Underwater sinkhole sediments sequester Lake Huron's

Lake Huron's submerged sinkhole habi- tats are impacted by high-conductivity groundwater that allows photosynthetic cyanobacterial mats to form over thick, carbon-rich sediments. To better understand nutrient cycling in these habitats, we measured the stable isotopic content of carbon and nitrogen in organic and inorganic carbon pools in Middle Island sinkhole, a *23 m deep feature influenced by both groundwater and overlying lake water. Two distinct sources of dissolved CO2 (DIC) were available to primary producers. Lake water DIC (d 13 C =- 0.1 %) differed by ?5.9 % from ground- water DIC (d 13 C =- 6.0 %). Organic carbon fixed by primary producers reflected the two DIC sources. Phytoplankton utilizing lake water DIC were more enriched in 13 C( d 13 C =- 22.2 to -23.2 %) than mat cyanobacteria utilizing groundwater DIC (d 13 C = -26.3 to -30.0 %). Sinkhole sediments displayed an isotopic signature (d 13 C =- 23.1 %) more similar to sedimenting phytoplankton than the cyanobacterial mat. Corroborated by sediment C/N ratios, these data suggest that the carbon deposited in sinkhole sedi- ments originates primarily from planktonic rather than benthic sources. 210 Pb/ 137 Cs radiodating suggests rapid sediment accumulation and sub-bottom imaging indicated a massive deposit of organic carbon beneath the sediment surface. We conclude that submerged sinkholes may therefore act as nutrient sinks within the larger lake ecosystem.