{"id":13145,"date":"2022-06-07T00:00:00","date_gmt":"2022-06-07T00:00:00","guid":{"rendered":"https:\/\/kcsensor.com\/index.php\/2022\/06\/07\/1-about-dissolved-oxygen-in-water\/"},"modified":"2026-09-23T06:45:27","modified_gmt":"2026-09-23T06:45:27","slug":"1-about-dissolved-oxygen-in-water","status":"publish","type":"post","link":"https:\/\/www.kcsensor.com\/it\/1-about-dissolved-oxygen-in-water\/","title":{"rendered":"Ossigeno sciolto in acqua: Fonti & #038; Deplezione Spiegata"},"content":{"rendered":"\n

Dissolved oxygen (DO) is the amount of free, molecular oxygen dissolved in water, usually expressed in milligrams of oxygen per liter (mg\/L). It is one of the most important indicators of water quality, and it directly determines whether fish, shrimp, and beneficial bacteria can survive in a pond, tank, or waterway. This guide explains where dissolved oxygen in water<\/a> comes from, what consumes it, and how temperature, pressure, and salinity affect how much oxygen water can actually hold.<\/p>\n\n\n\n

\"1.1<\/figure>\n\n\n\n

Where Does Dissolved Oxygen in Water Come From?<\/strong><\/h2>\n\n\n\n

Oxygen enters water through two main natural pathways, plus a smaller contribution from water exchange.<\/p>\n\n\n\n

1. Atmospheric Diffusion<\/strong><\/h3>\n\n\n\n

When the water surface is in contact with air, oxygen molecules diffuse across the surface and dissolve, at a rate that depends on how far the water is from saturation. The more \u201cunsaturated\u201d the water is, the faster this diffusion happens. Wind, wave action, and a large surface-area-to-volume ratio all speed up the process \u2014 this is why shallow, wind-exposed ponds pick up oxygen faster than deep, still ones.<\/p>\n\n\n\n

2. Photosynthesis<\/strong><\/h3>\n\n\n\n

Aquatic plants, algae, and phytoplankton release oxygen as a byproduct of photosynthesis whenever sunlight is available. In productive aquaculture ponds<\/a>, this is typically the single largest source of oxygen, often accounting for more than half of total DO input during the day \u2014 but it stops completely at night, which is why oxygen levels swing so much between day and night (more on that in Part 2 of this series<\/a>).<\/p>\n\n\n\n

3. Water Exchange<\/strong><\/h3>\n\n\n\n

Adding fresh, oxygen-rich water \u2014 through rain, tidal exchange, or intentional water replacement in a recirculating system \u2014 can raise DO levels. In practice, this contributes far less than diffusion and photosynthesis; in static (non-flow-through) ponds, its effect on oxygen is usually minor.<\/p>\n\n\n\n

\"1.2<\/figure>\n\n\n\n

What Causes Dissolved Oxygen Depletion?<\/strong><\/h2>\n\n\n\n

On the consumption side, several processes compete for the oxygen that photosynthesis and diffusion put into the water.<\/p>\n\n\n\n

Respiration of fish, shrimp, and other aquaculture animals.<\/strong> Oxygen consumption per unit of body weight is higher in smaller individuals and rises with water temperature \u2014 meaning warm-weather stocking density is far more sensitive to oxygen shortfalls than the same density in cooler water.<\/p>\n\n\n\n

Microbial and plankton respiration.<\/strong> Bacteria, zooplankton, and phytoplankton all consume oxygen through respiration, and bacteria also consume oxygen while decomposing organic matter (uneaten feed, waste, dead plankton). This decomposition demand is what biochemical oxygen demand (BOD5<\/sub>) actually measures.<\/p>\n\n\n\n

Sediment (benthic) oxygen demand.<\/strong> Oxygen is also consumed at the pond bottom by benthic organisms and by the decomposition of settled organic matter and reduced inorganic compounds \u2014 a demand that grows as bottom sludge accumulates over a production cycle.<\/p>\n\n\n\n

Chemical oxygen demand.<\/strong> Reduced substances such as ammonia, nitrite, sulfide, and ferrous iron all consume oxygen as they\u2019re oxidized (e.g., ammonia \u2192 nitrite \u2192 nitrate). Chemical oxygen demand (CODCr<\/sub>) is the standard measurement of this reducing load, and \u2014 alongside BOD5<\/sub> and total organic carbon (TOC) \u2014 is one of the three common ways labs quantify organic pollution. Facilities that need to track this alongside DO can pair a dissolved oxygen sensor with a COD sensor<\/a> for a fuller picture of oxygen demand.<\/p>\n\n\n\n

Escape to the atmosphere.<\/strong> When surface water becomes supersaturated, oxygen slowly escapes back into the air; wind speeds this up, but it is generally the smallest term in the oxygen budget.<\/p>\n\n\n\n

Dissolved salts.<\/strong> Salts and other dissolved solids physically occupy space between water molecules, which reduces how much oxygen the water can hold \u2014 one reason seawater carries less DO than fresh water at the same temperature. This is also why salinity monitoring<\/a> is often paired with DO monitoring in brackish or marine systems such as seawater desalination<\/a> intake water.<\/p>\n\n\n\n

How Temperature and Pressure Control Oxygen Saturation<\/strong><\/h2>\n\n\n\n

The saturation point \u2014 the maximum amount of oxygen water can hold at equilibrium with the atmosphere \u2014 is governed mainly by temperature, with smaller effects from atmospheric pressure and salinity. This relationship follows Henry\u2019s Law:<\/p>\n\n\n\n

[O2<\/sub>] = KH<\/sub> \u00d7 pO2<\/sub><\/strong><\/p>\n\n\n\n

where [O2<\/sub>] is the dissolved oxygen concentration, pO2<\/sub> is the partial pressure of oxygen in the atmosphere, and KH<\/sub> is Henry\u2019s constant, which falls as temperature rises. In practice, this means:<\/p>\n\n\n\n