Ingestion and digestion of epilithic algae by larval insects in a heavily grazed montane stream

1. Epilithic algae grown on elevated or non-elevated ceramic tiles were exposed (to produce assemblages with different grazing histories) in a heavily grazed, montane stream in New Mexico, U.S.A. to Ameletus nymphs (Ephemeroptera) and Ecclisomyia larvae (Trichoptera) and the algal composition in insect faeces was compared to that on the tiles. Differences in grazing and digestion efficiency between grazers were then assessed and also differences in susceptibility to ingestion and digestibility among common algae. 2. Ordination of tile and faecal samples, using the relative abundance of common algae, revealed that: (i) algal assemblages on elevated vs. non-elevated tiles differed only slightly; (ii) the taxonomic composition of algae in faeces of both caddis and mayflies differed substantially from that on the tiles, indicating low grazing efficiency for some algal taxa; and (iii) the algal composition of faeces produced by caddis larvae and mayflies was similar, indicating little difference in grazing efficiency between them. However, some algal taxa were more susceptible to ingestion by caddis fly larvae when occurring on elevated tiles than on non-elevated tiles, suggesting that previous exposure to caddis grazing influenced assemblage attributes. 3. Although Ameletus and Ecclisomyia differed little in grazing efficiency, the percentage of diatoms that were dead after passage through the gut was greatest in the mayfly treatment, suggesting that mayflies digested diatoms more efficiently than the caddis. Analyses of differences in the condition of chloroplasts within diatoms in tile and faecal samples showed that losses of 'live' diatom cells (i.e. those containing full chloroplasts) during gut passage through mayflies equalled the increase, in faeces, of 'dead' (empty frustules) cells of all common diatoms. In contrast, some diatoms were digested inefficiently by caddis larvae. 4. Algae on elevated tiles contained a higher proportion of dead diatoms than those on non-elevated tiles, possibly because mayflies visited raised tiles more often and, consequently, ingested and defaecated cells at a higher rate in the absence of caddis larvae. Moreover, diatom taxa differed in the percentage of cells that were dead within tile assemblages, with populations of typically grazer-resistant taxa (e.g. Achnanthidium minutissimum, Planothidium lanceolatum and Cocconeis placentula var. euglypta) containing significantly more dead cells than grazer-susceptible taxa [e.g. small, chain-forming Fragilaria (= Staurosirella)]. This result suggests that a trade-off exists between ingestion vs. digestion resistance of microalgae. Both the ingestion and digestion efficiency of algivorous macroinvertebrates could influence the structure and function of algal assemblages. In heavily grazed systems, where algal cells are probably processed through grazer guts repeatedly, differential resistance to digestion among algae may be particularly important.