Leaves in Streams II
Presiding: C J LeRoy, Department of Biological Sciences, Northern Arizona University; J Kominoski, University of Georgia
NB42A-01 10:30h
Plant genes link forests and streams
Recent terrestrial research demonstrates the importance of genetic variation within tree species such as oaks, aspen and cottonwoods in affecting the function of forest ecosystems. We show similarly that genetic variation within cottonwoods can affect stream ecosystem function through litterfall. The genetic makeup of cottonwood leaf litter directly affects in-stream leaf decomposition rates, aquatic fungal accumulation and macroinvertebrate assemblages. This genetic variation is especially important in the western United States because cottonwoods are a dominant riparian tree and are currently in dramatic decline. In western rivers, cottonwood genetic diversity may be elevated due to naturally-occurring hybridization zones. We collected litter from five genotypes of each of four cottonwood cross types from common garden trees and measured decomposition rates using litterbag techniques in the Weber River (UT). Among the cottonwood genotypes decomposition rates ranged on average from 0.0077 ± 0.0003 day-1 for backcross to P. angustifolia hybrids to 0.0105 ± 0.0003 day-1 for P. fremontii. Similar and substantial differences among F1 and backcross hybrids provide evidence for genetic control over in-stream decomposition rates. Prior studies have shown that species diversity influences litter quality and stream function. This study extends this by linking genetic diversity to stream ecosystem function.
NB42A-02 10:45h
Twenty-eight Years of Stream Invertebrate Abundance, Biomass, and Secondary Production From Three Headwater Streams
We analyzed 28 years of abundance, biomass and secondary production data from 3 headwater streams at the Coweeta Hydrologic Laboratory, NC, USA. These data include years of extreme drought and precipitation (78-y record) and 8 years of reduced litter inputs (litter exclusion) and wood removal for one stream. Analysis of functional feeding group (FFG) proportions revealed strong habitat-specific preferences for either mixed substrates or bedrock outcrop habitats, with remarkable between year similarities. For both reference streams and litter reduction streams there was a significant relationship between annual CPOM standing crop and secondary production for the dominant mixed substrate habitats. Habitat-weighted production across reference streams averaged 8.2 g AFDM/m2/y (range = 4.6 to 13.2) versus 1.7 g AFDM/m2/y (range = 0.9 to 3.5) for litter exclusion years. Predator production was strongly correlated (P<0.001) with total secondary production over all years, and based on known bioenergetic efficiencies, indicate the importance of predators in these streams. Our study suggests that trophic interactions, including standing crop of CPOM as a food source, strongly influence secondary production in these headwater streams.
NB42A-03 11:00h
Leaf Litter Decomposition and its ETS (Electron Transport System) Activity in two Stream Ecosystems (Austria, Poland)
Biofilms are involved in decomposition of leaf litter, contributing to self-purification capacity of streams. Total community respiration was measured as ETS-activity (indicator of potential respiration). Objectives: to examine respiration rates in decomposing leaves in over-flown stream bottom, to trace the breakdown processes and to assess patterns of nutrient parameters in decomposing litter. Two 2nd order streams contrasting in geomorphology, hydrology, and climate regime were selected: an alpine stream (Oberer Seebach, Austria) and a submountain one (Goscibia, Poland). Alder leaves were exposed in coarse mesh bags in stream riffles and removed 5 times between November and March. The TOC content in leaf litter was lower in Oberer Seebach than in Goscibia. No significant differences were found in the decomposing time. The ETS activity was significantly higher in the Oberer Seebach than in the Goscibia, indicating higher density and activity of biofilms in the Austrian stream. The higher TON content also confirms it. Consequently, the leaf litter decomposition in Oberer Seebach was faster than in Goscibia. It can be explained by higher water temperature and velocity. In general, differences in decomposition of alder leaves and its ETS activity are caused by hydrological and thermal regimes in studied streams.
NB42A-04 11:15h
Effect of Elevated Etmospheric CO2 on Litter Breakdown in a Stream
Climate change scenarios predict a continuing increase in atmospheric CO2 concentration in the future. One potentially important effect on plants is an increase in leaf lignin concentration, which in turn is critically important in controlling breakdown rates of leaves. Given the power-function type relationship between leaf lignin concentration and breakdown rate, breakdown of leaf species with a low lignin concentration is expected to respond sharply to CO2-induced changes in litter quality, whereas species with high concentrations should be insensitive. To test this hypothesis, we conducted a litter-bag experiment with leaves from six deciduous species differing in litter chemistry which were collected from trees exposed to either elevated or ambient CO2 levels. The 12 types of leaves were enclosed in coarse-mesh and fine-mesh bags and placed in a softwater mountain stream in November 2004. The bags were retrieved after 7 weeks and leaf mass loss determined. Consistent with our hypothesis, our preliminary results suggest that leaves subjected to elevated CO2 concentration showed indeed an overall tendency to decompose more slowly, with the CO2 effect being greatest for the species decomposing most rapidly.
NB42A-05 11:30h
Implications of Transgenic Corn Cultivation on the Ecology of Agricultural Streams
Corn has been genetically-modified by introducing a gene that codes for a toxic protein from a bacterium, Bacillus thuringiensis (Bt), into corn DNA. Genetically-modified crops provide internal resistance to herbivorous pests like the European Corn Borer (Ostrina nubilalis). With the use of transgenic crops on the rise, research is being done to consider its environmental effects on non-target taxa and ecosystems. Stream ecosystems occupy topographic low points in the landscape and thus are affected by agricultural land use. In many temperate streams, the main energy source is from terrestrial organic detritus, mostly in the form of dead leaves and wood, delivered via wind or natural leaf fall. Stream insects consume this material, contributing to organic matter breakdown and creating biomass for predators. With the heightened practice of no-till agriculture, crop detritus remaining on fields as a by-product of harvesting has been documented to enter adjacent streams. Given insect larvae are critical to the transformation of energy from detritus to higher trophic levels, we explored the implications of detritus containing Bt on both insect performance and litter decay in six streams. The presence of Bt in senesced corn leaf litter resulted in significant reductions in both insect feeding rate and organic matter breakdown. Furthermore, colonization of corn litter containing Bt by detritivorous insects was significantly reduced when compared to non-Bt isoline litter controls. We conclude that detritus generated from harvesting transgenic corn negatively impacts insect feeding behavior and colonization dynamics, and may contribute substantially to the reduction of organic matter breakdown rates in agricultural streams.
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