FOOD WEB STRUCTURE IN TEMPERATE FLOODPLAIN RIVERS:
STABLE ISOTOPE ANALYSIS.

Michael D. Delong1 and James H. Thorp2
1Large River Studies Center, Biology Department, Winona State University,
Winona, MN 55987 2Large River Program, Department of Biology, University of
Louisville, Louisville, KY 40292.

The relative importance of various forms of organic matter in the food webs of
large rivers has been long debated during attempts to better understand the
functioning of these important ecosystems.  Three theories (river continuum
concept, flood pulse concept, and the riverine productivity model) describe the
potential importance of organic matter, with each citing organic matter of a
specific origin as being most important.  Despite the continuing debate over the
strengths and weaknesses of each model, very little empirical research has been
attempted which specifically tests the applicability of these models on large river
ecosystems.  We examined food web structure of the Missouri, Ohio, and upper
Mississippi Rivers using stable isotopes of carbon and nitrogen to determine the
importance of allochthonous and autochthonous organic matter as potential food
sources.  Samples were collected for different size fractions of transported and
benthic particulate organic matter, dissolved organic matter, terrestrial leaf litter, 
and benthic algae, in addition to fish and invertebrate representatives of major
functional feeding groups.  Samples were analyzed for the ratio of 13C/12C
(represented as ë13C) and 15N/14N (represented as  ë15N).  Isotopic signatures were
compared to develop linkages between sources and consumers.  Consumers in
the Missouri River exhibited the greatest variability in ë13C, ranging from -32 to -18 ppt and exhibited the lowest levels of nitrogen enrichment, with most
organisms ranging from 8-14 ppt.  Isotopic signatures for consumers in the upper
Mississippi and Ohio Rivers differed from those observed in the Missouri but
were comparable to each other.  In both instances, consumers appeared to cluster
into two groups, suggesting at least two possible food sources.  Consumers in all
three rivers were most closely associated to the isotopic signatures of fine
transported organic matter, ultra-fine transported organic matter, and dissolved
organic matter.  The ë13C enrichment of sources and consumers in the Missouri
River suggest that C4 terrestrial plants or POM from upstream reservoirs may be
an important part of the food web of this heterotrophic system.  Although the
ë13C of FTOM, UTOM, and DOM in the other two systems is similar to that of
terrestrial C3 plants, the level of nitrogen enrichment indicates that another
source, possibly phytoplankton or detritus originating from aquatic macrophytes,
are a more likely component of the transported organic matter.  Further studies
which incorporate separation of transported organic matter into living and detrital
components are needed to provide additional resolution to this study.

Keywords: food webs, stable isotope, organic matter, ecosystem theory, trophic
structure

Michael D. Delong
Large River Studies Center, Biology Dept., Winona State University, Winona,
MN 55987
FAX 507-457-5681; Office 507-457-5484
mdelong@vax2.winona.msus.edu
Preference would be for a platform presentation but could change if it were really
needed