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As of 2004, this project is no longer current. Please see the Research Programs page for a list of current research projects.
Temporal and Spatial Variation in Lipid Content of the Mayfly Hexagenia
This project was completed in 2003.
The
burrowing mayfly Hexagenia was once very abundant in many
nearshore regions of the Great Lakes including Green Bay, Saginaw
Bay, and western Lake Erie. Because of eutrophication,
populations of this pollution-sensitive organism declined
dramatically in the 1950s and remained at low levels through the
1980s. Recent evidence indicates that populations are now
returning in some areas, and in the western basin of Lake Erie,
abundances have increased every year since 1991 according to
published reports, and individual Hexagenia have also been
reported in the lower Fox River in Green Bay. At present, the
environmental changes that led to this recolonization are
unknown. In the western basin, the return of Hexagenia may
be a response to declines in algal blooms and anoxia events
resulting from several decades of nutrient abatement programs.
However, some scientists have pointed out that these abatement
programs are not necessarily the sole reason, since the
recolonization process began in earnest only after zebra mussels
became widespread and abundant in the basin.
Hexagenia will likely have profound impacts on energy-flow pathways in areas where it becomes re-established. This organism is the preferred prey of many species of fish including perch and walleye. Some of the highest growth rates for perch have been associated with high consumption of Hexagenia. As a prey item, Hexagenia can also be an important means by which contaminants are cycled through the food web.
The objectives of this project are to:
1. Determine seasonal changes in lipid content and lipid class composition in Hexagenia at several different sites with contrasting environmental conditions (western Lake Erie, Lake St. Clair, and the Straits of Mackinac). Lipids have nearly twice the metabolic energy as proteins or carbohydrates, and lipid accumulation is necessary for proper metamorphosis and reproduction in aquatic insects such as Hexagenia.
2. Document changes in lipid content as individuals develop through the various life stages. Changes in lipid levels throughout an organism's life stages may strongly affect its uptake and elimination of lipophilic organic contaminants. Since lipid content in benthic organisms is generally a function of food availability, comparing lipid values from several different sites offers a means to assess the "health" of a given population relative to variations in environmental conditions.
3. Assess populations in the vicinity of the Straits of Mackinac. Hexagenia were found at selected sites in the straits region in the mid-1950s, but there is speculation that populations have since declined. In addition to collecting samples for lipid analyses, we will conduct a quantitative survey of Hexagenia populations at the same sites that were sampled in the mid-1950s.
2002 Plans
Complete and submit a manuscript on length-weight, lipid content, and glycogen content of Hexagenia collected in Lake St. Clair and Lake Erie in 1997-99.
2001 Accomplishments
Progress was made on a manuscript that documents length-weight, lipid content, and glycogen content of Hexagenia collected in Lake St. Clair and Lake Erie in 1997-99.
2000 Accomplishments
Completed the collection of Hexagenia nymphs (calendar year 1999) from a site in western Lake Erie and from a site in Lake St. Clair. Individuals were immediately identified, sexed, assessed for developmental stage, and measured for length. Individuals were prepared for analysis of lipid and glycogen content. This is the same sampling scheme that was employed in 1997 and 1998.
The measurement of lipid and glycogen levels for individuals collected in 1999 were completed. Trends in 1999 were similar to those in the previous two years. The data was entered in spreadsheets and the writing of a manuscript was initiated.
1999 Accomplishments
Collected Hexagenia nymphs on a monthly basis from a site in western Lake Erie and from a site in Lake St. Clair. Individuals were immediately identified, sexed, assessed for developmental stage, and measured for length. Individuals were prepared for analysis of lipid and glycogen content. This is the same sampling scheme that was employed in 1997 and 1998.
Lipid and glycogen levels were measured in Hexagenia collected in 1998. Trends in 1998 were similar to those in 1997. In Lake St. Clair values were high in the spring and then declined, whereas in western Lake Erie values were lower in the spring but then increased in summer.
1998 Accomplishments
This project compares the physiological condition, growth dynamics, and production of Hexagenia at two sites, one in western Lake Erie and the other in Lake St. Clair and is a joint effort between GLERL and Great Lakes Science Center-US Geological Survey. The Hexagenia population in western Lake Erie is rapidly expanding after a three-decade absence, while the population in Lake St. Clair has remained generally stable over the years. In 1998, Hexagenia nymphs were collected from the two sites on a monthly basis, and the following variables will be measured: lipid content, lipid classes, and glycogen content. These variables will also be measured in subimagos and imagos that were collected during the period of emergence (late June). In addition, quantitative samples were taken for estimates of nymph abundances, length-weights, size-frequencies, and developmental stages. Such information will be used to determine and compare growth dynamics and production of the two populations. Preliminary analysis of 1997 data indicates that there are major differences between the two sites. In the spring, lipid and glycogen levels were significantly higher in Lake St. Clair than in western Lake Erie, while levels in the fall were similar. Also, growth and size-frequencies were different, with larger individuals more prominent in western lake Erie. Sampling was continued at these sites in 1998 to extend our data set another year pursuant to the life cycle of the organism.
Publications
Cavaletto, J. F., Nalepa, T. F., Fanslow, D. L., and Schloesser, D. W. 2003. Temporal variation of energy reserves in mayfly nymphs (Hexagenia spp.) from Lake St. Clair and western Lake Erie. Freshwater Biology 48: 1726-1738.
Schloesser, D. W, and Nalepa, T. F. 2001. Changing abundance of Hexagenia mayfly nymphs in western Lake Erie of the Laurentian Great Lakes: impediments to assessment of lake recovery. Internat. Rev. Hydrobiol. 2001. 86: 87-103.
Last updated: 2004-04-23 mbl