Highly reducing marine sediments have received much less research attention than studies of typically more oxic/hypoxic sediments. When the oxygen demand caused by input of organic matter exceeds the oxygen diffusion rate from overlying waters, sediments become anoxic and anaerobic processes dominate. In marine systems, sulphate reduction is the most important anaerobic process for the degradation of organic material. This is critical for the benthic faunal community as the end product, sulphide, is toxic. Benthic fauna contribute significantly to organic matter degradation in the sediments and to the maintenance of a “healthy” sediment environment through mixing and irrigation processes. Sulphide can have several fates in sediments with a limited amount being stored as insoluble metal sulphides and most of the remainder undergoing repeated oxidation-reduction cycles. Sulphur cycling is therefore intimately linked to sedimentary oxygen demand. This study aimed to quantify sulphur biogeochemistry in the highly reducing sediments typically found beneath fish farms and improve our understanding of the role that sulphur plays in sediment recovery processes. The following relationships between sediment properties were considered; pore water sulphide concentrations, metal sulphide species, pH, organic matter content and oxygen demand. The effect of seasonal temperature change and increasing organic matter input on these properties was examined in detail during a short term period after an enrichment pulse. A high resolution data set was obtained using state-of-the-art micro electrodes. These measurements gave information on the assimilative capacity of sediments for fish farm wastes over a short incubation periods (72 and 92 days), and established the relative effect of temperature change and organic matter input on key biogeochemical parameters used to measure organic enrichment in fish farm monitoring programmes.