SHELL MICROLAMINATIONS OF THE FRESH-WATER BIVALVE HYRIDELLA-DEPRESSA AS AN ARCHIVAL MONITOR OF MANGANESE WATER CONCENTRATION - EXPERIMENTALINVESTIGATION BY DEPTH PROFILING USING SECONDARY-ION MASS-SPECTROMETRY (SIMS)

Specimens of the freshwater unionid bivalve Hyridella depressa were ex perimentally exposed to a synthetic river water containing an elevated Mn water concentration (20 mg l(-1)) for 2 or 6 days. SIMS depth prof iles through the incremental nacre microlaminations or tablets (simila r to 0.6 mu m breadth) of the shells of these bivalves showed increase s in the signal intensity of Ca-normalised Mn that corresponded to the period of exposure. These results support the proposition that bivalv e shells can be used as retrospective monitors of water chemistry. The y also indicate that I) there is a lag phase between exposure to the e levated Mn water concentration and its expression in the shell, and 2) the period for Mn in the shell to reach equilibrium with the aquatic medium is greater that 2 to 6 days.