5.3: Profiles of Carbon Dioxide, Oxygen, and Nutrients

In addition to being influenced by ocean-atmospheric interactions and by solubility, three of the major gasses in the ocean: oxygen (O2) carbon dioxide (CO2) co-vary. They are linked through the biological process of photosynthesis. As a reminder, plants (both on land and in the ocean) use carbon dioxide and nutrients (like nitrogen) during photosynthesis and create oxygen and organic matter. In fact, ocean plants produce ~50% of the oxygen that we breathe! The relationship between oxygen, carbon dioxide, and nitrogen gas is a bit more complex. Nitrogen gas must first be converted to biologically usable nitrogen (nitrate) before it can be used in photosynthesis. 

 

Oxygen in the Ocean

Typical oceanic dissolved oxygen profiles are shown in Figure \(\PageIndex{1}\). The shape of the profile is determined by the various processes that add or remove oxygen from the water at different depths.

Oxygen content is highest at the surface for two main reasons; this is where oxygen dissolves into the ocean from the atmosphere, and the surface water is where oxygen is produced by phytoplankton through photosynthesis. Respiration is also occurring in the surface waters, but the rate of photosynthetic oxygen production is greater than the rate of removal through respiration. It should be noted that even though dissolved oxygen is highest at the surface, there is still far less oxygen in the water than is found in the air. Well-oxygenated surface water may only contain around 8 mg O₂/l, while the air contains 210 mg O₂/l.

As depth increases, dissolved oxygen declines, reaching a minimum between a few hundred meters and 1000 m deep, the aptly-named oxygen minimum layer. At these depths and below, the water is too far removed from the surface for any atmospheric exchange, and there is not enough light to support photosynthesis, so there is little if any oxygen added to the water. At the same time, oxygen is removed from the water through the respiration of deep water organisms, and the decomposition of organic material by bacteria as it sinks to depth.

Below the oxygen minimum layer there is often an increase in dissolved oxygen at the greatest depths. This bottom water is usually colder than the surface water and is under enormous pressure; as stated above, lower temperatures and higher pressure increase the solubility of dissolved gases. But there is another reason that bottom water contains more oxygen than mid-water depths that has to do with the way water circulates throughout the deep ocean. In polar regions, the cold surface water absorbs lots of oxygen. This cold, oxygen-rich water sinks to the bottom due to its high density, taking the oxygen with it. The oxygen-rich bottom water will then spend the next thousand years or so moving over the seafloor throughout the major ocean basins. This deep water circulation is the source of oxygen for bottom-dwelling (benthic) organisms. The oxygen-rich bottom water forms in the polar regions of the Atlantic, and slowly makes its way to the Pacific, with oxygen being removed for respiration along the way. This is why dissolved oxygen levels in Pacific deep water are generally lower than in the Atlantic (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\) Representative dissolved oxygen profiles for the Pacific and Atlantic oceans (PW).

Figure \(\PageIndex{2}\) Dissolved oxygen profile from a transect across the Atlantic Ocean from Florida to the coast of Africa (inset). The oxygen minimum layer is visible between 500-1000 m (eWOCE, http://www.ewoce.org/gallery/eWOCE_T....html#Atlantic).

Areas where dissolved oxygen levels are too low to support most life are referred to as hypoxic zones (they are experiencing hypoxia, or low oxygen). Hypoxia is usually defined as oxygen levels below 2 mg/L. Anoxic zones (anoxia = without oxygen) show more severe forms of hypoxia, with oxygen below 0.5 mg/L. Some parts of the oceans may experience seasonal or temporary periods of hypoxia, while in other areas these conditions may last much longer. These hypoxic conditions often lead to mass die-offs of marine organisms who struggle to survive without sufficient oxygen.

 

Carbon Dioxide in the Ocean

Oxygen and carbon dioxide are involved in the same biological processes in the ocean, but in opposite ways; photosynthesis consumes CO₂ and produces O₂, while respiration and decomposition consume O₂ and produce CO₂. Therefore it should not be surprising that oceanic CO₂ profiles are essentially the opposite of dissolved oxygen profiles. At the surface, photosynthesis consumes CO₂ so CO₂ levels remain relatively low. In addition, organisms that utilize carbonate in their shells are common near the surface, further reducing the amount of dissolved CO₂.

In deeper water, CO₂ concentration increases as respiration exceeds photosynthesis, and decomposition of organic matter adds additional CO₂ to the water. As with oxygen, there is often more CO₂ at depth because cold bottom water holds more dissolved gases, and high pressures increase solubility. Deep water in the Pacific contains more CO₂ than the Atlantic as the Pacific water is older and has accumulated more CO₂ from the respiration of benthic organisms.

Figure  Representative carbon dioxide profiles for the Pacific and Atlantic oceans (PW).

 

Nitrogen in the Ocean

Nitrogen is the most abundant gas in the atmosphere, and like the other atmospheric gases it dissolves into the surface layers of the ocean. But most marine organisms cannot directly utilize dissolved nitrogen in the form in which it exists in air (N₂), so it must first be converted into other nitrogenous products. These conversions are undertaken by marine bacteria, who take the dissolved N₂ and convert it into ammonium (NH₄⁺), nitrite (NO₂^2-) or nitrate (NO₃^–). These bacteria also take the ammonium excreted by marine organisms and the byproducts of decomposition and ultimately convert them into nitrate. Nitrate is the main nitrogenous compound utilized by primary producers in the ocean; it is a major nutrient required for photosynthesis. Note that in this context, a nutrient refers to a chemical needed to support photosynthesis and primary production. It does not refer to the nutritional needs of consumer organisms. Since nutrients are rapidly used in biological processes, they are non-conservative, and their concentrations vary regionally and seasonally. They are generally found in low quantities in the oceans because they are quickly consumed by primary producers.

Since nitrate is one of the most important nutrients, for now we will focus only on nitrate as we discuss general nutrient patterns in the ocean. Other important nutrients, such as phosphate and silica, show similar patterns to nitrate, and will be discussed in the section on primary production.

A representative nutrient (nitrate) profile is shown below. Nutrient concentrations are low at the surface, because that is where the primary producers are located; the nutrients are rapidly consumed and they do not have the chance to accumulate. Nutrient levels increase at depth, as they are no longer being consumed by producers, and they are being regenerated through the decomposition of organic material by bacteria.

Figure  Representative nutrient (nitrate) profile for the open ocean (PW).

 

 

Additional links for more information:

• NOAA site on oceanic hypoxic zones: http://oceanservice.noaa.gov/hazards/hypoxia/