TEMPORAL RELATIONSHIPS IN THE ASSIMILATION OF TRANSPORTED ORGANIC MATTER BY THE 
FILTERING HYDROPSYCHIDAE 

Cheng Xiong, Paul Bates, and Michael Delong
Large River Studies Center and Biology Department, Winona State University, 
Winona, Minnesota 55987

Recent work in large rivers has linked algal transport organic matter (TOM) as a 
major food source for the primary consumers, including Hydropsychidae, using 
stable isotopes.  It is possible, however, that stable isotope ratios of TOM 
will change temporally at a rate more rapid than tissue turnover would change SI 
ratios of consumers.  This situation would require that models be developed to 
account for temporal differences between sources and consumers when using stable 
isotopes to examine riverine trophic dynamics.  To address this issue, temporal 
patterns in the isotopic ratios of TOM and Cheumatopsyche sp. and H. orris were 
examined in the Upper Mississippi River.  Water samples were collected at 
multiple points across a transect by pumping water from a depth of 1 m at 4 - 5 
transects from April - October 2004.  TOM was partitioned into fine TOM (FTOM, 
100 ?m – 1 mm) and ultrafine TOM (UTOM, 1 ?m-100 ?m).  FTOM and UTOM were 
further separated into algal and detrital fractions by colloidal silica 
centrifugation.  The ?13 C and ??15 N were determined for all TOM fractions.  
Hydropsychidae were collected from rocks and processed to ascertain ? 13 C and ? 
15 N.  We hypothesize that physical and chemical dynamics of large rivers are 
stable over relatively long time periods.  During these periods, phytoplankton 
SI ratios will accurately reflect the ratios of its consumers.  There has, 
however, been an indication that SI ratios of algal TOM change temporally in 
response to hydrological changes.  Assessment of consumer-source linkages over a 
broader temporal scale is needed to better define this relationship.

Keywords: stable isotope; trophic linkages; transport organic matter; 
Hydropsychidae, season