Implementing Closed Circuit Rebreathers as an Underwater Science Tool: A Case Study from the Middle Island Sinkhole in Thunder Bay National Marine Sanctuary
The Technology 
Since the introduction of closed-circuit rebreathers (CCR) to the commercial market in the mid 1990s, this technology has opened new frontiers to diving scientists while dramatically expanding the capacity for conducting underwater research within recreational diving limits of 60 to 130 feet. Though commonly associated with technical diving applications, CCRs deployed in recreational depths can greatly increase available bottom time when compared to open circuit (OC) diving modes, especially if used beyond no-decompression limits. This additional bottom time provided by CCRs enables scientific divers to increase in-water productivity and accomplish more of the myriad tasks associated with their research.
Two other benefits resulting from CCR design also benefit scientific divers operating within recreational depths. CCRs only require a fraction of the compressed gas supply consumed during OC dives of similar duration. As a result, dive support logistics for CCR operations do not require lengthy gas blending and fills. In the US Great Lakes where water temperatures remain around 39° F (4° C) at depth year-round, CCRs also help reduce thermal stress experienced by divers. Reduction of thermal stress, combined with proper utilization of thermal protection, is critical for scientific divers to safely conduct multi-hour diving operations in cold environments.
Between 2015 and 2017, NOAA’s Thunder Bay National Marine Sanctuary (TBNMS) began using the CCR diving mode to support an ecological research project at an ~80-foot deep collapsed sinkhole within the sanctuary. Site investigations had been continuous since 2001, led by an interdisciplinary team of investigators from NOAA, Grand Valley State University, and the University of Michigan. CCR technology allowed TBNMS divers to more than double the daily productivity of diving operations, providing increased return in numbers of observations, samples, and data from deployed scientific instruments.
Submerged Sinkholes in Lake Huron Support a Unique Microbial Ecology
The Great Lakes of North America comprise the world’s largest freshwater system by area. The five lakes, formed thousands of years ago under the advance and retreat of massive glaciers, contain almost a quarter of the surface freshwater on the entire planet. Over time, limestone substrate developed numerous inland sinkholes as well as networks of submerged sinkholes and groundwater resurgences dotting the lake floor across central Lake Huron. While nearly all of Lake Huron’s known submerged sinkholes are hundreds of feet deep, the Middle Island Sinkhole rests only 80 feet beneath the lake’s surface, 2 miles offshore.
Here, sulfate-rich and oxygen-depleted groundwater pours from a small karst window situated amid an inverted cone of collapsed limestone boulders, exiting into a large sinkhole basin nearly 290 feet in diameter. The ground water’s density, temperature, and chemical composition differ dramatically from the ambient waters of Lake Huron. As a result, a distinct layer over the lake floor defined by a mesmerizing thermo-pycno-chemocline maintains a year-round temperature of 49° F (9.5° C). Within this thin groundwater lens live complex benthic communities of filamentous cyanobacteria, descendants of early life forms present on Earth billon of years ago. Investigators examine the influence of environmental factors on mechanisms in these communities which permit oxygenic photosynthesis, anoxic photosynthesis utilizing sulfide from the groundwater, as well as chemosynthesis in the absence of light. These mats are freshwater analogs to marine vent ecosystems with potential for novel organisms and ecosystem function.
Expanded Research Capacity Using CCR Technology
Each year teams of investigators visit the sinkhole to collect bacteria, water and sediment samples while also deploying an array of underwater instruments. TBNMS scientific divers constitute the team’s primary capacity for underwater work. Up until 2015, however, these teams operated according to bottom time and gas supply limitations imposed by OC diving modes. Namely, maximum available no-stop dive time for a diver breathing NOAA Nitrox 36 gas mixture at a depth of 80 feet is 60 minutes. Underwater science work at the sinkhole, however, requires long swimming transits and working against the constant flow of groundwater. OC divers breathing NOAA Nitrox 36 from manifolded dual tanks would reach their minimum gas supply limits somewhere between 40 and 50 minutes and terminate their dives. Repetitive dives were also limited both in terms of available bottom time and room for additional OC diving cylinders onboard a small research vessel.
Starting in 2015, however, TBNMS began deploying CCR teams. Initially, to test protocols mixed CCR and OC teams (2 OC divers with a single CCR diver) were deployed during sampling operations in June and July. The mixed team ran a dive profile defined by the OC no-stop bottom times and gas supply limitations. In October 2015, the first dedicated CCR team was deployed at the Middle Island Sinkhole achieving an 80 minute bottom time on a single dive without any decompression obligation.
During the 2016 and 2017 campaigns, this three-diver CCR team was again utilized. Another OC team consisting of either 2 or 3 divers was independently deployed, with the OC and CCR operations being supervised and logged separately. The table provides a summary and comparison of OC and CCR diving modes used at the Middle Island Sinkhole from 2001 to 2017. While OC times remained fixed, CCR bottom times expanded greatly via deployment of exclusive CCR teams, addition of repetitive dives, and practice of air-diluent decompression dives planned along an 80-foot profile.
To summarize, the increase in available bottom time available to the CCR teams facilitated a broad increase in the extent and complexity of scientific sampling and observation within the sinkhole area. This, in turn, supported an expanded collaborative effort among the investigative team while enabling utilization of scientific tools and methods that would not have been possible were the diving operations constrained by OC limitations.
Author: John C Bright, MA, RPA.
Mr. Bright is a maritime archaeologist and scientific diver. He was Unit Diving Supervisor for NOAA’s Thunder Bay National Marine Sanctuary from 2015 to 2018.
He is now an Operations Supervisor at CSA Ocean Sciences Inc., an AAUS Organizational Member.
Originally printed in ECO Magazine
http://digital.ecomagazine.com/publication/?i=667968&ver=html5&p=42