PRELIMINARY FOOD WEB MODELS FOR THREE LARGE TEMPERATE
RIVERS

Michael D. Delong 1 and James H. Thorp2
1Large River Studies Center, Biology Department, Winona State University,
Winona MN 55987
2School of Science, Clarkson University, Potsdam, NY 13699-5800

Stable isotopic ratios of carbon and nitrogen in addition to carbon:nitrogen ratios
were used to create linkages between potential sources of organic matter and
invertebrate and fish consumers in three large, temperate rivers.  Samples of
different size fractions of transported and stored organic matter, dissolved organic
matter, benthic algae, and floodplain leaf litter from C3 and C4 terrestrial plants
were collected from the lower Ohio, lower Missouri, and upper Mississippi Rivers
in July 1994.  Representatives of invertebrate and fish functional feeding groups
were also collected.  Samples were analyzed to determine ratios of 13C/12C
(reported as ?13C), 15N/14N (?15N), and percent carbon/percent nitrogen were
determined for all sample units.  Results of the study indicate that the lower Ohio
and upper Mississippi River have similar food web structure.  Isotopic and C:N
ratios of coarse and fine stored organic matter and coarse transported organic
matter were most similar to ratios of C3 terrestrial plants.  Ratios of all fine
transported organic matter fractions and DOM were most similar to benthic algae,
suggesting they are primarily autochthonous in origin.  Values for ?13C of some
invertebrate consumers were similar to C3 terrestrial plants; however, consumers
were substantially more 15N-enriched, indicating that C3 terrestrial plants are
probably not a major food source in the Mississippi and Ohio Rivers.  Carbon and
nitrogen ratios of invertebrate consumers correlated most strongly with fine
transported organic matter and dissolved organic matter, indicating that these two
fractions are an important carbon source.  We suggest that the food webs of the
lower Ohio and upper Mississippi Rivers, in the areas studied, are driven either
directly or indirectly by autochthonous organic matter.  Two possible pathways
exist.  One possible pathway is through the consumption of the phytoplankton
component of FTOM and through the microbial loop through which DOM
excreted by autotrophs passes.  The second possible pathway is detrital, by which
invertebrate consumers are processing FTOM containing organic matter
consisting of decomposing algae and macrophytes and through the microbial loop,
which would be assimilating DOM released through decomposition.  The food
web of the Missouri River, an ecosystem severely altered by human activity, is
less clear; however, it appears that allochthonous organic matter is driving the
food web of this system.  Further development of these food webs in necessary. 
These preliminary results do suggest, however, an important role for organic
matter originating from autochthonous sources in some large river ecosystems.

Keywords: food webs, stable isotopes, floodplain rivers, large rivers, riverine
productivity model


Michael D. Delong, Large River Studies Center, Biology Department, Winona
State University, Winona MN 55987; 507-457-5484; FAX 507-457-5681;
mdelong@vax2.winona.msus.edu

Poster but will do platform if needed (this really is different from last year's)