IDENTIFICATION OF CRITICAL RESOURCES IN LARGE RIVER FOOD WEBS THROUGH SEPARATION 
OF ALGAL AND DETRITAL COMPONENTS OF TRANSPORTED ORGANIC MATTER

Michael D. Delong
Large River Studies Center, Biology Department, Winona State University, Winona, 
MN 55987

Past studies of large river food webs using natural stable isotope ratios of 
carbon and nitrogen as tracers indicated that fine organic matter (particles 1 
mm - 100 µm in diameter) transported in the water column and dissolved nutrients 
(DN) were primary food resources for consumers.  C/N ratios of these components 
suggested that these food resources were primarily autochthonous in origin.  
Greater resolution, however, was necessary to elucidate their composition and to 
develop better functional models of large river food webs.  This study used a 
colloidal silica extraction technique to separate the living and detrital 
components of both fine (FTOM) and ultrafine (100  - 1 µm in diameter; UTOM) 
transported organic matter.  Carbon and nitrogen isotopic ratios were determined 
separately for each component so that linkages with consumers could be 
established.  Samples were collected July 2001 over a 12-km long section of 
Reach 6 of the Upper Mississippi River.  A single transported organic matter 
sample consisted of an 80-L sample collected across a single perpendicular 
transect at four points.  Equal volumes of water were drawn at four depths at 
each point and all water samples on a transect were pooled to create a 
composite.  In addition to FTOM, UTOM and DN, samples were partitioned into 
coarse transported organic matter (> 1mm diameter) and colloidal dissolved 
organic matter (1 - 0.5 µm in diameter).  Only FTOM and UTOM were separated 
using colloidal silica.  A duplicate water sample was collected along each 
transect so that the isotopic ratio of whole FTOM and UTOM could be compared to 
values obtained in past studies.  Other potential food sources collected from 
nearshore areas included: benthic algae, aquatic macrophytes, and terrestrial C3 
leaves from the floodplain forest floor.  Benthic invertebrates were collected 
by hand-picking snags and rock.  Insects were placed in aerated chambers for 2 d 
to allow clearance of their digestive tract.  All samples were ground to a fine 
powder and sent to the University of Alaska-Fairbanks Isotope Ratio Mass 
Spectrometry Laboratory for determination of carbon and nitrogen stable isotope 
ratios and C/N mass ratio.  Separation using colloidal silica revealed that FTOM 
consists of approximately 70% living phytoplankton and 30% detritus.  Detailed 
analysis of actual isotopic ratios allowed for a firmer association between 
phytoplankton component of available organic and consumers, thus allowing for 
the creation of more realistic food web models.

Keywords: food web, trophic relations, stable isotope, riverine productivity 
model