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Submerged Sinkholes Trap Lake Huron's Carbon
By Bopi Biddanda, Annis Water Resources Institute
Brilliantly colored purple photosynthetic cyanobacterial mats – found nowhere else in the Great Lakes or in the northern hemisphere - thrive under low-oxygen and high-sulfur conditions in Lake Huron’s sinkholes (Figure 1). Here, dissolution of ancient (~400 mya, Paleozoic) aquifer in the Lake Huron Basin has produced karst formations in the limestone bedrock (sinkholes), through which dense salt-rich groundwater emerges onto the lake floor fueling the growth of microbial communities on the lake bottom. These underwater ecosystems offer fascinating opportunities for scientific exploration. For example, inexplicably large amounts of carbon-rich sediments occur beneath the sinkholes as revealed by sub-bottom acoustic studies (Figure 2). The burning question is – where does it all come from?
Figure 1. Underwater imagery of benthic microbial mats in Middle Island Sinkhole in the Thunder bay National Marine Sanctuary, Lake Huron. Underneath the mats is buried over 15m of carbon-rich sediment. Where does it all come from?
Tracing carbon flow through food webs to understand the workings of ecosystems is a fundamental goal of ecology. On a planetary scale, following carbon helps us figure out the role of biology in regulating the carbon cycle – including the uptake of CO2 produced by anthropogenic activity. During 2008 to 2012, researchers and students from Grand Valley State University, the University of Wisconsin, and NOAA’s Great Lakes Environmental Research Laboratory collaboratively addressed tracking the source of the massive sedimentary carbon reserve in the Middle Island Sinkhole in Thunder Bay National Marine Sanctuary, Lake Huron. The study set out to find out whether the source of sedimentary carbon was the lake’s plankton or the benthic cyanobacteria. Our favorite hypothesis was that successive benthic cyanobacterial mats (Figure 1) were getting buried under the sediments (Figure 2). The alternate hypothesis was that plankton production in the overlying water column settles down and gradually accumulates in the sediments.
Figure 2. Acoustic sub-bottom profile of Middle Island sinkhole showing sediment surface (thick line at 22 m water depth), rocky substratum (fainter hemispherical line beneath sediment surface) and extent of sediment accumulation (ca. 17 m) within the sinkhole.
In order to answer this question, we deployed a sediment trap in the water column to catch settling phytoplankton, diver-sampled benthic cyanobacterial mats and used naturally found stable (non-radioactive) isotopes of carbon (ratios of 13C and 12C) found in both plankton and benthic biomass in distinctive ratios to track down the flow of carbon into sedimentary carbon (Figure 3). Measurements of carbon isotope ratios in the mass spectrometer showed that plankton growing in lake water have distinctively more of the heavier isotope (-22.7 parts per thousand or ppt, measured against a reference standard of zero value) than benthic cyanobacteria growing on groundwater (-28 ppt), whereas sedimentary carbon displayed values quite similar to that of phytoplankton (-23.1 ppt). Two lines of evidence suggest that carbon in sinkhole sediment originates from water-column phytoplankton rather than benthic cyanobacteria. First, the 13C content of organic carbon in Middle Island sediments was similar to the carbon in sediment traps. Second, the cyanobacterial mat13C signature does not appear in sediments below the mat, suggesting that the majority of sediment carbon is derived from phytoplankton biomass falling from above. Such sequestration of planktonic carbon may be facilitated by the rapid phototactic mobility by micrometer sized cyanobacterial filaments of several millimeters per hour (personal observations in the laboratory). Mat filaments climbing over settled phytoplankton in search of light may help bury phytoplankton carbon within anoxic sediments underneath and facilitate both their accumulation and preservation over time.
Additional markers such as comparisons of carbon to nitrogen ratios of the plankton, mat and sediment also corroborate our findings that submerged sinkhole sediments trap planktonic carbon. Furthermore, sub-bottom imaging and 210Pb dating of sediments suggest that the massive deposits of organic carbon beneath the sinkhole may be due to rapid rate of sediment accumulation here (~17 m of sediment accumulation over 5,000-6000 years amounting to approx. 5 million Kg carbon) compared to the to lake environments of similar depth. If confirmed by future studies, sinkholes may have one of the highest rates of sedimentation of any natural environment on Earth. Additionally, the “bowl-shaped” geology of sinkholes may facilitate sediment focusing of materials from outside the basin (Figure 3). What is also of significant note is that whereas most of the sediments found in the Great Lakes and the oceans are about 5% carbon, Lake Huron’s sinkhole sediments are ~15% carbon. Sinkholes are rapidly accumulating large amounts of carbon-rich sediments – making it a massive reservoir of plankton-derived carbon.
Figure 3. Profile view of Middle Island sinkhole along a NW transect from Middle Island showing site of ground water source (Alcove), the region where it spreads out on the lake floor (Arena) and the moorings of the mid-water sediment trap. Inset represents a bathymetric contour map of the sinkhole with pink areas indicating uncertainty of measurements at the described location. Analyses of the carbon signature of the samples showed that the primary origin of sedimentary carbon is distant lake plankton from overlying waters and not the proximal benthic cyanobacteria.
All this leads to the other unanswered but obvious question. If the carbon in the sediments is of planktonic origin, then what happens to all that cyanobacterial mat production occurring right on the sinkhole sediment? Thus, the exploration of these underwater sinkhole ecosystems in the Great Lakes present exciting challenges and opportunities for future interdisciplinary studies with implications for our understanding of life processes in extreme ecosystems, microbial diversity and evolution, carbon sequestration and climate change.
This research was supported through funds from National Science Foundation and National Oceanic and Atmospheric Administration. Results are published in the journal Biogeochemistry (Nold, S. C., M.J. Bellecourt, S.T. Kendall, S.A. Ruberg, T.G. Sanders, J.V. Klump and B.A. Biddanda 2013: Underwater sinkhole sediments sequester Lake Huron’s carbon. Biogeochemistry 115: 235-250).
Page last modified November 5, 2013