net oxygen consumption

【Amount of oxygen consumed by breathing】

　Let's estimate the amount of oxygen consumed through respiration (ΔO₂) between time t =0 and t=T.

【O₂】_t is the oxygen concentration at time t.

　How should we determine【O₂】_t=0and【O₂】_t=T in the actual ocean?

　One is to sample the same water at different times and measure the oxygen concentration. All you have to do is take your time and make observations in a place where water bodies are guaranteed to remain. In Funka Bay, Hokkaido, we know from experience that water at a certain depth stays there for a certain period of time, so we can plan such observations.

　The other is an area with constant water flow, where water from upstream and downstream can be collected in a short period of time. As shown in the figure below, if the flow rate is constant and the flow route is clear, it will be possible to obtain seawater samples that can be assumed to have passed over a certain period of time. This type of analysis may be possible for slow, large-scale flows such as deep water flows. What you observe depends on your ideas.

　In the above two examples, the location is too limited and the effort is too much. I would like to analyze it a little more easily. It can be expected that the oxygen concentration in the surface water of the ocean has reached an equilibrium state with the atmosphere, so this equilibrium state is set as the initial value of oxygen concentration [O₂]₀. Once the surface water is cut off from contact with the atmosphere, the oxygen concentration continues to decrease due to oxygen consumption in seawater. Although the elapsed time is unknown, it is possible to determine the amount of oxygen consumed.

　For example, let us assume that the oxygen concentration when a water mass exists at the surface is the initial concentration [O₂]₀ , and that the water mass moves to a depth of 3000 m after a certain time (T). Then, we conducted ocean observations and collected water at a depth of 3000 m and measured the oxygen concentration [O₂ 3000m]. The amount of oxygen consumed by the water body through respiration over time (T) is

【amount of oxygen consumed by breathing】＝ [O₂]₀ － [O₂ 3000m]

　[O₂]₀  can be calculated by determining the atmospheric oxygen concentration (a constant value) and water temperature. The temperature of ocean water only changes due to cooling and heating from the atmosphere, so the temperature of water in the middle and deep layers is the same as the temperature of the water at the surface. Therefore, all you have to do is give the water temperature measured through observation.

　Note that an equilibrium state is a state in which the surface water and the atmosphere are in contact for a sufficiently long period of time so that no oxygen appears to enter or exit, and there is no change in oxygen concentration. The oxygen concentration in seawater that is in equilibrium with the atmosphere is determined by the laws of physical chemistry. In other words, it's a matter of solubility, but it's important to understand that the colder the water, the more oxygen it can dissolve. The oxygen concentration in seawater that is in equilibrium with the atmosphere is also called the oxygen saturation concentration. In reality, there is also a salt effect, so we use an experimentally determined relational formula between oxygen saturation concentration, water temperature, and salinity (the calculation method is introduced on the next page).

It is also possible to somehow estimate the time it took to collect data from the time it was on the surface. For example, if we know that the water sinks due to vertical mixing every winter, and we are further assured that the water sank during the previous season's winter, then we can determine the time. Such an analysis is possible in Funka Bay, where the water is replaced twice every year.