Hydrogen sulphide (H2S) is produced in ponds by chemical reduction of organic matter accumulating in pond bottom, bottom soil often turning black and emanating a rotten smell. The gas can be detected by its characteristic odour and is often associated with black mud and sand and anoxic conditions. The gas is toxic to fish.
Hydrogen sulphide is toxic to aquatic animals because it interferes with reoxidation of cytochrome a3 in respiration. This effect is caused almost entirely by H2S, while HS- is essentially non-toxic.
Sulphide in sediment
Hydrogen sulphide formation in sediment is mainly the result of sulphate reduction by microorganisms. Sulfate reduction occurs at a lower oxidation-reduction (redox) potential than is necessary for the reduction of iron and manganese by microorganisms.
Hydrogen sulphide in sediment can enter overlying water by diffusion. It also can be mixed into the water column by biological activity and sediment disturbances by seine hauls and strong water currents caused by wind or mechanical aeration. If the rate at which hydrogen sulphide enters the water exceeds the rate of its oxidation, there will be a detectable concentration of this potential toxin in the water column – especially in the layer a few centimeters above the sediment-water interface.
Measurable hydrogen sulphide in water usually means a low dissolved-oxygen concentration in the water or at the sediment-water interface, and aeration should be increased.