(CHO) Dissolved Oxygen Lesson

Dissolved Oxygen

Dissolved Oxygen (DO) is the amount of oxygen dissolved in the water. DO is measured in units of parts per million (ppm) or milligrams per liter (mg/L). DO levels in ocean water range from about 1 to 12 ppm. Just for comparison, the amount of O2 in the air is 200 ppm!

The minimum DO level needed by fish and other aquatic animals to breathe is 4 ppm; below this level, a condition called hypoxia occurs. Hypoxia occurs when oxygen levels in the water decrease causing fish and other aquatic animals to suffocate. Hypoxia can result from algal blooms (anaerobic bacteria) and sludge dumping.

There are two major chemical reactions that contribute to maintaining levels of dissolved oxygen in ocean water. These chemical reactions are the same reactions that contribute to maintaining oxygen levels within the atmosphere; photosynthesis and cellular respiration.

Oxygen is a diatomic gas, which means gaseous oxygen moves in pairs as O2. Photosynthetic algae and plants take in carbon dioxide, water, and solar energy to produce carbohydrates and oxygen. Marine organisms then consume this oxygen either by diffusion or through gills. These organisms use oxygen and carbohydrates for cellular respiration and release carbon dioxide. Once again, the plants and algae take up the carbon dioxide and the cycle continues.

As depth increases, the DO decreases. The DO levels will continue to decrease with depth until the measured DO is zero. The depth at which there is the lowest amount of oxygen is called the O2 minimum zone. This is typically found at a depth of 1000 meters.

The Water Column

The makeup of the ocean varies depending on the latitude of the location, but some general statements can be made about how the ocean is structured vertically, which is known as the water column.

Temperature

graph of thermocline: comparing Depth (m) to Temperature (°C), thermocline between 200m and 100m, epipelagic zone less than 200mThe temperature of the ocean is highly dependent on the latitude and location. For example, an area of ocean off of the coast of Georgia warmed by the Gulf Stream current will be much warmer than an area of ocean off of the coast of California. The temperature of the ocean also changes seasonally, just like the air temperature.

However, in general, the surface of the ocean is much warmer than the bottom of the ocean. This is because the surface of the ocean is warmed by the Sun. For the first 200 meters, the temperature of the ocean is relatively constant, and warmer, than lower layers because the Sun can penetrate the first 200 meters of the ocean quite well. This area of the ocean is known as the epipelagic zone because the Sun can penetrate well enough for plants and algae to perform photosynthesis. Past this zone, between approximately 200 and 1000 meters, the temperature drops quickly. This area of the ocean where the temperature drops quickly is known as the thermocline. This rapidly changing temperature is due to several factors, but the biggest one is the rapidly decreasing availability of light, or warmth from the Sun.

Past the thermocline, the temperature begins to stabilize again because no light reaches past the thermocline. This means that below around 1000 meters, the ocean is uniformly cold and dark. The temperature stabilizes around 2-4 degrees Celsius. Regardless of the location or surface temperature of the ocean, the deep ocean is the same temperature. This means that very deep water in the tropics is the same temperature as very deep water in the Arctic or Antarctic.

Salinity

The salinity of the ocean refers to the salt content of the ocean. Typically, the open ocean has a salinity of 35 ppt, which means parts per thousand. However, the ocean is not 35 ppt and the coast 0 ppt. Usually, the salinity of the ocean gradually increases as you move away from the shore until you reach full salinity, or the open ocean.

types of water briny water brine pools 50+ parts per thousand Red Sea - 40 parts per thousand Mediterranean Sea - 38 parts per thousand Average sea water - 34.7 parts per thousand 30ppt saline water seawater, salt lakes 30-50 parts per thousand brackish water estuaries, mangrove swamps, brackish seas and lake, brackish swamps .5-30 parts per thousand Black Sea 18 parts per thousand Baltic Sea 8 parts per thousand Limit on agriculture Irrigation - 2 ppt parts per thousand Drinking water - .1 parts per thousand .5 parts per thousand *traditional ways to express salinity is in "parts per thousand" or ppt fresh water ponds, lakes, rivers, streams, aquifers 0-.5 parts per thousand

Areas that do not have full salinity, but are partially salty are considered brackish and areas that have salinities less than 5 ppt are considered fresh, even though they may have a salinity above 0 ppt.

Sometimes, areas of the ocean will have a sharp change in salinity. This is known as a halocline.

Ocean Circulation

Temperature and salinity work together to circulate our oceans and to make sure nutrients, oxygen, and water is moved around, keeping the ocean from becoming stagnant.

At the poles, salty ocean water freezes, leaving behind the salts since only fresh water freezes. This increases the salinity of the ocean at the poles. As the water gets saltier, the density of the water increases. This means that an area of super salty water is heavier than the same sized area of fresh or less salty water. Heavier, or more dense water sinks.

Cooler, less salty water rises to the surface to replace to salty water that sank. This continual movement of ocean water drives the global climate and is known as thermohaline circulation. For example, in areas where warm surface currents are at the surface, the climate will be more mild than in areas where there are no warm surface currents. For example, warm surface currents move past England and Ireland, which mediates the climate, making these areas much warmer than other areas (Canada, for example) at the same latitude.

As you can see, both the temperature and the salinity of the ocean contribute to oceanic circulation, which drives our climate.

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