Presentation on theme: "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."— Presentation transcript:
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 Waterloo Creek 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. 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. 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. 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. 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. 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. RESULTS 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. 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. CONCLUSIONS 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. Insect Abundance (Figure 1) Aquatic insect abundance at the water’s surface was greatest between midnight and 8:00 AM, peaking at 4:00 AM. Terrestrial insect abundance at the water’s surface was greatest between 4:00 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 water’s 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 4:00 PM and rose again between midnight and 4:00 AM. Taxonomic richness of terrestrial insect inputs into the stream was greatest during the day from 4:00 PM to 8:00 PM. 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 8:00 PM and dropped steeply through the night, following the trends seen with terrestrial insect abundance. 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. 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. *NOTE: On Figures 1-3, all times on the X-axis are sample period ending times.