DIRECT AND INDIRECT EFFECTS OF GEOLOGY ON THE DISTRIBUTION, BIOMASS, AND PRODUCTION OF THE FRESH-WATER SNAIL ELIMIA

We studied the relationship between geology and lotic secondary produc tion by analyzing geographical patterns of the distribution, biomass, and production of the freshwater snail Elimia (Pleuroceridae) in Alaba ma. Nine streams were selected for study, three each in three physiogr aphically distinct regions of uniform climate but contrasting litholog y. Our objectives were to assess: 1) the production and biomass of Eli mia among streams with contrasting alkalinities due to differences in regional geology-sandstone (5.7 mg/L as CaCO3) < phyllite (19.4) < car bonate (103.4); p < 0.05-and 2) geologically mediated variation in the rmal regime as an alternative factor contributing to the widely observ ed correlation between alkalinity and productivity. Although conspicuo us in phyllite and carbonate streams, Elimia was absent from sandstone streams. Biomass (B) and annual production (P) of Elimia were signifi cantly lower (p less than or equal to 0.03) in streams draining phylli te (B = 1102 mg/m(2), P = 1565 mg/m(2)) compared with carbonate catchm ents (B = 2990 mg/m(2), P = 2501 mg/m(2)). However,whereas the correla tion between biomass and alkalinity was consistent throughout the year , the correlation between production and alkalinity was not. Productio n did not differ significantly during the summer months (April-October , p = 0.41), and differences in annual production between regions were attributable to near cessation of production in phyllite streams duri ng the winter months (October-April, p = 0.04). Although mean annual s tream temperature was not significantly different between regions (p = 0.51), a simulation showed that the low winter temperatures of phylli te streams (much less than 10 degrees C) should cause the cessation of production and high net losses of snail biomass and result in insuffi cient post-winter biomass for rapid compounding of production during s pring and summer Consequently, biomass should be regulated at relative ly lower levels in phyllite than in carbonate streams where production occurs year-round because of a more moderate thermal regime (e.g., wi nter minima similar to 10 degrees C).