Food web structure and the role of epilithic biofilms in cave streams
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.