Food Web Structure and the Role of Epilithic Biofilms in Cave Streams

• Published in: Ecology (Durham) 2003-09, Vol.84 (9), p.2395-2406
• Main Authors: Simon, K. S., Benfield, E. F., Macko, S. A.
• Format: Article
• Language: English
• Subjects: 13C; 15N; Acetates; Animal and plant ecology; Animal, plant and microbial ecology; Animals; Biofilms; Biological and medical sciences; Caecidotea holsingeri; Canyons; cave; Caves; Creeks & streams; epilithic biofilms; Fontigens tartarea; Food chains; Food webs; Fresh water ecosystems; Freshwater; Freshwater ecology; Fundamental and applied biological sciences. Psychology; Gammarus minus; Gyraulus parvus; Macrocotyla hoffmasteri; microbial loop; Particular ecosystems; Physa; Predators; Product labeling; stable isotope; stream; Streams; Stygobromus emarginatus; Stygobromus spinatus; Synecology
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/02-334

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 (13C and15N) and13C tracer additions. Natural abundance15N ratios showed that cave animals occupied two trophic levels: primary consumers and predators. Epilithic biofilms and animals were highly enriched in15N, 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-day13C-acetate tracer additions to trace the use of epilithon carbon in the food webs. The tracer13C acetate was rapidly taken up in the streams at uptake velocities (0.5-22.8 × 10-5m/s) and uptake rates (0.04-0.23 mg m-2h-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 tracer13C. 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 with13C, 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.