Freshwater process conditions

Iron sulphides are ubiquitous in marine and freshwater sediments. They are usually present either as pyrite or as monosulphides, which can be liberated by hydrochloric acid. These acid volatile sulphides give rise to an intense black colour that is characteristic of anoxic sediments. They play an important role in recent diagenetic processes in sediments and the ratio of pyrite to acid volatile sulphides has been used as an historical indicator to determine whether sediments were formed in marine or freshwater conditions. They can be present over a wide range of... 

Even if rate measurements in sediments are made using whole core incubations, e.g., when the inhibitor is a gas, it is still difficult to obtain a depth distribution of the rate (usually, an areal rate is obtained). A sophisticated measurement and model based system that avoids direct rate measurements has been used to overcome this problem. Microelectrodes, which have very high vertical resolution, are used to measure the fine scale distribution of oxygen and NOs" in freshwater sediments. By assuming that the observed vertical gradients represent a steady state condition, reaction-diffusion models can then be used to estimate the rates of nitrification, denitrification and aerobic respiration and to compute the location of the rate processes in relation to the chemical profiles (e.g., Binnerup et ai, 1992 Jensen et ai, 1994 Meyer et ai, 2001 Rysgaard et ai, 1994). Recent advances and details of the microelectrode approach can be found in the Chapter by Joye and Anderson (this volume). 

An example from a slowly-accumulating (0.01-0.1 cmyr ) sediment profile in a freshwater lake in Scotland (Williams, 1992) should suffice to illustrate the formation of diagenetic metal profiles. Early diagenetic processes, such as those described before, have promoted extensive metal enrichment immediately beneath the water-sediment interface. The oxic conditions, near the water-sediment interface, that promote metal precipitation and enrichment (Mn, Fe, Pb, Zn, Cu, Ni) are entirely confined to strata of post-industrial age (Williams, 1992). 

Much of the chemistry of the oceans and of freshwater systems depends on the kinetics of various physical and chemical processes and on biochemical reactions rather than on equilibrium conditions. The simplest model describing systems open to their environment is the time-invariant steady-state model. Because the sea has remained constant for the recent geological past, it may be well justified to interpret the ocean in terms of a steady-state model. 

Due to concerns over potential exposure of aquatic organisms to NP, a number of acute and chronic toxicity tests have been conducted for both freshwater and saltwater species of invertebrates, fish, and aquatic plants. NP is considered an endocrine dis-ruptor chemical and induces production of vitellogenin in male rainbow trout, a process that normally occurs only in female fish in response to estrogenic hormones during the reproductive cycle. NP also induces precocious development of ovaries and an intersex condition in some fish species. 

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