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Manoomin 2011 - Plant Macrofossil Analysis
Research Interns: Lynda Morrison, Mary Anderson
Research Mentors: Vania Stefanova, Amy Myrbo, Christa Drake
Sponsoring Organizations: Fond du Lac Tribal & Community College, Fond du Lac Resource Management, University of Minnesota – LacCore
Funded by the National Science Foundation (NSF)
Study sites location ↓
Introduction
Plant macrofossils are plant remains (seeds, fruits, cones, leaves, buds, needles,
wood, and sporangia) that are visible to the naked eye and can be manipulated
by hand (Birks 2001). They are often studied in conjunction with pollen studies
because they help provide more conclusive look at the local record of vegetation,
owing to their shorter dispersal range. When analyzed though a sequence of lake
sediments macrofossils can reveal vegetation changes over time as well as past
lake level fluctuations and climatic change. Macroremains can often be identified
to a higher taxonomic resolution (more specific) than pollen – often to the species
level.
Materials and Methods:
Samples of 87 cm3 were taken at 3cm intervals from the top and bottom
sections of each core. Using four sieve layers for each interval (1 millimeter, 500
micron, 250 micron, 125 micron), the samples were washed with low-purity DI
water. The remains from the top two screens – 1 millimeter and 500 micron –
were examined with use of a light microscope, and macrofossils were picked
with fine tweezers and organized onto a paper-lined Petri dish coated in glycerol
to keep specimens moist and in place.
1
Pictures: 1) Floating-leaf pondweed
seeds 2) Birch seed, 3) Wild rice
seeds, 4) Najads seeds, 5) Chara sp.
oogonia
5
MID PORTAGE LAKE
2
3
Study sites
Core 3A – water depth 30 cm
4
Core 3A – water depth 131 cm
Core 4A – water depth 62 cm
Discussion & Results
Mid Portage Lake
The core taken from the shoreline (4A) of Mid Portage had more plant macrofossils than the core taken in deeper water (3A),
which had more aquatic and terrestrial macrofossils in the bottom section of the core, possibly meaning that the lake was
shallower when that sediment was deposited.
In core 4A wild rice occurs in both the top and bottom sections indicating that the water depth in this part of the lake has remained
favorable for wild rice. The top section examined shows evidence of pondweed, which indicates a slight rise in water depth.
Wild Rice Lake
Cores 3A and 5A, being closer to the shoreline, both contain much higher levels of tree and sedge macrofossils than core 4A
which is located near the center of the lake. Also present are Cinquefoils and Pea seeds, which are absent in core 4A – indicating
that the substrate (vegetation mat) is firm enough to support these plants.
The Chara (algae) oogonia and Najad seed patterns reflect greater water depths in core 4A/5A than in 3A.
Comparison
While it is evident that Wild Rice Lake does not support its namesake – perhaps because of greater-water level fluctuations – Mid
Portage, with a larger drainage system has does favorable conditions for wild rice. Pine macrofossils are present there but absent
from Wild Rice Lake. Birch macrofossils are found in both lakes, but spruce is found only in Wild Rice Lake.
Wild Rice Lake also sustains aquatic mosses consistent throughout in all of the cores, but only in core 4A in Mid Portage Lake.
Likewise Mid Portage holds more pondweeds than Wild Rice Lake, pointing to a greater water depth.
References:
1. Birks, H H. (2007). Plant Macrofossil Introduction. Elsevier B.V.
Core 4A – water depth 91 cm
Core 5A – water depth 82 cm
WILD RICE LAKE