Effects of grazing on the quantity and quality of freshwater Aufwuchs

Abstract

Qualitative and quantitative measures of the Aufwuchs (scum flora) on artificial substrates in situ were used to evaluate the effects of grazing by freshwater pulmonate snails in a shallow pond in southeastern Michigan . Grazer densities of-216 snails/ M 2 marked reduced standing crop so that after 45 days grazed substrata had 6 .46 mg dry weight, 604 µg C and 4.18 µg chlorophyll a as compared to controls with 30 .62 mg dry weight, 3699 µg C and 6 .29 µg chlorophyll a, all on a per dm 2 basis .Grazing did not change carbon per mg dry matter but caused significant increases in both µg chlorophyll a (control, 0 .2o6 ; and grazed, 0.649 µg chlorophyll a/ mg dry weight, P < o .o1) and nitrogen (control, 8 .3 ; and grazed, 24 .2 µg N/ mg dry weight, P < 0.00,) after 45 days . Both abundance and diversity of the attached community was reduced by grazing from 24 taxa and 80,889 individuals/ cm2 on control to 8 taxa and Sot individuals/ cm 2 on grazed substrates . Mean productivity of the Aufwuchs was significantly (P <o.ool) reduced by grazing from 76.3 µg C/(dmZ-day) on control to 17 .7 µg C/(dmZ -day) on grazed substrata. Snails were very efficient at clearing smooth surfaces of living cells, detritus, and particulate inorganic matter . There was little evidence of selectivity except for an apparent inability to remove some of the smallest cells (e .g. Cocconels sp .) probably for mechanical reasons .

Food selection by freshwater snails in the Gezira irrigation canals, Sudan

“…The present results confirm that freshwater pulmonate snails are generalist feeders grazing on epiphytic algae, decaying macrophytes and fine detritus as has been demonstrated for several species belonging to the Lymnaeidae, Physidae and Planorbidae (Pimental & White, 1959;Gohar & El-Gindy, 1961;Hunter, 1961;Malek, 1958;Clampitt, 1970a, b;Storey, 1971;Reavell, 1980;Hunter, 1980). No fresh fragments of aquatic macrophytes were found in the present studies and animal remains were found only on a few occasions.…”

Herbivore-mediated increase in the photosynthetic capacity of marine biofilms: indirect effects of changing microalgal assemblage composition

“…A comparatively large number of investigations have shown that, in freshwater habitats, grazers can control microalgal biomass, affect productivity and species richness, and change the composition of the biofilm (e.g. Hargrave 1970, Cooper 1973, Hunter 1980, Lowe & Hunter 1988, Swamikannu & Hoagland 1989, McCormick & Stevenson 1991, Rosemond et al 1993). In contrast, marine studies have in the past focused primarily on the effects of grazers on microalgal biomass and composition (Castenholz 1961, Nicotri 1977, Hunter & Russell-Hunter 1983, Mak & Williams 1999, Williams et al 2000.…”

Differences in snail ecology lead to infection pattern variation of Echinostoma spp. larval stages

“…However, lymnaeid snails rely on the aquatic environment for food, and L. columella frequently returns to the water to forage. Their feeding behavior is similar to that of the physid snails, which have a huge impact on plant and macrophyte communities (Hunter 1980) and exhibit significantly greater Patterns of Echinostoma spp. infections in snails 507 vagility than Helisoma spp.…”

“…employ a basic system of 'give up time' in which they continually change patches when resources become depleted (Charnov et al 1976, Dillon 2000 Lymnaeid snails, similar to the other pulmonates, show a dietary preference for algae (Bovbjerg 1965, 1968, Hunter 1980, Reavell 1980. These snails typically possess sand in their stomachs, which they use to aid in the digestion of more complex food items such as algae and live plant tissue (Storey 1970, Reavell 1980.…”

“…These snails typically possess sand in their stomachs, which they use to aid in the digestion of more complex food items such as algae and live plant tissue (Storey 1970, Reavell 1980. Similar to physids, lymnaeids can also have a large impact on the macrophyte diversity (Hunter 1980, Cuker 1983. They use a feeding system of 'give up time' (Charnov et al 1976, Dillon 2000, i.e., when food is abundant, lymnaeids are typically associated with rocks, plants, and other hard substrata, but when food is scarce, they can also be found foraging in the mud substratum (Walter 1980).…”