(Figure 1) Map of the North American Laurentian Great Lakes Basin showing regions of karst limestone formations and regions of above ground limestone formations in Alpena County, MI and submerged sinkholes (including the study sites – Misery Bay containing the El Cajon Bay Blue Hole, Middle Island Sinkhole and Isolated Sinkhole) in the Thunder Bay National Marine Sanctuary (TBNMS), Lake Huron. Click image for larger view and image credit.
Mission Plan
September 2 - 12, 2008
The Thunder Bay Sinkholes 2008 project will explore shallow and deep coastal sinkholes in order to understand the chemical and physical properties that contribute to the unique ecology found in these systems. This multi-investigator team will gather data on the hydrology (the flow rates and dispersion of groundwater), biology and chemistry from submerged groundwater vents located within the boundaries of the Thunder Bay National Marine Sanctuary, in northwestern Lake Huron (Figure 1). During the project, the team will collect benthic (bottom) samples of the unusual microbial mats growing in very low oxygen conditions to determine how groundwater chemistry allows them to thrive. Samples of these microbial mats will also be analyzed for their potential application as pharmaceuticals. A long-term understanding of the groundwater’s chemical and physical characteristics such as conductivity (an indication of the presence of ions), temperature, and flow will be obtained from a moored hydrographic instrument array. Mapping of chemical and biological characteristics over large spatial areas will done using underwater remotely operated vehicles (ROVs) and diver-led sample collection work. Sinkholes characterized by active groundwater venting occur here in shallow.
Beneath Lake Huron there are 350-430 million year old bedrock aquifers deposited by an ancient shallow seas during the Silurian and Devonian ages. This rocks in this layer consist of limestone (karst), shale and sandstone with fresh and saline water, containing sulfates, chlorides and iron. Surface waters seep into the limestone layer and cause it to dissolve producing caves and caverns. Caverns that reach the surface collapse becoming sinkholes. Groundwater of unknown age continues to flow from these systems creating habitat for extremophile bacteria capable of thriving in the very low oxygen, high sulfate environment.
(Figure 2) On the left, from inside the Alcove nearly pure groundwater, fueling purple cyanobacterial mats, spills over the wall into the Arena (photo by Russ Green, Thunder Bay National Marine Sanctuary). On the right with 9 meter Whaler for scale, the Middle Island sinkhole is open to Lake Huron creating a gradient of biological activity (aerial photo by Scott Kendall and Bopi Biddanda, Grand Valley State University). Click image for larger view and image credit.
One type of extremophile forms mats made up of filamentous cyanbacteria (Oscillatoria) in the extremely low oxygen, high conductivity groundwater at Middle Island. These dark purple mats seen from the inside of the sinkhole in Figure 2 (left) from a well of groundwater called the Alcove carry out photosynthesis without oxygen. Water from the Alcove spills over the wall and into the broader open area of the sinkhole called the Arena. The water in the Alcove consists of almost pure groundwater. The higher density of the water allows an underwater waterfall to cascade over the Alcove wall and into the Arena. The turbulence from this fall allows the groundwater to mix into Lake Huron waters increasing the oxygen and reducing the conductivity. This change in the chemical and nutrient makeup of the water causes a gradient of biological activity as the groundwater moves further away from the source and mixes with the waters of Lake Huron.
(Figure 3) CTD mapping by ROV just over the Isolated sinkhole (at 91 m over a ~35m x 55m area) showing enhanced conductivity and temperature at sites of venting in September 2003. Click image for larger view and image credit.
(Figure 4) Isolated Sinkhole: ROV-video still images of conspicuous benthic white and dark mats (top, composition unknown) and 1-2 m thick nepheloid-like plume layer (potential microbial hotspot) prevailing just over the lake floor (bottom) at the Isolated Sinkhole in September 2003. Click image for larger view and image credit.
A pilot study of the Isolated sinkhole (93 meters depth) funded by NOAA-OE during September 2003 found that conductivity levels in the proximity of the sinkhole were an order of magnitude higher than ambient lake water warmer water (by up to 4 oC) relative to surrounding lake water at depth (maps, Figure 3). White benthic mats interspersed with the brownish mats characterized the lake floor in the vicinity of the sinkhole, and a dark cloudy nepheloid-like plume layer prevailed just over the site of submarine groundwater seepage (Figure 4). The images of benthic mats recorded at the Isolated Sinkhole in western Lake Huron are remarkably similar in appearance to those commonly characterizing cold seep and thermal vent habitats in the ocean. Preliminary results suggested that the Isolated Sinkhole environment was a hot spot of nutrients and microbial activity. In comparison to surrounding deep water, the nepheloid-like layer prevailing just over the site of venting was characterized by higher conductivity and temperatures, higher concentrations of chloride, sulfate, phosphorus, dissolved organic carbon, particulate organic carbon and bacteria.
These nearshore and offshore sinkhole systems provide gradients of light, oxygen and chemical nutrients resulting in a variety of biological habitats. We will map these underwater habitats (ROV and diver mapping), evaluate the variable physical and chemical conditions for one year (continuous time-series exploration), measure the rate of groundwater discharge using radiochemistry techniques, quantify groundwater nutrient chemistry, describe the fine-scale vertical organization of the microbial mats, identify metazoan sinkhole inhabitants, study mat growth, and collect mat samples to determine if they have medicinal value. The proposed study is expected to provide a clear working picture of the bathymetry, physical and chemical conditions, groundwater flow processes, and the variety and distribution of life in a range of submerged sinkhole ecosystems in Lake Huron sinkholes.
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