Posts tagged: UM REUs

Mini-REU Abstracts

By , August 4, 2010


For the diatom analysis we focused on the sediment cores from Perch Lake; two samples were analyzed near the surface of the core (representing recent conditions) and three samples were analyzed from deeper sediments (representing approximately 3,000 years before present).   Diatoms were identified to genus and also grouped as benthic or planktonic.  Benthic diatoms live on the lake bottom, attached to sediments or aquatic plants, planktonic diatoms live suspended in the water column.  Since diatoms are photosynthetic, a low of benthic diatoms indicate the lack of light penetration to the lake bottom which would indicate turbid conditions.  Our first four samples were dominated by benthic Fragilaria species; these species tend to dominate shallow lake systems and can live either attached to bottom substrates or suspended in the water column.  We found the largest shift in diatom species assemblage between 319 cm and 379 cm sediment depth.   In the bottommost sample, there was decrease in benthic  Fragilaria and an increase in Navicula and Achnanthes species, as well as in the planktonic genus Cyclotella.  This suggests that water level may have been deeper at this time, due to the presence of the benthic genera Navicula and Achnanthes as well as the increase in the plantonic diatom species.


Why is pollen so important to our community? We found pollen in lake sediments from 10,000 years ago to the present time. We found a high abundance of spruce pollen at 600 to 700 cm, around the time the lake was first formed after the glaciers melted. This was followed by a high abundance of red-jack pine pollen from 425 to 700 cm. White pine pollen increases about 550 cm, estimated to be about 7000 years ago. Dates are estimated pending radiocarbon dates. There is a decrease of red-jack pine and an increase of white pine during the last ~4000 years.  This increase in white pine is usually thought to be caused by a change to cooler or wetter climate. Another interesting change is the increased abundance of grass pollen in the last several thousand years. Wild rice is a grass and this could represent an increase in wild rice abundance in the lake, but its pollen cannot be distinguished from other grasses so it is not clear what type of grass increased. Our data show that the forests of the region have change a lot over thousands of years and raise many questions about the role of climate and native people in these changes.


The results from the macrofossil analysis of Perch lake core 1C show the development of the aquatic and terrestrial vegetation since the last glaciation. Needles of spruce in the oldest sediments indicate the late glacial spruce stands around the lake. In the same period, the aquatic vegetation was represented by slender naiat (Najas flexilis) and pondweed (Potamogeton sp.). Seeds of birch and needles of pine between 600 and 350 cm indicate the arrival of these trees around the lake. In the littoral zone, spikerush (Eleocharis sp.) and common cattail (Typha latifolia) appeared. High amounts of seeds from slender naiat and pondweed between 250 and 350 cm indicate most probably a change from oligotrophic to mesotrophic conditions in the lake. However, after this change occurred, the amount of seeds from the slender naiat and pondweed has decreased. In addition to this, macrofossils of Chara sp. and Bryales sp. have been found in the sediment between 150 to 0 cm.

The top 90 cm of core 1A from Rice Portage lake were studied for macrofossils in order to determine whether or not wild rice (Zizania sp.) was present in the past. Our results show macro-remains of wild rice between 30 and 60 cm. Macrofossils of pine, spruce, birch, johnswort, common arrowhead, rush, sage, pondweed, slender naiat, and aquatic mosses were found as well.


The glumes (seed cases) of different grasses produce different assemblages of phytoliths. Phytoliths are silicon-dioxide accumulating cells found in all grasses. Phytoliths were recovered from glumes of zizania palustris from several localities: Perch Lake, Koochiching County, St. Louis County, and Hubbard County. A river rice sample was obtained from Clay County. A sample of Zizania aquatic was collected from Ohio, and Zea mays from North Dakota. The method we used to describe the phytholith assemblages was initially developed by Dr. Susan Mulholland, and refined by Dr. Thompson. This method consisted  of: taking images of the phytoliths, identifying the phytolith physical structure, measuring the structure, entering data from each sample (50 phytolith forms each)  and combining into a database allowing comparison of the samples. A program called PAST was used for statistical analysis. We used principal components analysis, multi-dimentional scaling, and cluster analysis to determine the differences between the samples. We saw that Z. palustris aquatic, and Zea mays each produced different assemblages of phytoliths. This establishes the utility of this method to identify wild rice from sediments or other contexts.

Research Topics – Summer 2010

By , April 10, 2010

What research study would you be interested in working on this summer at the University of MN?

Phytoliths– working with microscopes, computers and statistics

Diatoms – working with microscopes, ecology and chemistry

Magnetics – working with instruments, computers and physics

Pollen – working with microscopes and ecology

Plant macrofossils – working with microscopes and ecology

Ostracodes – Working with microscopes, ecology and chemistry

X-radiography and XRF elemental analysis – working with scientific  instruments, computers, x-ray images, and chemistry

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