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Entering the Twilight Zone (Adapted from the 2002 Gulf of Mexico Expedition)

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Entering the Twilight Zone (Adapted from the 2002 Gulf of Mexico Expedition) Thunder Bay Sinkholes 2008 Entering the Twilight Zone (adapted from the 2002 Gulf of Mexico Expedition) FOCUS TEACHIN G TI M E Deep water habitats Two 45-minute class periods, plus time for indi- vidual group research GRADE LEVE L 5-6 (Life Science) SEATIN G ARRAN G E M ENT Groups of four students FOCUS QUESTION What organisms are typical of deep water habi- MAXI M U M NU M BER O F STUDENTS tats, and how do they interact? 32 LEARNIN G OBJECTIVES KEY WORDS Students will be able to describe major features Cold seeps of cold seep communities, and list at least five Methane hydrate ice organisms typical of these communities. Chemosynthesis Brine pool Students will be able to infer probable trophic Trophic level relationships within and between major deep-sea Pelagic zone habitats. Epipelagic zone Mesopelagic zone Students will be able to describe the process of Bathypelagic zone chemosynthesis in general terms, and will be able Hadopelagic zone to contrast chemosynthesis and photosynthesis. Benthic zone Intertidal zone Students will be able to describe major deep-sea Subtidal zone habitats and list at least three organisms typical Bathyal zone of each habitat. Abyssal zone Hadal zone MATERIA L S Hydrothermal vent 5 x 7 index cards Drawing materials BAC kg ROUND IN F OR M ATION Corkboard, flip chart, or large poster board In June, 2001, the Ocean Explorer Thunder Bay ECHO Expedition was searching for shipwrecks AUDIOVISUA L MATERIA L S in the deep waters of the Thunder Bay National None Marine Sanctuary and Underwater Preserve in Lake Huron. But the explorers discovered more 1 Thunder Bay Sinkholes 2008 - Grades 5-6 (Life Science) Focus: Deep water habitats oceanexplorer.noaa.gov than shipwrecks: dozens of underwater sinkholes Minnesota and New York. Over thousands of in the limestone bedrock, some of which were years, sand, minerals, and sediments accumu- several hundred meters across and 20 meters lated on the seafloor, and were gradually com- deep. The following year, an expedition to sur- pressed to form sandstone, limestone and shale. vey the sinkholes found that some of them were About 1.8 million years ago, the Great Ice Age releasing fluids that produced a visible cloudy of the Pleistocene epoch began and continued layer above the lake bottom, and the lake floor until about 10,000 years ago. During this time, near some of the sinkholes was covered by con- four major periods of glaciation occurred, sepa- spicuous green, purple, white, and brown mats. rated by three interglacial periods. As the final glacial period came to a close, retreating glaciers Preliminary studies of the mats have found that along the U.S.-Canadian border revealed five where water is shallow (≤ 1.0 m) the mats are huge lakes that we now know as the Laurentian composed of green algae. In deeper (about 18 Great Lakes. In the Great Lakes region, aquifers m) waters, mats are formed by filamentous purple are found in deposits of sand and gravel left by cyanobacteria. Mats near the deepest (93 m) glaciers, as well as in porous bedrocks (limestone sinkholes are white or brown, but their composi- and sandstone) that were formed much earlier tion is presently unknown. The appearance of in geologic time. Five major aquifers are recog- mats near the deepest sinkholes is very similar to nized in this region: one near the land or lake mats observed in the vicinity of cold seeps and floor surface (the surficial aquifer) and the others hydrothermal vents in the deep ocean, which are in deeper bedrock named for the geologic time often formed by chemosynthetic bacteria. These periods when they were formed (the Cambrian- bacteria are able to obtain energy from inorganic Ordovician, Silurian-Devonian, Mississippian, and chemicals, and are a food source for a variety Pennsylvanian aquifers). The bedrock that forms of other organisms that inhabit cold seep and the Silurian-Devonian aquifer is primarily lime- vent communities. Biological communities whose stone and mineral formations from evaporating primary energy source comes from chemosyn- seawater. Both fresh and saline water are found thesis are distinctly different from more familiar in the Silurian-Devonian aquifer. biological communities in shallow water and on land where photosynthetic organisms convert the Sinkholes are common features where limestone energy of sunlight to food that can be used by is abundant, because limestone rocks are soluble other species. Hydrothermal vent and cold seep in acid. Atmospheric carbon dioxide often dis- communities are home to many species of organ- solves in rainwater to form a weak acid (carbonic isms that have not been found anywhere else on acid). Rainwater flowing over land surfaces may Earth, and the existence of chemosynthetic com- also pick up organic acids produced by decay- munities in the deep ocean is one of the major ing leaves and other once-living material. The scientific discoveries of the last 100 years. resulting weak acid can slowly dissolve limestone rocks to form caves, springs, and sinkholes. Scientists hypothesize that the source of the Sinkholes on land are known recharge areas for fluids venting from the Lake Huron sinkholes is the Silurian-Devonian aquifer (areas where water the Silurian-Devonian aquifer beneath the lake’s flows into the aquifer). But very little is known sediments. Aquifers are rocks and sediments that about the chemistry, geology, and biology of contain large amounts of water. Between 350 submerged sinkholes that may serve as vents for and 430 million years ago, during the Paleozoic groundwater in the aquifer. Water samples col- era, shallow seas covered what is now the border lected near these sinkholes is very different from between Canada and the United States between the surrounding lake, with much higher concen- 2 Thunder Bay Sinkholes 2008 - Grades 5-6 (Life Science) Focus: Deep water habitats trations of sulfate, phosphorus, and particulate communities they may support; but since their organic matter, as well as ten times more bacteria appearance is very similar to mats found in compared to nearby lake water. These observa- some deep ocean habitats, these habitats may tions suggest that submerged sinkholes may be provide clues for explorations of the Thunder biogeochemical “hot spots” inhabited by unusual Bay sinkholes. and possibly unknown life forms. At the same time, water flow through submerged sinkholes Lead a discussion of the major categories depends upon recharge from land. This means that of ocean habitats, and introduce deep-sea sinkhole ecosystems are likely to be very sensitive chemosynthetic communities (hydrothermal vents to changes in rainfall patterns that may accom- and cold seeps). pany climate change, as well as human alterations of these landscapes surrounding recharge areas. Ocean habitats are usually categorized into These factors make understanding sensitive sink- zones: hole ecosystems an urgent necessity. I. Pelagic zones are found in the water column above the bottom. Organisms that inhabit Because little is presently known about these sys- pelagic zones are divided into plankton that tems, information from chemosynthetic communi- drift with the ocean currents and nekton that ties in the deep ocean may provide a useful start- can swim and control their motion in the ing point for exploration of sinkhole ecosystems. water (at least to some extent). This activity focuses on major ocean habitats, A. The epipelagic zone includes surface organisms typically found in these habitats, and waters where light is adequate for pho- the interactions that take place within and among tosynthesis (about 200 m, maximum). these habitats. Phytoplankton are the dominant primary producers in this zone. LEARNIN G PROCEDURE B. The mesopelagic zone (about 200 m - 1. To prepare for this lesson, visit http://www.bio.psu. 1,000 m) is the twilight zone. Because edu/cold_seeps for a virtual tour of a cold seep there is not enough light for photosyn- community, http://www.bio.psu.edu/hotvents for a thesis, much less energy is available to virtual tour of a hydrothermal vent community, support animal life. Bacteria and detritus and introductory essays for the Thunder Bay (pieces of dead plants and animals that Sinkholes Expedition at http://oceanexplorer.noaa.gov/ slowly settle to the bottom) are the pri- explorations/08thunderbay/welcome.html. You may also mary sources of food for animals like want to watch video clips linked from http://www. jellyfishes that are confined to this zone. glerl.noaa.gov/res/Task_rpts/2006/eosruberg06-1.html. Other animals, including squids, fishes, and shrimps can move up and down 2. Briefly introduce the Thunder Bay Sinkholes through the water column, and have a Expedition, highlighting the discovery of flu- wider range of food available to them. ids emerging from sinkholes on the lake floor, C. The bathypelagic zone (sometimes divid- and the variety of mats found in the vicinity of ed further into an additional abyssope- these sinkholes. Be sure students understand lagic zone) has no light at all. Deep-sea the concept of an aquifer, and that the mats organisms are dependent upon produc- are likely to be living organisms (algae and/or tion in other zones. The base of bathy- bacteria) that can serve as food for many other pelagic food webs may be primary pro- organisms. Point out that very little is known duction in shallower water (obtained by about the mats in Lake Huron or the biological feeding on detritus or on other animals 3 Thunder Bay Sinkholes 2008 - Grades 5-6 (Life Science) Focus: Deep water habitats oceanexplorer.noaa.gov feeding in shallower water) or chemosyn- and oil seep out of sediments. These areas, thetic communities like hydrothermal vents known as cold seeps, are commonly found or cold-seeps. along continental margins, and (like hydrother- D.
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