Daily Patterns of Aquatic Insect Activity at the Surface of a Northeastern Iowa Trout Stream Katie J. Hopp and Kirk J. Larsen Luther College, Department of Biology, 700 College Drive, Decorah, Iowa

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Daily Patterns of Aquatic Insect Activity at the
Surface of a Northeastern Iowa Trout StreamKatie
J. Hopp and Kirk J. LarsenLuther College,
Department of Biology, 700 College Drive,
Decorah, Iowa
RESULTS
CONCLUSIONS
Waterloo Creek
Table 1. Combined list of taxa present in June,
July and August 2004 samples and relative
abundance of each taxa of a total of 13,435
insects.

• Relative Abundance (Table 1).
• The most abundant aquatic insects at the stream
  surface were mayflies (Ephemeroptera Baetidae
  and Ephemeroptera Tricorythidae).
• These are most likely what the trout were feeding
  on during dawn and dusk, when they were most
  abundant at the surface.

INTRODUCTION Northeastern Iowa is home of native
brook trout, and hosts a large number of trout
streams with naturally-reproducing populations of
brown trout. Currently, a habitat restoration is
under way on the banks of Waterloo Creek in
Allamakee County, Iowa. This project involves
the conversion of corn fields and riparian areas
into tallgrass prairie, overgrown woods into oak
savanna, and stream habitat improvements to
reduce streambank erosion. Luther College is
involved in documenting changes to the
terrestrial and aquatic insect communities
associated with this terrestrial and aquatic
habitat restoration. It is hoped that the
habitat improvement will ultimately result in
increased insect abundance and diversity,
improving potential food sources for the trout
population within Waterloo Creek. The goal of
this study was to monitor and quantify daily
aquatic insect patterns at the surface of
Waterloo Creek over a 24-hr cycle and to compare
these results with terrestrial insect inputs into
the stream. There is expected temporal resource
partitioning at the surface of the stream between
aquatic insects in the morning and terrestrial
insects during the evening.

• Insect Abundance (Figure 1)
• Aquatic insect abundance at the waters surface
  was greatest between midnight and 800 AM,
  peaking at 400 AM.
• Terrestrial insect abundance at the waters
  surface was greatest between 400 PM and
  midnight.
• These results suggest that aquatic insects are
  coming to the surface during night hours and then
  retreating to the substrate during mid-day hours,
  while terrestrial insects are falling onto the
  waters surface during the day time hours,
  indicating temporal resource partitioning at the
  surface of the stream between aquatic and
  terrestrial insects.

• Taxonomic Richness (Figure 2)
• Taxonomic richness of aquatic insects at the
  stream surface was greatest at 400 PM and rose
  again between midnight and 400 AM.
• Taxonomic richness of terrestrial insect inputs
  into the stream was greatest during the day from
  400 PM to 800 PM.

METHODS Aquatic insects near the surface of
water and terrestrial insects falling into
Waterloo Creek were sampled over 24-hour periods
during June, July and August of 2004. Two 10 m
reaches were selected in the stream (average
width of 10.5 m) and pairs of collecting nets
established in each reach for four sample
replicates.
NOTE On Figures 1-3, all times on the X-axis
are sample period ending times.

• Biomass (Figure 3)
• Biomass of aquatic insects was greatest between
  midnight and noon, dropping off during the
  afternoon, following the trends seen with aquatic
  insect abundance.
• Biomass of the terrestrial insect inputs into the
  stream peaked at 800 PM and dropped steeply
  through the night, following the trends seen with
  terrestrial insect abundance.

This picture shows one of the two pairs of
clearing and collecting nets designed to limit
insect collections to a 10 m long by 0.5 m wide
surface area of stream for each replicate sample.
At the upstream end of the reach, two clearing
nets 2.3 m wide removed all insect inputs from
further upstream.
DISCUSSION These results support the hypothesis
that aquatic insects are more abundant at the
stream surface from dusk until dawn and
terrestrial insects are most abundant during the
mid-day when it is warmest. However, based on
observations in the field, trout were surfacing
at dusk and dawn to feed. This provides evidence
that although trout do feed on some terrestrial
insects that are falling onto the stream surface,
they are primarily feeding on aquatic insects
that are at the surface or emerging from the
stream. Therefore, although the newly planted
prairie is expected to increase terrestrial
insect abundance, this may not be a crucial
factor for trout feeding as they primarily feed
when aquatic insects are most abundant at the
stream surface. However, studies on gut contents
of the trout would be needed in order to
determine what insects the trout are feeding on.
Two 0.5 m wide collecting nets were then placed
10 m downstream from the clearing nets to capture
all aquatic insects near the water surface or
emerging and terrestrial insects falling onto the
water surface between the clearing and collecting
nets. Nets were placed perpendicular to the
water current. A second pair of clearing and
collecting nets were placed approx. 20 m
downstream from the first pair of collecting nets
in the same manner.
Figure 2. Taxonomic richness (mean SE) of
aquatic and terrestrial insects for each 4 hr
time period over 24-hours for combined taxonomic
richness of June, July and August 2004.
Figure 1. Insect abundance (mean SE) of aquatic
and terrestrial insects for each 4 hour time
period over 24-hours for combined insect
abundance of June, July and August 2004.
Two of the four 0.5 m wide collecting nets used
to remove aquatic and terrestrial insects just
below or floating on the surface of the water.
Samples were collected every 4 hours beginning at
12 noon for 24 hours.
ACKNOWLEDGEMENTS Generous financial support for
this project came from the R.J. McElroy Trust
Student/Faculty Research Fund and Dr. Michael
Osterholm. Collaborators on this project include
the Iowa DNR, USDA-NRCS, Trout Unlimited, and
Driftless Land Stewardship.
Insects removed from the sample nets were rinsed
and placed in 70 ethanol for later separation
and identification in the lab. Aquatic and
terrestrial insects were separated, identified to
order and family. Insect taxonomic richness,
abundance, and biomass were quantified.
Figure 3. Insect biomass (g) of aquatic and
terrestrial insects (mean SE) for each 4 hour
time period over 24-hours for combined insect
biomass of June, July and August 2004.