Benthic Sediment Oxygen Demand

Benthic sediment oxygen demand (SOD) is the rate at which the pond bottom consumes dissolved oxygen to decompose accumulated organic matter through microbial respiration, typically expressed in grams of O2 per square meter per day. In productive aquaculture ponds, SOD ranges from about 0.5 g O2/m2/day in lightly loaded extensive ponds to over 5 g O2/m2/day in organically enriched intensive ponds, and it often exceeds the water-column respiration of the entire phytoplankton bloom. SOD rises as sludge depth, feed inputs, and dead-algae deposition increase, and it peaks in warm conditions when microbial activity accelerates. Its operational importance is greatest at dawn: night-time SOD, combined with bloom respiration, is what pushes dissolved oxygen to its daily minimum of 3-4 mg/L or below, triggering fish kills in shrimp and fish ponds. Managing SOD means managing the bottom: regular sludge removal, oxidizing treatments (lime, nitrate), probiotics, avoiding overfeeding, and maintaining aerobic conditions near the sediment-water interface keep decomposition efficient and non-odorous. A pond with high SOD shows blackening sediment, rising alkalinity loss, and unstable morning oxygen. Farmers who track SOD indirectly through dawn oxygen records can time aeration, water exchange, and harvest decisions far more accurately.