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Episodic
Events:
Great Lakes Experiment
The Impact of Episodic
Events on the Nearshore-Offshore Transport and Transformation of
Biogeochemically Important Materials in the Great Lakes
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Episodic Event: Recurrent Coastal Plume
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| In 1996, GLERL scientists monitoring NOAA satellite images tracked the
development of a massive turbidity plume that ultimately extended over 10
miles off-shore and 200 miles along the southern coastline of Lake Michigan.
While previously known to occur, our satellite "view" of the annual
late winter/early spring plume in years past has been obscured by cloud
cover typical for the Great Lakes region during that time of year. The unusually
clear conditions in 1996 enabled scientists to see the full extent of the
plume for the first time. The plume is believed to consist of newly eroded
bluff material from the western shore near Milwaukee, Wisconsin as well
as materials washed into the lake over the last several decades. These very
small clay and silt particles then slowly settle into temporary reservoirs
all along the coastline. Each year the ice that builds along the coastline
breaks up, and large storms generates wind and waves strong enough to stir
these particles back off the lake bottom, resuspending them into the water
column. A conservative estimate is that the 1996 plume moved over one million
tons of material. Offshore eddies in the southeastern portion of the plume
coincide with the area of maximum sediment accumulation in the lake, implying
that this event plays an important role in how particles are moved around
in the lake (nearshore-offshore transport) and where they eventually accumulate
on the lake bottom. |
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Ecological Importance
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| Although relatively short in duration, lasting on the order of one month,
the annual plume is hypothesized to have a profound impact on the overall
ecology of Lake Michigan. While the plume is hypothesized to contain virtually
100% of the new "mud" that is washed into the lake each year,
some of the material that is stirred up off the bottom in this event has
been in the lake for decades and is being "reinjected" into the
water column. Nutrients and contaminants adhere to the fine-grained materials
and are transported by the plume. Contaminants such as PCBs may be scoured
from the water column by particles in transit and ultimately buried in high
depositional zones where they are no longer a threat to the food chain.
The high levels of phosphorous (P) in the plume, a nutrient that is necessary
for primary production, likely plays a critical role in the development
of Lake Michigan's spring diatom bloom. The plume eventually settles, creating
a cloudy bottom layer that is important in chemical interactions between
the surface sediments and water throughout the year. |
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Study Approach
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| One of the challenges of studying episodic events is "unpredictable"
timing of the particular event. The onset of the annually recurrent southern
Lake Michigan plume has been documented as early as February (1995) and
as late as May (1994). A team of over 40 environmental scientists from federal
agencies and universities have put together a comprehensive interdisciplinary
5 year research program to study the Lake Michigan plume. Teams of specialists
in remote sensing, physical oceanography, HF radar, hydrodynamic, meteorological,
and sediment transport modeling, post-depositional sediment behavior, environmental
radiotracers, phosphorous processes, phytoplankton processes, copepod reproduction,
and lower food web structure are all coordinating their efforts. The simultaneous
efforts being conducted across several different disciplines, focusing on
the same region, provides a unique opportunity for new insights into coupling
between biological, chemical, and physical processes. In early fall of 1997,
the first large array of scientific equipment will be deployed in a 1,500
square mile region off of St. Joseph, Michigan. Some of these moorings will
measure water velocities and temperatures throughout the water column, others
will collect the plume materials as they sink towards the lake bottom. These
measurements will be complemented by satellite-tracked drifters used to
measure the large scale circulation and to track the plume itself when it
occurs. For the first time anywhere in the Great Lakes, two coastal over-the-horizon
radar sites will also be installed and used to study surface currents, winds,
and wind waves. During the plume, multiple shipboard surveys, including
rare late-winter cruises, are planned for collecting data and samples for
further analysis. |
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Visible band satellite imagery of southern Lake Michigan AVHRR Channel
1. 3/16/96: Shore-fast ice along eastern shore, large ice floes in the
south. 3/22/96 & 4/1/96: Highly reflective plume (+) = stations sampled
by helicopter). 4/10/96: Off-shore eddying of plume coincides with zone
of maximum sediment deposition in lake (+) = stations sampled by small
boat. 4/24/96: Dissipation of plume. (Clouds are masked out on 3/22/96,
4/10/96 and 4/24/96 images.)
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Expected Products
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| The research program, which
includes three seasons of intensive data collection starting fall
1997, is expected to be completed in 2002. Expected research products
include the most extensive Great Lakes data collection in over 25
years, and the development of the most sophisticated research models
ever created for the Great Lakes. These will incorporate hydrodynamic
models (currents, temperature, wave, and ice), sediment transport
data, and lower food web simulations. These will be useful in evaluating
future lake management options, providing a more realistic assessment
of how nutrients and contaminants in the sediments continue to recycle
within the lake. |
Eleven of these sequential sediment samplers developed at GLERL
will be strategically deployed in Lake Michigan offshore from St.
Joseph and Michigan City to collect plume particles as they settle
towards the lake bottom. The traps are programmed to collect 23
samples over a period of 10 days each.
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| Participants
Include: |
- Academy of Natural Sciences, Estuarine Research Center
- Argonne National Laboratory
- Canadian Centre for Inland Waters
- Cooperative Institute for Limnology and Ecosystem Research
- Michigan Technological University
- National Oceanic and Atmospheric Administration (NOAA) COP
- National Oceanic and Atmospheric Administration (NOAA) NOS ORCA
- NOAA/Great Lakes Environmental Research Laboratory (GLERL)
- National Science Foundation (NSF) CoOP
- Ohio State University
- Rutgers University
- State Univ. of NY at Buffalo
- USGS - St. Petersburg
- University of Akron
- University of Georgia
- University of Iowa
- University of Michigan
- University of Minnesota
- University of Texas
- University of Wisconsin-Milwaukee
- University of Southern Mississippi
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The Plankton Survey System (PSS) is towed in a vertically undulating
(tow-yowed) mode as the ship travels at about 2.5 m/s. Data collected
will determine vertical structure of plankton and related physical
variables along the ship's path.
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| Text last revised June 1998 |
