Grazing and nutrient interactions in controlling the activity and composition of the epilithic algal community of an arctic lake1

Abstract

The effects of grazing by the snail Lymnaea and enrichment with N and P were examined in the epilithic algal community of an arctic lake. Increased nutrients raised chlorophyll CL levels and primary production and favored the growth of filamentous green algae and coccoid blue-greens.Intensive grazing reduced algal concentration and activity. Grazing increased the relative abundance of small green and blue-green coccoid cells at the expense of larger (>20 pm) diatoms.Photosynthetic activity of snail feces was 77-79% that of ungrazed algae per unit mass. Split chamber experiments and measurements of dissolved phosphorus indicated that nutrients released in grazing may be tightly recycled within snail guts or on the rock surface, rather than transferred into the water. Radioisotope tracing was used to develop a simple energy budget for Lymnaea. Despite viable gut passage of some of the algae. the snail was an efficient grazer, assimilating 68% and indbrp&ating 25% of ingested c&b&.

The effects of grazing by the snail, Lymnaea elodes, on benthic N2 fixation and primary production in oligotrophic, arctic lakes

“…Cuker (1983) found that L. elodes egest viable cells that photosynthesize at 80% the rate of ungrazed substrates and hypothesized that these viable cells may take up inorganic N within the gut. This process would lead to the low concentrations of inorganic N observed in excretion and could result in a lowered N demand and a reduced need to fix N 2 .…”

Concurrent grazing effects of two stream insects on periphyton1

“…Periphyton grazed by snails is characterized by reduced three-dimensional structure brought about by the disproportionate mortality of overstory algae (Sumner and McIntire 1982;Hunter and Russell-Hunter 1983). Small, adnate algal cells that are not cropped by the snails often dominate these snail-grazed communities (Patrick 1970;Cuker 1983;Hunter and Russell-Hunter 1983 Creek, resulting in a ratio of ambient biomasses that approximated 2 to 1 (Neophylax to Ameletus). The 2 : 1 ratio implies that the absolute value of Ameletus regression coefficients had to be twice those of Neophylax for Ameletus to have had an equivalent ambient effect.…”

Phosphorus uptake by Microcystis during passage through fish guts

“…Miura and Wang (1985) observed a doubling of the photosynthetic activity of cyanobacteria excreted by bighead (Aristichthys nobilis). From investigations of snails fed cyanobacteria, it had been supposed that undigested cyanobacterial cells can take up nutrients from the alimentary canal during passage (Cuker 1983). Phosphorus uptake was demonstrated by microautoradiography for the green algae Sphaerocystis after passage through Daphnia (Porter 1976) and by uptake of radiolabeled P for the green algae Scenedesmus in Daphnia (Boersma pers.…”