WHY DO SOME WATERS IN CLOSE PROXIMITY HOST DIFFERENT MOLLUSCAN ASSEMBLAGES?

The solute composition (the relative proportion of dominant ions, or solutes, within natural waters) of hydrologic settings may change (solute evolution) in space and time owing to various physical and chemical processes operating within the water. This is commonly determined by hydrologic setting and process, local geology, and climate variation. Solute composition is relatively stable in waters measuring between approximately 200 and 1,200 mg/l TDS, even though Ca and HCO3 (CO3) are precipitating. However, between approximately 1,200-1,600 mg/l TDS waters balance between three major water solute types that are of biologic importance: Ca = HCO3 (CO3), Ca < HCO3 (CO3), and Ca > HCO3 (CO3). Furthermore, this TDS range transitions from water dominated by CaCO3 to water dominated by solutes other than CaCO3. Solute composition affects the distribution and occurrence of fresh- and slightly saline-water mollusks. Therefore, the occurrence of aquatic mollusks can be defined in terms of solute composition which has implications for evaluating past hydrology.

The effect of solute composition on the distribution of mollusks is explored here using assemblages collected from pairs of lakes with differing hydrochemistry. The first example compares two adjacent lakes in North Dakota with Ca < HCO3 (CO3), however one lake is dominated by CaCO3 and the other is dominated by solutes other than CaCO3. The second example compares two nearby lakes in Wyoming that have similar TDS values but one is dominated by HCO3 relative to Ca and the other vice versa. All four of these lakes host different molluscan assemblages. Although the molluscan assemblages found in the differing solute environments support the solute composition model, it is possible that the difference in the distribution of taxa in these two examples may be due to other environmental parameters and not solute composition alone.