Lab 10 ESCI

3 Each varve pair represents a year’s deposition. In Minnesota lakes, a varve’s dark layer forms during the winter when the lake is ice covered while its light layer forms from late spring through early fall when the lake is ice-free. Human Impacts on Lake Systems Human activities affect lake systems in many different ways. Agriculture & Deforestation – The nutrients in a lake arrived by one of two processes. Water-soluble nutrients (like nitrogen, silica, and sulfur) came in as dissolved material in incoming water. But phosphorus and iron arrived as crystals attached to sediment grains so the movement of the latter nutrients is tied to sediment transport. Since prairie grasses and tree roots once minimized soil erosion, in the past Minnesota lakes were phosphorouslimited. However, as agricultural activities and deforestation led to more soil erosion, phosphorus runoff increased. If farmers raise more livestock or use more fertilizer to increase crop yields, phosphorus runoff becomes high enough to harm lake ecosystems. Wetland Drainage – Draining wetlands or converting them into farm fields dramatically altered lake drainage basins. Natural wetlands trap sediment and filter water which helps protect lake systems. However, as wetlands were drained, flooded, or converted into fields, this protection vanished. Sediment and other materials now flow directly into our lakes with little filtration. Urbanization – High-density housing also transforms lake systems. Yard waste, fertilizer runoff, or household sewage can deliver overwhelming amounts of nutrients to lakes. In addition, urban landscapes dramatically increase surface runoff. In urban areas, roads, pavement, and buildings cover much of the land. Now, instead of seeping into the ground to slowly move into lake systems, rainfall or snowmelt water rushes across impermeable surfaces to flush more sediment and nutrients into lakes. Turbidity (or Water Clarity) – Increasing lake sediment or nutrients decreases water clarity. While this is an aesthetic concern for homeowners along the lakeshore, it is a matter of life or death for many lake organisms. Photosynthesis only occurs in a lake’s clear sunlit shallow water layer – its photic zone. In clear ocean water, the photic zone may extend to 200 meters deep. However, in most Minnesota lakes suspended sediment, nutrients, and algae limit the depth sunlight reaches. If nutrient and sediment levels become too high, sunlight is blocked and photosynthetic generation of oxygen decreases. Organisms living on lake bottoms begin to decline or die out. In severe cases, all life below the sunlit surface layer decreases in abundance, including the many fish cherished by Minnesota anglers. Algal Blooms & Lake Anoxia – When people think of lake organisms, they often imagine fish, birds, turtles, crayfish, or multi-cellular plants. However, folks living along urban lakes are likely to add algae to their list, often with mixed feelings. Photosynthetic algae are necessary for a healthy lake since they produce the dissolved oxygen that other lake organisms depend on. However, increased nutrient levels due to human activities trigger immense algal blooms. These blooms cover the lake, making it less attractive for recreation. More importantly, because algae are very short-lived, these blooms soon die and decay. While this only creates unpleasant smells for people living along the lake, its impacts on aquatic life can be catastrophic. Decay requires oxygen. So large masses of decaying algae can remove all dissolved oxygen from the lake water, turning much of the lake into a barren anoxic (without oxygen) zone devoid of most life.

6 Forty years later, homes extended along most of the lakeshore, transforming Lake McCarron into an urban lake system. Due to combined nutrient runoff from agriculture, household, and recreational activities, Lake McCarron’s bottom waters became persistently anoxic (oxygen depleted) and phosphorous released from anoxic sediments accumulated in the lake’s deep water. When the lake seasonally overturned, this excess phosphorus triggered algal blooms. As early as 1928, even before Lake McCarron became a fully urban lake, water quality was so poor that Ramsey County initiated an expensive copper sulfate treatment program to kill its algae (Myrbo, 2008). % Year Male Female Total Increase Increase 1850 1350 877 2227 1860 6260 5890 12150 9923 446% St. Paul growth 1870 12021 11064 23085 10935 90% 1880 24877 21013 45890 22805 99% 1890 73292 66505 139797 93907 205% 1900 88496 82058 170554 30757 22% 1910 116730 106945 223675 53121 31% 1920 122601 121953 244554 20879 9% 1930 139547 147174 286721 42167 17% suburbs growth 1940 149004 160931 309935 23214 8% 1950 171030 184302 355332 45397 15% 1960 202880 219645 422525 67193 19% 1970 227675 248580 476255 53730 13% 1980 219086 240698 459784 -16471 -3% 1990 231984 253781 485765 25981 6% 2000 246288 264747 511035 25270 5% Table 1. Changes in Ramsey County population over time (census data from the Minnesota Historical Society). The first population boom (1860-1910) was primarily in the city of St. Paul. McCarron Lake remained a rural area until the second population boom (1930 to 1970), which reflected the growth of suburban communities. 1 1 Note that Ramsey County’s historic gender balance reflected its frontier territory origin. Not until the 1920s were there equal numbers of women and men – so the area’s Norwegian bachelor farmer tales had a historical basis. Figure 7. Bathers crowd Lake McCarron’s beach a few years after the beach’s 1926 opening. During its heyday, thousands of people visited the beach every year. (Photographs courtesy of Minnesota Historical Society.)

9 2 This authigenic carbonate is a reflection of the close tie between lake and ground water systems. As groundwater moves through the subsurface, it dissolves carbonate rock. The resulting dissolved ions later precipitate in the lake to form carbonate crystals. This authigenic carbonate once comprised the bulk of Lake McCarron’s sediment before the lake’s drainage basin became an agricultural area.

12 Question 2 (1 point) If the start of farming and varve sedimentation coincided, how might farming have triggered that change? If they do not coincide, then what change in the basin’s use might have led to varve sedimentation? ☐ – It coincided with farming because of increased soil erosion. ☐ – It coincided with farming because of increased fertilizer runoff. ☐ – It came after farming began as wetlands were drained. ☐ – It came after farming began as housing subdivisions were built. Note that for the first fifteen years of varve deposition, varve sets were remarkably uniform in thicknesses. From that you can calculate the average yearly sediment thickness in the lake before human activities significantly changed deposition rates. You can then use that yearly average to estimate the amount of time represented by the non-laminated intervals in the lower parts of the core. Of course, lake researchers would use isotope dating to confirm ages, but for our lab exploration rough estimates are good enough. Question 3 (1 point) For the first fifteen years of varve deposition, how much sediment was deposited each year? In other words, how thick is each varve (not a set of five varves)? ☐ – 1 mm ☐ – 2 mm ☐ – 3 mm ☐ – 4 mm ☐ – 5 mm ☐ – 6 mm The dark-colored non-laminated sediment interval is 13 cm thick. The light-colored nonlaminated interval is 72.5 cm thick. As mentioned varves began to form around 1915. Question 4 (1 point) Using your average varve thickness as an estimate of yearly sedimentation rates, when did the shift from light colored to dark colored non-laminated sediment occur? ☐ – ~1870 ☐ – ~1876 ☐ – ~1882 ☐ – ~1888 ☐ – ~1902 ☐ – ~1908 Question 5 (1 point) From Table 2, which change in land use in the lake basin is most likely responsible for this change from light-colored to dark-colored sediment? ☐ – This change occurred as the first farm was built in the area. ☐ – This change occurred as more farms were built in the area. ☐ – This change occurred as ditch digging and wetland draining began. ☐ – This change occurred when the first housing subdivision was built. Question 6 (1 point)

15 Earlier, we noted that temperature differences could separate warm less-dense shallow water from underlying cool more-dense water. In a similar manner, density differences due to salinity also lead to lake stratification. Figure 12. Impacts of winter road salt use on lake overturn. Without road salt runoff, Minnesota lakes tend to completely overturn during the spring when the lake is cool enough that temperature stratification is reduced (A). However, saline runoff sinks to the lake bottom creating a salinity stratification that limits lake overturn (B). In addition, phosphorous from older lake sediments leach into this anoxic water layer – so when the lake does more completely overturn immense algal blooms are triggered by the excess phosphorous. As saline meltwater from winter roads flows into a lake, it sinks to form a dense bottom water layer. Even when surface waters cool in the fall or spring, this salinity-generated stratification may be great enough to delay or prevent the lake from completely turning over and mixing (Figure 12). Consequently, a lake’s bottom layer may remain persistently anoxic (without oxygen) for much of the year. This harms bottom-dwelling organisms but also alters water composition. Phosphorus escapes from anoxic sediment faster than from aerated (oxygenated) sediment. So during periods of bottom water anoxia, phosphorous builds up in deeper water layers. When the lake finally overturns, this excess phosphorous triggers massive algal blooms. As a result, even if people manage to eliminate current phosphorus runoff into salinitystratified lakes, phosphorus from older lake bottom sediments may continue to trigger algal blooms for decades afterwards. While road salt impacts tend to create local problems that affect adjacent lakes, other changes in nutrient supply can have more extensive impacts. See Part D for examples of how Minnesota land use affects downstream water bodies all the way to the Gulf of Mexico. Exercise B – ‘A Road Runs Through It’ Road Salt Use and Lake McCarron Highway 36 runs across the northern part of Lake McCarron’s drainage basin (Figure 8) so winter road salt has washed into the lake. As a result, Lake McCarron now exhibits a strong seasonal cyclicity in salinity (salt) levels, with higher salinity during the winter and spring thaw and lower salinity in the summer and fall. Table 4 shows chloride (Cl - ) concentrations measured over a four-year interval as reported by Novotny, Murphy and Stefan, 2008 . Table 4. Chloride levels in Lake McCarron Min Max Year Summer-Fall Winter-Spring % seasonal Cumulative Increase in

(from Myrbo, 2008). As welcome as that improvement was, the sewer system did not fully restore Lake McCarron’s ecology. Continued high nutrient levels and algal blooms led to construction of the McCarrons Wetland Treatment System (MWTS) northwest of the lake, along the main stream that flows into the lake (Figure 15). In the 1800’s, farmers had drained the original wetlands that were here and converted them to farmland. A century later, Ramsey County began transforming them back to wetlands. In 1985, the county built two sediment basins to trap sediment before it reached the lake and developed six wetland areas (called Villa Park Ponds) – planting vegetation designed to reduce phosphorus levels in water flowing into the lake (Figure 16). The Capital Region Watershed District reengineered the wetland system in 2004 to improve its efficiency. Figure 15. Location of the MWTS (after Myrbo, 2008) Figure 16 – Photographs of the sediment containment basin wall (left) and a restored wetland area (right). Both are part of the MWTS designed to reduce the lake’s sediment and nutrient influx. While remediation efforts helped Lake McCarron, restoring lake systems’ health may take a long time. In a study of a Swiss alpine lake that became eutrophic (subject to algal blooms) because of 14 th and 15 th century animal pasturing, it took 85 years after people abandoned the area before the lake returned to its original condition (Hausmann et al. , 2002). A lake that remains vulnerable to urban pressures, like Lake McCarron, may take even longer to recover. While that is discouraging, Lake McCarron is responding to remediation rather than getting worse. Remediation efforts, even in urban settings, can improve lake quality. We just need to be patient. Eventually, as local lake systems improve, we should see changes in larger water systems. Exercise C– Lake Remediation Efforts Sanitary Sewer System (~1960)

1990-1996 5.15 9.20 12.23 14.79 20.31 8.64 875576 1,607,538 1980-1990 4.10 6.48 9.89 12.24 17.98 7.35 744661 1,368,774 1970-1980 3.63 6.26 9.91 12.23 15.67 6.06 613840 1,157,650 1960-1970 3.22 5.34 9.45 12.74 14.56 7.09 718628 1,302,040 1950-1960 2.23 3.81 9.57 10.38 12.32 6.04 611792 1,093,263 1940-1950 2.04 3.19 9.56 7.78 11.07 4.62 467922 847,897 1930-1940 1.56 3.15 7.72 5.52 6.29 3.13 316680 601,994 1910-1930 1.22 2.16 4.58 4.22 6.42 2.63 266210 497,136 1890-1910 1.25 1.54 2.86 2.84 4.73 1.96 198210 373,843 1860-1890 1.21 1.63 2.65 2.97 3.82 1.86 188684 361,494 1830-1860 0.87 1.16 2.65 2.10 3.82 1.50 151625 287,346 Before 1830 0.54 0.70 1.18 1.24 2.14 0.78 79304 151,654 Table 5. Sediment accumulation rates in Lake Pepin from the five cores shown in Figure 17 and for the lake as a whole (box on the right). Data compiled by Dan Engstrom (St. Croix Watershed Research Station, Science Museum of Minnesota) as research reported in Engstrom et al. (2009). a = abbreviation for year (annum) Figure 18. Graphic representation of data from Table 5, showing rates of sediment accumulation in Lake Pepin. Table 5 and Figure 18 show changes in the lake’s sediment accumulation rate over time. Not only did the volume of sediment change but also its composition. Figure 19 shows the relative abundance of different biologic components in the lake sediments. Diatoms are the most abundant algae present. These single-celled organisms include forms that live on the lake sediment surface (benthic diatoms) and organisms that float in the lake’s shallow water (planktonic). Both groups flourish when the lake water is clear and sunlight reaches the lake bottom floor. However, if muddy water or algal blooms prevent sunlight from reaching the lake bottom, planktonic forms will dominate the lake community. As shown in Figure 19, as Lake Pepin’s sedimentation rate increased, a pronounced shift in dominance occurred from benthic forms to planktonic forms. This occurred as increased erosion from agricultural activities raised phosphorous levels to trigger algal blooms that decreased water clarity. Regional Impacts – Further Afield (Gulf of Mexico) Figure 1 9 . Graph showing relative proportions of planktonic diatoms, benthic diatoms, and chrysophyte cysts in Lake Pepin’s basin sediment. Higher levels of cysts and benthic diatoms reflect periods of clear water conditions with sunlit lake bottom (from Engstrom et al. , 2009). 0 200000 400000 600000 800000 1000000 1200000 1400000 1600000 1800000 >1830 1830-1860 1860-1890 1890-1910 1910-1930 1930-1940 1940-1950 1950-1960 1960-1970 1970-1980 1980-1990 1990-1996 Sediment Deposition in Lake Pepin over Time ) m3/a (

The Minnesota River Valley tile drainage systems not only affect Lake Pepin but the Gulf of Mexico. However, Lake Pepin is an effective sediment trap so the tile drainage systems’ impacts to the south are tied to increased nutrient runoff rather than sediment deposition. Phosphorous and nitrogen are two of the most important nutrients in aquatic systems and the scarcity of one or the other is typically the main limiting factor on aquatic productivity. However, phosphorous and nitrogen arrive in aquatic systems by different paths. Phosphorous typically moves as salt crystals attached to sediment particles while nitrogen is water-soluble and moves as dissolved ions in flowing water. Consequently, the increased sedimentation in Lake Pepin also increased phosphorous in the lake, which triggers algal blooms and subsequent fish kills. Phosphorous is also the main nutrient responsible for Lake McCarron’s algae production. However, because phosphorous moves as crystals attached to sediment, it is less mobile than dissolved nitrogen. While Lake Pepin traps phosphorous-laden sediment, dissolved nitrogen can flow downstream to enter the Gulf of Mexico. There, increased nitrogen levels trigger immense algal blooms along the Gulf coast especially during summer months. As these short-lived algal blooms die and decay, their decay consumes the water’s dissolved oxygen, making it hypoxic (oxygen depletion). This results in large areas of the Gulf Coast becoming ‘dead zones’ devoid of multicellular life. The nitrogen responsible for the Gulf Coast ‘dead zone’ comes from waters across the Mississippi River drainage basin but the greatest contributors are Minnesota River Valley tile drainage systems. Figure 20. Map of a typical Gulf of Mexico ‘dead zone’ overlaid across the Upper Midwest area to illustrate the dead zone’s scale. The core of the ‘dead zone’ (red) ranges from 7000 to 8000 square miles, an area larger than all of Minnesota’s lakes combined.

Figure 20 overlays the Gulf’s summer ‘dead zone’ on a map of the Upper Midwest. Although algae and bacteria flourish in this dead zone, all commercial fisheries have vanished. Hence, agricultural practices that increase crop yields in the river’s headwaters are destroying the fishing industry at its mouth. Balancing these intertwined processes is a formidable legal and environmental challenge. Exercise D – Regional Impacts Deposition rates are greater upstream (near cores E & D of Figure 17) and decrease downstream (towards cores B & A). This is because most sediment drops out when the river flow slows down as it enters Lake Pepin. However, to answer the questions below, you should use the whole lake figures in the far right column of Table 5 (shown in m 3 /a). Question 17 (1 point) Roughly how many times greater is the recent sediment deposition rate (1990-1996) than the deposition rate before Euro-American agriculture began in the region (~1830)? ☐ – 2x ☐ – 5x ☐ – 10x ☐ – 20x ☐ – 50x ☐ – 100x The initial increase in sediment deposition occurred as farmers converted prairies to farm fields. However, by 1940 farmers had created most of our present agricultural areas. Consequently, changes in sediment loads after 1940 are primarily due to the use of tile drainage systems to drain wetland areas rather than to an increase in cultivated acreage. Question 18 (1 point) From Figure 18, which factor played the greater role in increasing sediment deposition: increased cultivated acreage (pre-1940) or tile drainage system use (1940 on)? ☐ – increased acreage under cultivation ☐ – tile drainage system use Let’s play with some numbers. Before 1830, Lake Pepin had an estimated volume of 653,000,000 m 3 and each year upstream streams brought ~151,650 m 3 of sediment into the lake. Presently, the remaining volume of Lake Pepin is 553,000,000 m 3 and upstream rivers now annually deliver ~1,607,500 m 3 of new sediment (Engstrom et al. , 2009). Question 19 (1½ points) How long would it have taken to fill Lake Pepin if sedimentation levels had remained at their pre-1830 level? _______ years If sedimentation rates stay at the 1990-1996 level, how many years will it take to fill Lake Pepin’s remaining volume? (Note that this is a maximum estimated lifespan, as sedimentation rates are still climbing.) _______ years Your answers above are for the whole lake basin. In contrast, the shallow, northern part of the lake (shaded areas around cores E & D of Figure 17) is estimated to fill in less than a fourth of that time as deposition rates are higher where the river first begins to slow. If this ¼ estimate is correct, if you have children or grandchildren are they likely to see the northern part of Lake Pepin significantly fill in?

☐ – yes, it could be in their lifetime ☐ – no, it shouldn’t happen in their lifetime Question 20 (2 points) Why is phosphorous the main fertilizer that threatens the environmental health of Lake Pepin yet nitrogen is the primary cause of the Gulf’s ‘dead zone’. Part E: Continental Impacts – A Recent Megafauna Mass Extinction Figure 21. Mammoth (left) and Mastodon (right) by Dantheman9758 Wikipedia Well, let’s finish by exploring the megafauna extinction that transformed the world’s terrestrial ecosystems. Until relatively recently, mammoths and mastodons roamed North America (Figure 21). Then they abruptly died out. As early as the 1840s, when scientists first realized there had been a past ice age, two ideas have been forwarded for the demise of these beasts and other recent megafauna: climate change and human hunting. More recently, some have suggested a meteorite impact around 12,900 years ago was responsible for their extinction. One of the difficulties in settling this question is timing. The megafauna fossil record is not continuous through time and even when fossils are found, we have no way of knowing how many others were present. Surprisingly, lake sediment records provide a way around this dilemma. Obviously, we do not find megafauna in lake sediment cores, but we do find Sporormiella spores (Figure 22). This fungus lived in megafauna digestive tracts and its spores were expelled in their dung. So the presence and abundance of Sporormiella spores in lake sediment records reflects the abundance of megafauna living in the lake basin. Figure 22. Sporormiella spores courtesy of Alain Brissard

Roughly the same time the megafauna declined, there was a pronounced shift in plant pollen records to plant communities that have no modern analogs. There was also a significant change in the number of wildfires. Hence, this was a time of change, but how can we sort out the timing of each change relative to one another? As you have probably already guessed, we will turn to lake sediment cores. Figure 24 shows the distribution and abundance of sediment types through time in cores taken from Appleman Lake, Indiana (modified from J.L. Gill, et al. 2009) . At first the figure may appear daunting but the first five columns simply show the distribution and abundance of five tree pollen types through time while column G shows the abundance of Sporormiella spores through time. The number of charcoal pieces found in the sediment (column I) reflects the fire or burn pattern of the area over time. Depth is recorded on the right and the time before present is indicated in thousand-year increments (ka) on the left. Figure 24. Distribution through time of various tree pollen types (A, B, C, E, F), Sporormiella (G), and charcoal counts (I). Picea – spruce, Pinus - pine, Quercus – oak, Fraxinus Nigra – black ash, and Ostrya/Carpinus - ironwood. From a plant community perspective, the most unusual part of this diagram is the interval from 12,000 to 13,700 years ago (12 to 13.7 ka) when the surrounding forests consisted of a

mixture of spruce, ash, and ironwood trees. In the present, that particular mix of trees is rare. Spruce forests now tend to be boreal, distinct from the oak, ash, and ironwood trees that form the region’s modern deciduous forest cover. Megafauna Extinction The North American megafauna extinction did not occur at the same time in every area so we will only concentrate on their disappearance from this one lake basin. Several factors have been proposed for their extinction. Climate Shifts A warming period began about 14.7 ka, but was abruptly interrupted by a rapid shift back to glacial conditions known as the Younger Dryas from 12.9ka to 11.7 ka. Because climate shifted so abruptly, the start of the Younger Dryas climate change has been proposed as a possible cause for the megafauna extinction. Plant Community Shifts Although changes in plant communities might have driven megafauna extinction, realize the reverse may also be true. In Africa, elephants and other large animals strongly impact plant communities, keeping savannah grasslands open by preventing forests’ spread. When North American bison herds were exterminated, the loss of bison meant that woodlands were no longer held in check and many prairie grasslands were lost to woodland expansion. Meteorite Impact Although the idea remains controversial, some researchers claim a meteorite impact occurred about 12.9 ka that was responsible for both the Younger Dryas cooling event and the megafauna extinction as well as the decline of the Clovis culture. Human Impacts Humans may have played a role both in intensifying fire regimes (the number and scale of fires) and in contributing to the megafauna extinction. In southeastern Wisconsin, butchered mammoth bones found at one site date from 14.8 to 14.1 ka. Exercise E – Megafauna Extinction Well, with the increased temporal resolution lake sediment cores provide, let’s examine how the timing of these changes relate to one another in order to figure out which proposed extinction cause makes the most sense. So when did the megafauna die out, fires become abundant, and plant communities change? Question 21 (1 point) Based on spore and pollen patterns, is it more likely that a changing plant community led to megafauna extinction or did the loss of megafauna lead to changes in the plants? ☐ – changes in plant communities led to megafauna extinction ☐ – megafauna extinction led to changes in plant communities Question 22 (1 point) From the pollen patterns, which type of plants do you think the megafauna fed on (and kept in check)?

☐ – needle trees (spruce & pine) ☐ – broadleaf trees (oak, ash, and ironwood) An area’s fire patterns depend on which plants are present, fuel loads (dead plants), and human activities. The types of plants present can be affected by the feeding patterns of megafauna while the loss of megafauna could lead to the buildup of fuel loads across an area. Question 23 (1½ points) Based on the spore, pollen, and charcoal patterns, when did the first significant fires begin? ☐ – ~9.3 ka ☐ – ~10.8 ka ☐ – ~11.3 ka ☐ – ~12.8 ka ☐ – ~14.3 ka When did fires become common? ☐ – ~15.8 ka ☐ – ~9.3 ka ☐ – ~10.8 ka ☐ – ~11.3 ka ☐ – ~12.8 ka ☐ – ~14.3 ka ☐ – ~15.8 ka Which factor is most closely associated with this change to fires becoming common? (Note that we do not know when humans first became a significant part of this regional ecosystem.) ☐ – the loss of megafauna ☐ – a switch in plant communities Question 24 (2 points) From the patterns you see, which of the following hypotheses are possible or unlikely? A meteorite impact at 12.9 ka played a significant role in the megafauna extinction. ☐ – possible ☐ – unlikely Climate warming beginning ~14.7 ka played caused the megafauna extinction. ☐ – possible ☐ – unlikely Human hunting played a significant role in the megafauna extinction. ☐ – possible ☐ – unlikely An abrupt shift to colder conditions (12.9-11.7 ka) caused the megafauna extinction ☐ – possible ☐ – unlikely

Lab Conclusion The question of to what extent humans are responsible for the megafauna extinction seems like a good ending point. Humans are now one of the most potent geologic forces affecting the Earth’s surface systems. We now move more sediment than rivers or glaciers, transform ecosystems on an unprecedented scale, and are even altering Earth’s climate. Environmentally, one of our greatest challenges going forward is to determine how to best manage our many impacts on Earth Systems. This laboratory exercise is based upon three primary references: Myrbo, A., 2008, Sedimentary and historical context of eutrophication and remediation in urban Lake McCarrons (Roseville, Minnesota), Lake and Reservoir Management, v. 24, p. 349-360 Engstrom, D.R., Almendinger, JJ.E. and Wolin, J.A., 2009, Historical changes in sediment and phosphorous loading to the upper Mississippi River: mass-balance reconstructions from the sediments of Lake Pepin, Journal of Paleolimnology, vol. 41, p. 563-588 Gill, J.A., Williams, J.W., Jackson, S.T., Lininger, K.B., Robinson, G.S., 2009, Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America, Science, Vol. 326, Issue 5956, p. 1100-1103 Additional References: Hausmann, S., Lotter, A.F., van Leeuwen, J.F.N., Ohlendorf, C., Lencke, G., Grönlund, E., and Sturm, M., 2002, Interactions of climate and land use documented in the varved sediments of Seebergsee in the Swiss Alps, The Holocene, vol. 12, p. 279-289 Novotny, E.V., Murphy, D., and Stefan, H.G., 2008, Increase of urban lake salinity by road deicing salt, Science of the total environment, 406(1-2), 131.