Bonanza for Lake Superior: Seiches Do More Than Move Water
by Ben Korgen
Researchers Steve Eisenreich and Joel Baker used a submersible, the Johnson Sea-Link II, to explore the depths of Lake Superior and seiches in 1985 and 1986. Photo courtesy of the Minneapolis Star-Tribune
If you’ve ever experienced a large seiche (pronounced “saysh”) on one of the Great Lakes, it’s something you’ll not soon forget. That so much water can be moved in a relatively short period of time is astounding. Before you know it you’re either left high and dry or inundated with water.
Whenever ice is not an inhibiting factor, wind or air pressure changes can cause the entire surface of a lake to rhythmically rock back and forth in the physical process that can form a seiche. According to David Schwab, a scientist with the Great Lakes Environmental Research Lab, “seiche” is a French word that means “to sway back and forth.” It was first applied by a Swiss lake scientist, Francois-Alphonse Forel in the late 1800s. Forel is also known as the founder of limnology, or the study of lakes.
In Lake Superior, small seiches occur almost continuously. These go largely unnoticed. However, the biggest seiches can bang ships together in harbors, snap their mooring lines, and buckle their plates.
On July 13, 1995, a big Lake Superior seiche left some boats hanging from the docks on their mooring lines when the lake water suddenly retreated. In that seiche, lake water went out and came back within fifteen to twenty minutes at Ashland, Wisconsin, Marquette and Point Iroquois, Michigan, and Rossport, Ontario. People who witnessed it were amazed. In just a few minutes, water levels changed about three feet.
In 1998 a seiche occurred in Two Harbors, Minnesota, that caused several hundred thousand dollars of damage to vessels loading iron ore at the Duluth Missabe Iron Range Railway Company docks. “We’ve had two seiches in the past that have surged a vessel almost 12 to 15 feet, causing the ship to damage shuttles on our shiploader, and the vessel incurred some damage,” said company dock manager David VanBrunt. Just last December, another large seiche occurred in the Duluth-Superior harbor.
Seiches generate currents within the lake’s water column. The strengths of these currents depend on the sizes of the seiches driving them. The smallest seiches drive currents with no significant effect on vessel navigation. The biggest seiches drive strong and unpredictable currents that can move vessels off course and aground or otherwise into harm’s way.
Seiche-driven currents do more than disturb moored vessels and influence navigation. They can impact lake biology. Upper lake organisms eventually die. Bacteria decompose these dead organisms, converting them into nutrients or fertilizer for future generations. To support life, a lake needs mechanisms for lifting nutrients from deeper waters into the sunlit surface waters where algae use photosynthesis to create new living tissue.
What can lift nutrients in this way? Sometimes winds move surface waters away from the shoreline. This causes upwelling, a process in which deeper waters rise to replace surface waters driven offshore. Upwelling does lift nutrients, but it isn’t as dependable as other mechanisms. Steady winds must blow in a direction that moves surface waters offshore. As you know, winds aren’t always steady, and they can blow in many directions.
In many of the shallower oceanic regions near land, tidal currents are strong enough to create eddies that lift nutrients from deeper waters to the surface independent of weather conditions. The Great Lakes have tidal currents too weak to lift significant amounts of nutrients. Knowing this has led scientists to ask the following question: In water bodies with low tidal ranges, can seiches act as tide-replacement mechanisms that lift nutrients from deeper waters to the surface?
In the 1980s, Sea Grant researchers Steve Eisenreich and Joel Baker decided to find out how seiches might influence the biological economy of water bodies with low tidal ranges. First, they needed to recognize the importance of what scientists call the nepheloid layer. This is a turbid, nutrient-loaded, particle-rich zone that hovers above the lake floor.
They lowered instruments from a ship to make measurements in Lake Superior. Their analysis of these measurements suggested that seiches either maintain the nepheloid layer or make it bigger by suspending previously-deposited sediments. Near Duluth during the summer, for example, they found near-bottom currents driven by a single seiche that were strong enough to triple the nepheloid layer’s vertical dimension from sixteen to forty-eight feet!
With financial support from Minnesota Sea Grant and the National Undersea Research program at the University of Connecticut, these scientists used a submersible to explore first Lakes Huron and Superior, then Lake Michigan. During dives to the floor of Lake Superior, Eisenreich and Baker were surprised by the abundance and variety of life in the sediments.
During their dives, Eisenreich and Baker watched organisms living in Lake Superior sediments stirring materials free and moving them up into the nepheloid layer. They found that these materials were loaded with nutrients that would otherwise be locked within the sediments, but now could be used by organisms that live higher in the water column. Eisenreich and Baker were immediately struck by the profound implications of what they had discovered: that a seiche-controlled nepheloid layer can be an important source of raw materials for the water column.
When you think about Lake Superior, remember that during most of the year, many small seiches repeatedly help to feed the lake’s living organisms. When you learn that another big seiche has occurred, rest assured that the living lake has been treated to a bonanza!