LOTIC SESTON QUALITY AND THE GROWTH OF A COMMOM FILTER-FEEDING CADDISFLY

Jason A. Veldboom and Roger J. Haro
River Studies Center, University of Wisconsin-La Crosse, La Crosse, WI 54601.

In order to maximize growth, organisms must acquire adequate amounts of carbon, 
nitrogen and phosphorus from their food.  The growth of organisms can be limited 
by the relative supply of these nutrients, especially in aquatic ecosystems.  In 
streams, growth of larval aquatic insects has been shown to be influenced 
primarily by density, thermal regimen, and food quantity.  Here we take a 
different approach, we assess the growth of a common filter-feeding caddisfly 
(Brachycentrus occidentalis Banks: Brachycentridae: Trichoptera) from a 
stoichiometric perspective by comparing the elemental composition (C:N:P) of the 
organism to its food resource.  We analyzed C:N:P ratios of B. occidentalis body 
tissue and its food resource, seston, from four streams near Coon Valley, 
Wisconsin.  Samples were collected in summer and fall of 2004 for C:N:P analysis 
and to estimate the instantaneous growth of B. occidentalis.  Our preliminary 
findings showed differences in the growth of this caddisfly among the four 
streams, even though the streams possessed very similar thermal regimes.  These 
differences appear to be correlated to the C:N ratio of the seston; B. 
occidentalis growth was higher in streams transporting seston of lower C:N 
ratio.  

Keywords:  seston, ecological stoichiometry, caddisfly growth, driftless zone, 
streams