services for their assistance in launching the gliders and the officers and crew of the Nathaniel B. Palmer for their assistance in recovering the gliders.

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