Browsing by Author "Simon, K. S."
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- Food web structure and the role of epilithic biofilms in cave streamsSimon, K. S.; Benfield, Ernest F.; Macko, S. A. (Ecological Society of America, 2003-09)Cave stream food webs rely exclusively on detritus from the surface for energy and represent a heterotrophic end point in the continuum of stream types. Both dissolved organic matter (DOM) and particulate organic matter, and associated microbes, are available in caves, but the relative importance of these foods is unclear. We examined trophic structure and the roles of particulate organic matter and DOM retained in epilithic biofilms in three cave streams using natural abundance isotopes (C-13 and N-15) and C-13 tracer additions. Natural abundance N-15 ratios showed that cave animals occupied two trophic levels: primary consumers and predators. Epilithic biofilms and animals were highly enriched in N-15, suggesting that dissolved organic matter from surface soils was incorporated into epilithon and supported stream food webs, even when coarse particulate organic matter (CPOM) from the surface was abundant. We conducted 28-day C-13-acetate tracer additions to trace the use of epilithon carbon in the food webs. The tracer(13)C acetate was rapidly taken up in the streams at uptake velocities (0.5-22.8 x 10(-5) m/s) and uptake rates (0.04-0.23 mg m(-2) h(-1)) similar to those in surface streams. Epilithon was highly labeled and epilithon turnover time was relatively long (10.0-16.7 d), showing that C from DOM was immobilized in biofilms and thus available to invertebrates, which also became labeled with tracer by the end of the experiment. Several snails (Fontigens tartarea, Gyraulus parvus, and Physa sp.) that fed directly on epilithon were highly enriched with tracer C-13. Other primary consumers (Gammarus minus and Caecidotea holsingeri) fed on a combination of epilithon and fine particulate organic matter. Predators in the streams (Stygobromus emarginatus, S. spinatus, and Macrocotyla hoffmasteri) also became labeled with C-13, indicating that biofilm C was passed through the entire food web. This study shows that DOC and epilithic biofilms are important energy sources for stream communities even when CPOM is an abundant resource.
- Variation in ecosystem function in appalachian streams along an acidity gradientSimon, K. S.; Simon, M. A.; Benfield, Ernest F. (Ecological Society of America, 2009-07)Acidification is a widespread phenomenon that damages aquatic systems, and it has been the focus of intensive management efforts. While most management has focused on community structure as an endpoint, ecosystem function is also sensitive to Acidification and important in stream health. We examined how a key ecosystem function in streams, leaf breakdown, varied along a gradient of pH resulting from acid deposition, natural conditions, and liming. We also measured how invertebrate and microbial assemblage structure and microbial function were related to altered leaf breakdown rates. Leaf breakdown rates declined more than threefold along a gradient of stream acidity from pH 6.8 to 4.9. The diversity of leaf-shredding invertebrates, bacteria, and fungi showed little response to variation in pH. The abundance of one acid-sensitive caddisfly, Lepidostoma, declined with Acidification, and Lepidostoma abundance explained 37% of the variation in leaf breakdown rates among sites. Microbial respiration was suppressed along the acidity gradient, although the pattern was weaker than that for breakdown rate. In short-term laboratory incubations, microbes at acidic and circumneutral sites demonstrated adaptation to ambient pH. The activity of microbial extracellular enzymes was strongly influenced by pH. In particular, the pattern of activity of phosphatase indicated increasing P limitation of microbes with increasing Acidification. Our results show that leaf breakdown is a sensitive tool for examining the response of stream function to Acidification and also for de. ning the mechanisms that drive functional response. Future management efforts should focus on key taxa that are particularly sensitive and effective at shredding leaves and also the role of shifting acidity in mediating the availability of phosphorus to microbial films that are important for stream function.