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 Β΅g C and 4.18 Β΅g chlorophyll a as compared to controls with 30 .62 mg dry weight, 3699 Β΅g C and 6 .29 Β΅g chlorophyll a, all on a per dm 2 basis .
Grazing did not change carbon per mg dry matter but caused significant increases in both Β΅g chlorophyll a (control, 0 .2o6 ; and grazed, 0.649 Β΅g chlorophyll a/ mg dry weight, P < o .o1) and nitrogen (control, 8 .3 ; and grazed, 24 .2 Β΅g 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 Β΅g C/(dmZ-day) on control to 17 .7 Β΅g 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 .