SEASONAL CHANGES IN SECONDARY PRODUCTION OF HYDROPSYCHIDAE IN
REACH 6, UPPER MISSISSIPPI RIVER.

L. Schuette, L. Paulson, D. Bruesewitz, and M. Delong.  
Large River Studies Center, Biology Department, Winona State University, Winona, MN 55987.

The theoretical models proposed to explain trophic dynamics in large rivers differ critically in the
relative importance of autochthonous vs. allochthonous organic matter.  Studies testing these
theories have focused on the role of different forms of organic matter.  An additional aspect of
these forms of organic matter is the timing of their deliver.  Allochthonous organic matter is
delivered either continuously via downstream transport or in conjunction with annual flood
periods.  Autochthonous matter is present in greatest quantities during low-flow periods when the
abundance of slackwater habitats is maximized and nutrient spiraling is slowed by reduced
current velocities.  We studied Hydropsychidae larvae to determine if seasonal changes occurred
in rates of secondary production.  Samples were collected every 3-4 wk from May-Oct 2000 by
removing cobble from wing dams in the upper area of Reach 6 near Winona, MN. 
Hydropsychids were removed from samples and divided into two size classes representing
different cohorts.  Biomass was determined and secondary production was estimated for each
sample interval using the increment-summation method.  Three species were identified:
Hydropsyche orris, Cheumatopsyche sp., and Potamyia flava.  Secondary production was highest
in August and September, which coincided with the low-flow period.  While annual spring flood
patterns deviated from the historical average, rates of secondary production would have been
predicted to remain constant if hydropsychids were relying on allochthonous organic matter
derived from upstream processing.  We propose that hydropsychid production was maximized
during this sample year through consumption of autochthonous materials typically present during
the period when production was highest.

Keywords: trophic dynamics, riverine productivity model, macroinvertebrate, Mississippi River,
secondary production