ocean oxygen

　Marine plants use a small portion of the vast amount of carbon dioxide and water present in the ocean for photosynthesis to synthesize organic matter, and in return they exhale oxygen molecules (O₂). On the other hand, plants and animals consume oxygen and organic matter through respiration and exhale carbon dioxide and water. Also, keep in mind that most of the oxygen in seawater is absorbed from the atmosphere.

海洋での光合成と呼吸による酸素の移動のイメージImage of oxygen transfer through photosynthesis and respiration in the ocean

　Let's assume that the oxygen concentration when a water mass exists at the surface is the initial concentration [O₂]₀ , and the water mass moves to a depth of 3000 m. 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 respiration while the water mass travels up to 3000 m 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.

　This is the formula for calculating the oxygen saturation concentration in seawater relative to atmospheric oxygen.

　The saturation concentration (solid line) and the oxygen concentration observed in surface seawater (arrow (↑)) are shown below.

表面水温と酸素濃度の関係Relationship between surface water temperature and oxygen concentration

実線は、水温から計算される酸素飽和濃度。The solid line is the oxygen saturation concentration calculated from the water temperature.

矢印は実際に観測される表面海水の酸素濃度。The arrow is Oxygen concentration in surface seawater actually observed.

　The amount of oxygen consumed in seawater can be determined by subtracting the "oxygen concentration at a certain time T" from the "initial concentration of oxygen at a certain time (0)" to find the amount of oxygen consumed at time 0 → T.  However, this only holds true if there is no oxygen production through photosynthesis (see the left diagram below).

　In seawater, oxygen consumption through respiration and oxygen generation through photosynthesis occur simultaneously. You can't tell them apart. In other words, it is impossible to know the ``true amount consumed'' or the ``true amount produced.'' If the direction of consumption is positive, if "unknown true consumption" > "unknown true production", then "net consumption" > 0. This means that respiration was superior to photosynthesis. The opposite is also true.

酸素消費量oxygen consumption　酸素の初期濃度initial concentration of oxygen　ある時刻Tの酸素濃度Oxygen concentration at a certain time T　負negative　発生occurrence　消費consumption　正Positive　実際に計測できる量Quantity that can actually be measured　観測濃度Observed concentration　水温から計算される飽和濃度を初期濃度とする。The saturation concentration calculated from the water temperature is set as the initial concentration.　光合成photosynthesis　呼吸breathing

　Weak light reaches even the subsurface layer, which has no contact with the atmosphere, and oxygen is produced through photosynthesis. Therefore, the "oxygen consumption" or "apparent oxygen consumption" calculated earlier is the amount obtained by subtracting the "amount of oxygen produced through photosynthesis" from the "amount of oxygen consumed through respiration." Here, the apparent oxygen consumption is called Apparent Oxygen Utilization (AOU). The figure below shows the reasons why the oxygen concentration in the mid-latitudes of the North Pacific is determined. The AOU corresponds to the length of the arrow in the right figure below.

待機と海洋表面で酸素が平衡Oxygen equilibration at the waiting and ocean surface　北大西洋高緯度深層水形成エリアNorth Atlantic high latitude deep water formation area　密度躍層pycnocline　北太平洋中緯度North Pacific mid-latitudes　酸素飽和度oxygen saturation　表層水surface water　中層水middle water　深層水deep water　酸素濃度oxygen concentration　酸素消費oxygen consumption