Direct and indirect effects of geology on the distribution, biomass, and production of the freshwater snail Elimia

We studied the relationship between geology and lotic secondary production by analyzing geographical patterns of the distribution, biomass, and production of the freshwater snail Elimia (Pleuroceridae) in Alabama. Nine streams were selected for study, three each in three physiographically distinct regions of uniform climate but contrasting lithology. Our objectives were to assess: 1) the production and biomass of Elimia among streams with contrasting alkalinities due to differences in regional geology-sandstone (5.7 mg/L as CaCO3) < phyllite (19.4) < carbonate (103.4); p < 0.05-and 2) geologically mediated variation in thermal regime as an alternative factor contributing to the widely observed correlation between alkalinity and productivity. Although conspicuous in phyllite and carbonate streams, Elimia was absent from sandstone streams. Biomass (B) and annual production (P) of Elimia were significantly lower (p less than or equal to 0.03) in streams draining phyllite (B = 1102 mg/m(2), P = 1565 mg/m(2)) compared with carbonate catchments (B = 2990 mg/m(2), P = 2501 mg/m(2)). However,whereas the correlation between biomass and alkalinity was consistent throughout the year, the correlation between production and alkalinity was not. Production 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 during the winter months (October-April, p = 0.04). Although mean annual stream temperature was not significantly different between regions (p = 0.51), a simulation showed that the low winter temperatures of phyllite streams (much less than 10 degrees C) should cause the cessation of production and high net losses of snail biomass and result in insufficient post-winter biomass for rapid compounding of production during spring and summer Consequently, biomass should be regulated at relatively lower levels in phyllite than in carbonate streams where production occurs year-round because of a more moderate thermal regime (e.g., winter minima similar to 10 degrees C).