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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

149: Lower Mississippi

Major Habitat Type:

temperate floodplain rivers and wetlands

Author:

Text modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC, USA.

Countries:

United States

Boundaries:

The Lower Mississippi is defined by the mainstem Mississippi below its confluence with the Ohio River. The entire ecoregion lies within the lowland Gulf Coastal Plain, and the Fall Line constitutes its northern boundary (Robison 1986). The ecoregion ranges across parts of southwestern Kentucky, southeastern Missouri, western Tennessee, eastern Arkansas, most of Mississippi, and the eastern half of Louisiana.

Drainages flowing into:

The Lower Mississippi drains an area from the Appalachian to the Ozark and Ouachita mountains, and empties into the Gulf of Mexico (Robison 1986).

Main rivers or other water bodies:

Other major rivers include the lower reaches of the Arkansas and White rivers in Arkansas, the Big Black River in Mississippi, and the Tensas River in Louisiana.

Topography:

Structurally, the Lower Mississippi is a trough between the Appalachians to the east and the Ozark and Ouachita mountains to the west, and this, combined with the Mississippi’s turbid character, has served as a barrier to the dispersal of upland fish species between river systems on either side (Robison 1986).

Climate:

The climate of this ecoregion is humid subtropical (Köppen 1936).Along the Lower Mississippi floodplain precipitation averages between 1150 and 1650 mm annually and temperature averages between 14 and 21 oC (McNab & Avers 1994).

Freshwater habitats:

The entire original extent of the Mississippi alluvial plain, stretching 1,120 kilometers from the northern portion of the ecoregion to the confluence with the Gulf of Mexico, occurs here. Swamps, marshes, and other wetland areas, including bottomland forests, were once dominant features throughout this ecoregion. Although these areas still exist in many places, they are not as extensive as in pre-settlement times (U.S. Geological Survey 1996).

Terrestrial Habitats:

The Lower Mississippi was once dominated by swamps, marshes and bottomland forests, primarily oak-hickory-pine forests (Ricketts et al. 1999). Today, the ecoregion is heavily converted, with just under half of the ecoregion covered by forest. One-third has been converted to agriculture and the remaining areas are comprised of water, wetlands, urban, and barren areas (Smith et al. 2002).

Fish Fauna:

The Lower Mississippi is distinguished by its extraordinary species richness, particularly in fish. The entire Mississippi basin has served as a center for fish distribution as well as a glacial refugium, and as such it is home to many of the species found in surrounding drainages. As a result it is the second richest ecoregion in North America, after the Tennessee [152]. The ecoregion is noted for its assemblages of large river fish, which include lamprey species, sturgeon, the only North American paddlefish, gar, and the bowfin. Many of these large river fish exhibit adaptations for the constantly turbid character of the Mississippi. Additionally, numerous marine species have been commonly recorded in the Mississippi’s lower reaches.

Description of endemic fishes:

Only 4% of fish species are endemic to the Lower Mississippi, and these are found in tributary drainages rather than in the Mississippi mainstem. These endemics include a shiner (Notropis rafinesquei), catfish (Noturus hildebrandi),  killifish (Fundulus euryzonus), and a number of darters (Percina aurora, Etheostoma chienense, E. pyrrhogaster, E. raneyi, E. rubrum, E. cervus and E. lynceum).

Other noteworthy aquatic biotic elements:

This ecoregion supports a moderate number of unionid mussel and crayfish species compared to the Tennessee [152], Cumberland [151] and Teays-Old Ohio [150] ecoregions to the north, but an impressive 58% of its crayfish species are endemic. With its warm, humid climate, it is also home to nearly 70 species of amphibians and aquatic reptiles, including the American alligator (Alligator mississippiensis). Two reptiles are endemic; the ringed map turtle (Graptemys oculifera) is restricted to the Pearl River system and the yellow-blotched map turtle (G. flavimaculata) is found only in the Pascagoula River system (Conant & Collins 1991).

Justification for delineation:

Ecoregion boundaries are taken from Abell et al. (2000) and are based on subregions defined by Maxwell et al. (1995).

References/sources:

Abell, R., Olson, D., et al. (2000). "Freshwater ecoregions of North America" Washington, D.C.: Island Press.

Conant, R.,Collins, J. T. (1991). "A field guide to reptiles and amphibians: eastern and central North America" Boston, Massachusetts, USA: Houghton Mifflin Company.

Maxwell, J. R., Edwards, C. J., et al. (1995) "A hierarchical framework of aquatic ecological units in North America (Nearctic Zone)". St. Paul, MN. North Central Forest Experiment Station, USDA Forest Service.

McNab, W. H.,Avers, P. E. (1994) "Ecological subregions of the United States". U.S. Forest Service, ECOMAP Team, WO-WSA-5. Online. http://www.fs.fed.us/land/pubs/ecoregions/index.html..

Ricketts, Taylor H. Dinerstein Eric Olson David M. Loucks Colby J. (1999). "Terrestrial ecoregions of North America: A conservation assessment" Washington, D.C.: World Wildlife Fund.

Robison, H. W. (1986)"Zoogeographic implications of the Mississippi River Basin" In Hocutt, C.H.;Wiley, E.O. (Ed.). The zoogeography of North American freshwater fishes. (pp. 267-285) New York, New York, USA: Wiley.

Smith, R.K., P.L. Freeman, J.V. Higgins, K.S. Wheaton, T.W. FitzHugh, K.J Ernstrom, and A.A. Das (2002) "Priority Areas for Freshwater Conservation Action: A Biodiversity Assessment of the Southeastern United States". The Nature Conservancy.

United States Geological, Survey (1996) "First Detailed ‘Report Card’ on Mississippi River Shows Movement of Contaminants. Online. http://www.usgs.gov/public/press/public_affairs/press_releases/pr106m.html".

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