January 2022 Bering Sea Mooring Recovery

By Natalie Monacci (UAF) and Shaun Bell (NOAA)

Natalie Monaci

NOAA’s biophysical mooring site 2 (M2), led by the Ecosystems and Fisheries-Oceanography Coordinated Investigations (EcoFOCI) co-lead Phyllis Stabeno, has been collecting seawater measurements in the southeastern Bering Sea since 1995. Measurements of temperature, salinity, nitrate, oxygen, chlorophyll and currents have been the cornerstone of many interdisciplinary objectives over the years that M2 has consistently been in operation.

Subsurface instruments monitor the water column at M2 year-round, supplemented by additional measurements from a surface buoy that must be recovered prior to the advance of seasonal sea ice. EcoFOCI partnered with the University of Alaska Fairbanks (UAF) in 2011 to add sensors to the M2 mooring site that would monitor surface carbon dioxide (CO2) dynamics and ocean acidification.

The global oceans absorb ~26% of human emitted carbon dioxide, causing seawater pH to decline. This global decline in pH, or Ocean Acidification (OA), is happening faster in the Arctic and subarctic, and can negatively affect important subsistence and commercial fisheries. The Bering Sea supports some of the most productive fisheries in the world, and the M2 site serves as the longest-running ecosystem monitoring station within this vital region.

We began what turned out to be a 5-month-long odyssey to get the surface equipment recovered before it would be destroyed by the advance of sea ice. 

The UAF CO2 sensors are housed in the seasonal surface buoy, nicknamed Peggy (photo on opposite page), which is typically deployed from May to September. In recent years, a second seasonal surface buoy, nicknamed Oscar, has been established at M2 to support the newly-developed ‘Prawler’, a seawater profiler that uses wave energy from the surface mooring to crawl up and down the anchor line, taking temperature, salinity, oxygen, and chlorophyll measurements along the way.

In September 2021, the annual fall mooring cruise — slated to handle the recovery of both Peggy and Oscar, among other tasks — was canceled due to COVID-19 and we began what turned out to be a 5-month long odyssey to get the surface equipment recovered before it would be destroyed by the advance of sea ice.

With the Bering Sea’s notorious autumn storm season fast approaching, our first obstacle was to secure a new charter while most vessels and their crews are heading for home or laid up in harbor. The first of many storms that fall took place in mid-October, bringing hurricane force winds to the Aleutians. The meteorological package at M2, ~325 kilometers north of Unalaska, experienced howling winds of 25 m/s, or about 56 mph, at one meter above the sea surface. Thankfully there were no weather-related injuries reported in the region, and the passing storm wound up driving an interesting oceanographic event that was recorded at M2.

Geoff Lebon

Our 10-year record of observing surface carbon dynamics at M2 shows a seasonal, biological drawdown of seawater CO2 , driven by the spring phytoplankton bloom, with a gradual return to near-equilibrium with air by the fall. As summer progresses, the biological pump on the continental shelf exports organic carbon from surface to bottom waters, where bacterial degradation remineralizes this organic matter, causing increases in CO2 at depth. During the October 2021 storm, deep, CO2 -rich water was mixed to the surface, and Peggy recorded a significant event with CO2 in the seawater greater than that in air by ~100 µatm. This causes the ocean to release CO2 back into the atmosphere. We have rarely had the opportunity to observe late-season outgassing events like this in the Bering Sea, since the surface mooring with its CO2 sensor package is usually recovered and removed much earlier in the season. Observing these late-season dynamics helps create a better understanding of the annual carbon cycle, which is critical for measuring future change.

That “bonus” data was exciting, but we were feeling increasing pressure to secure a charter for the M2 surface moorings recovery before seasonal sea ice advance could threaten the equipment. Two additional attempts to recover the mooring late in 2021 fell through, because of COVID-19, weather, or both.

Peggy’s mooring line is engineered to withstand strong storms, but the float is not designed to handle sea ice. In some winters, sea ice may not reach M2, but as the season progressed the forecast of relatively extensive ice in 2021/2022 was beginning to ring true, and our stress levels were rising. Finally, we were able to establish a charter with the M/V Resolve, and our team traveled to Unalaska in mid-January. As the crew waited in port for another storm to pass before beginning the roughly 16-hour trip from Unalaska to M2, the GPS transmitters on our CO2 sensors stopped reporting. Had we lost Peggy?

The forecast of relatively extensive ice in 2021/2022 was beginning to ring true, and our stress levels were rising. 

Now our group was faced with the dilemma of going ahead with the costly charter, in a narrow weather opportunity, knowing we may not find anything when we arrived at M2. The final call was made by CICOES Director John Horne and the chief scientist on the M/V Resolve charter, Geoff Lebon — we decided to go for it.

The ship departed six days after the last satellite transmission from Peggy was received. It was minus 9° C, and the winds were strong (12-13 m/s) and frigid, coming out of the north and pushing ice floes toward M2. Peggy is a roughly 3 m tall torrid-style surface float, weighing approximately 750 kg. Ideal recovery conditions have winds less than 10 m/s and a sea state of less than 2 m, although a capable vessel and skilled crew allow for recovery in less favorable conditions.

Geoff Lebon

Arriving at the M2 site as daylight was fading, after the long crossing from Unalaska, the crew immediately spotted the buoy Oscar, slightly damaged but still afloat. Peggy was nowhere in sight. The M/V Resolve set up to maintain station overnight, and begin operations at first light. As dawn broke the next day, the crew spotted Peggy. Sea ice had ridden over the top of her, tearing off the tower, meteorological package, and communication antennas and badly damaging the torrid, turning Peggy into a low-riding lump that was being claimed by fur seals as their couch.

After a difficult and challenging day of recovery operations by the Resolve and crew, both Oscar and Peggy — together with the 70 m of instruments below the surface — were recovered and brought back to Unalaska.

Now that the instruments and recorded data are safely back in Seattle at PMEL, these first-ever surface ocean measurements during freeze-up in the Bering Sea are providing a glimpse into a seasonal shift that is rarely captured. Climate change is faster paced in the sub-Arctic and Arctic, and the lack of baseline conditions make it hard to predict changes in primary production and the CO2 cycle. The autonomous observations collected at M2 can help address these gaps in our understanding. The M2 data, along with supplemental discrete samples collected during ship-based fishery assessments in the Bering Sea, will be used by CICOES researcher Darren Pilcher and colleagues to validate a carbonate chemistry package for the “Bering 10K” regional ocean model. These observations and model outputs ultimately benefit fishery biologists and managers, through the addition of an ocean acidification index in NOAA’s Ecosystem Status Report.

These first-ever surface ocean measurements during freeze-up in the Bering Sea are providing a glimpse into a seasonal shift that is rarely captured.  

Even before the recovered gear had been shipped out of Unalaska, planning for the 2022 spring mooring cruise was in full swing. Unfortunately, we no longer could rely on our flagship buoy: Peggy sustained irreparable damage to her fiberglass hull from the sea ice.

Thankfully, the PMEL Carbon Group loaned us a surface buoy and we deployed the replacement Peggy on schedule in April 2022. We then successfully recovered the buoy on September 8th, 2022, just in time — only one week before ex-typhoon Merbok entered the Bering Sea. ◆