Stream permanence influences microalgal food availability to grazing tadpoles in arid-zone springs

Primary production in many ephemeral waters peaks soon after inundation, bu t the extent to which the algal biomass generated by this process is immedi ately available to aquatic herbivores as a food source has not been extensi vely studied. To examine this, we exposed natural epilithon from two perman ent and two recently rewetted temporary reaches of an intermittent stream t o grazing by small, presumably newly hatched, Limnodynastes tasmaniensis ta dpoles and compared the algal content of tadpole feces to that of the assem blages on which they grazed. Rocks from the temporary sites, one colonized by tadpoles and one not, supported relatively flocculent, diatom-rich (79.7 -85.7%) epilithon of similar biomass and taxonomic content. Epilithon from the permanent sites (one with and one without tadpoles) were more cohesive. contained fewer diatoms (57.0-60.7%). and differed in species composition from that of the temporary sites, and from one another. Feces and epilithon were more taxonomically similar when epilithon originated from temporary r eaches than from permanent sites. This implies that grazing tadpoles access ed a greater percentage of the algal assemblages from recently rewetted sit es. Algal species differed in susceptibility to ingestion by small tadpoles , but these differences were not consistent among habitats, susceptibility to ingestion was not predictable based solely on species growth habit, but was likely also affected by physiognomic differences in mat structure among habitats. A large percentage of algal cells ingested by tadpoles survived gut passage. 'Live' cells (those with full chloroplasts) comprised 43.8-66. 6% of all diatoms from epilithic samples and 27.4-42.7% of those in feces o f small tadpoles. In contrast, only 12.8-14.9% of the diatoms in feces prod uced by large L. tasmaniensis tadpoles collected from the two tadpole-colon ized sites contained full chloroplasts, suggesting higher digestion efficie ncy in large tadpoles than in small ones. Distinct, gut-passage-induced tra nsitions from 'live' diatoms to empty frustules or single diatom valves ('d ead' cells) were evident when grazed material originated from temporary rea ches. in contrast 'live' diatoms in epilithon from permanent sites were mor e likely to emerge in tadpole feces with reduced or fragmented chloroplasts . Thus, algae from temporary reaches appeared to be more efficiently digest ed than those from permanent reaches. While digestibility of individual tax a varied among sites, some algae (e.g., Synedra ulna) were clearly more dig estible than others. Our results suggest that temporary stream reaches in a rid-zone catchments are important sources of readily digestible autotrophic biomass for anuran species in these regions.