Chemical Setting and Biogeochemical Reactions in Meromictic Lakes

Abstract

The chemical composition of meromictic lake waters varies widely. Concentrations of total dissolved substances (TDS ) range from very low ( 300 g L−1), i.e. saturated with respect to particular salts. pH varies from acidic ( 10), and redox conditions range from well oxygenated and dominated by high concentrations of dissolved ferric iron (Eh about 600 mV in iron-rich acidic pit lakes) to strongly reduced (Eh < −100 mV). While the ranges of TDS and pH apply to both mixolimnion and monimolimnion, redox conditions are typically oxic for mixolimnion (except for hypolimnion in some meromictic lakes during thermal stratification) and permanently anoxic for monimolimnion. Concentrations of reduced chemical species, e.g. ferrous iron, hydrogen sulphide and ammonia, vary over a wide range in monimolimnia. Chemical differences between mixolimnion and monimolimnion are the reason for density differences that keep the stratification stable. Several processes occur in the water column of meromictic lakes that are known from sediments of holomictic lakes. Permanently anoxic conditions above the monimolimnetic sediments of meromictic lakes provide better conditions for the conservation of settling organic material and prevent disturbance by bioturbation . Based on these special conditions, we divide Chap. 3 into four sections. After a brief introduction, we present ten selected examples to illustrate the variety of chemical conditions in meromictic lakes in Sect. 3.2. We refer also to appropriate case studies presented in Chaps. 5–12. Section 3.3 is devoted to biogeochemical processes that have the potential for creating and sustaining meromixis and that occur in the water column of meromictic lakes but usually not in the water column of holomictic lakes. Special biogeochemical aspects of monimolimnetic sediments in meromictic lakes are presented in Sect. 3.4 from a palaeolimnologic point of view.