CHAPTER 3. Long-term tendency of DO concentration
3.6. Long-term change of physical-biogeochemical environment in the lower layers
3.6.2. Physical and biological contributions to DO concentration over the EJS
Figure 3.18 shows the long-term variation of the DO concentration of the CTL experiment over each depth range. Each depth range of 516 to 1006 m, 1006 to 1618 m, 1618 to 2250 m, and 2250 m to the bottom can be regarded as the upper portion of the central water (CW), the lower portion of the CW, deep water (DW) and the bottom water (BW). Comprehensive review of Kim et al. (2004) and Yoon et al. (2018) shows that the water masses of the EJS can be roughly classified as the CW from 400 to 1500 m, the DW from 1500 to 2500 m, and the BW from 2500 to 3500 m.
The mean DO concentration averaged over the 23 years is extremely high in the upper portion of the CW (5.33 ml l−1) compared with that of the other depth range (lower portion of the CW: 4.83 ml l−1; DW: 4.86 ml l−1; BW: 4.90 ml l−1). The high DO concentration of the upper portion of the CW is due to the supply of the oxygen from the upper ocean by vertical mixing process. Below the depth of 1006 m, the DO concentration is basically higher in the BW than in the lower portion of the CW. This vertical structure results from more vigorous decomposition process of the DET in the CW than in the BW. The biological consumption becomes lower along with the decrease in DET concentration with increasing depth.
The DO concentration in the upper portion of the CW has decreased, superimposed with the conspicuous interannual variation. This interannual fluctuation is attributed to
0.30 ml l−1 in the depth range, respectively. The decreasing trend in DO concentration of the BW reproduced by the model (0.013 ml l−1 year−1) is almost as the same scale as that of the observation data (0.017 ml l−1 year−1) reported by Gamo (1999).
In this section, the effect of three possible causes of “P×B”, “B”, and “P” on the decline in DO concentration are quantitatively evaluated. We deal with the physical contribution to the DO variation in Section 3.6.2.1. The biological contribution is discussed in Section 3.6.2.2.
3.6.2.1. Physical contribution to DO variation
We quantitatively identify the physical contributions without biological activities (that is, the third possible cause: “P”) influencing on the DO variation by taking advantage of the NE experiment. Figure 3.19 shows the long-term variation in DO concentration and the DO change rate for individual years by the physical processes only over each depth range.
The DO concentrations by the physical processes without biological decomposition tend to increase gradually in all depth ranges, and the increasing trend is more distinct in the upper part (Fig. 3.19 (a)). The vertical distribution of the DO concentration in the beginning, which was low in the lower portion of the CW and high in the BW, is rapidly reversed without considering the biological consumption.
The physical effect of positive (negative) indicates the increase (decrease) in DO concentration (Fig. 3.19 (b)). The contribution of “P” to the DO concentration is largest in the upper portion of the CW, and concurrently has the significant interannual variation ranging from negative to positive values. The physical processes below the depth of
1006 m primarily perform to increase the DO concentration without biological decomposition. The increase in DO concentration across all depth ranges is mainly related to the vertical transport of the water masses. Basically, the supply of oxygen penetrated from the surface increases the DO concentration of the deep layers. The deep convection reaching the bottom appeared temporarily in the winter of 2000 to 2001, but the DO concentration of the DW and BW did not increase significantly because the water formation just occurred a few times in a few grids.
The deepening of the winter mixed layer also contributes to the increase in DO concentration of the deep water. Moreover, the recent change in density structure influences on the DO distribution. It will be discussed in Section 3.6.3 in detail. Because the source is located in the upper layer, the physical contribution is significant in the CW and becomes lower with increasing depth. The DO change by the physical processes without biological decomposition for the 23 years is +6.2×10−1, +4.2×10−1, +1.4×10−1, and +7.0×10−2 ml l−1 in each depth range from 516 to 1006 m, 1006 to 1618 m, 1618 to 2250 m, and 2250 m to the bottom, respectively.
3.6.2.2. Biological contribution to DO variation
The DO change by the biological activities is separated by the first (“P×B”) and the second (“B”) possible causes. The “P×B” indicates the change in DO concentration by
experiments. The difference between the CTL and CDF experiments provide us the influence of the second cause on the DO change.
Figure 3.20 indicates the long-term change in DO concentration obtained from the CDF experiment from 1990 to 2012 for each depth range from 516 to 1006 m, 1006 to 1618 m, 1618 to 2250 m, and 2250 m to the bottom, respectively. The results of the CTL experiment were plotted together for comparison. The constant flux of the CDF experiment is larger than the CTL experiment for the periods from 1992 to 1997 (Fig.
3.17). As a result, the DO concentration of the CDF experiment is slightly higher than that of the CTL experiment until the late 1990s in the all depth ranges (Fig. 3.20).
The sinking flux of organic matters has been larger in the CTL experiment compared the CDF experiment since 1998, and their difference grows larger as the time passed. As a consequence, the difference of DO concentration between the two experiments gradually becomes larger resulting from the significant biological consumption of the DO in the CTL experiment. The effect of the increase in DET flux influencing on the DO concentration is remarkable in the CW, where the decomposition process is active, and weak in the DW and BW, which are less sensitive to the upper environment.
The DO concentration of the two experiments is reversed around 2000 in the CW and DW, even though the DET flux of the CTL experiments has been greater than the CDF experiments since 1998. It suggests that the DO concentration responses to the changes in biogeochemical environment of the upper layer with the time lag of a few years. The response is more delayed as the depth goes deeper.
The time-series of the DO concentration due to the second cause (“B”) obtained from the difference between the CTL and CDF experiments, and its change rate for individual years are shown in Fig. 3.21. The enhancement in the biogeochemical environment of the upper layer induces the changes in DO concentration of the lower layers. The decrease in DO concentration due to the increase in DET flux is quantitatively large in the CW, but it has less contribution in the DW and BW, as we confirmed in Fig. 3.20. Thus, the decline in DO concentration by the second cause for the 23 years is 5.3×10−2, 4.3×10−2, 2.2×10−2, and 1.9×10−2 ml l−1 in each depth range from 516 to 1006 m, 1006 to 1618 m, 1618 to 2250 m, and 2250 m to the bottom, respectively.
The reduction in DO concentration by the first cause (“P×B”) is significant in the CW than in the DW or BW (Fig. 3.22 (a)). That is because the decomposition process accompanying the DO consumption is active in the upper part. The decreasing rate of the DO concentration in the CW has been weakened in recent years attributed to the enhancement of the vertical mixing as shown in Fig. 3.14 (b). On the other hands, the decrease in DO concentration due to the first cause is almost constant every year over the DW and BW. The fundamentally consumed DO by the biological decomposition process without the new water supply is 6.8×10−1, 5.9×10−1, 4.1×10−1, and 3.5×10−1 ml l−1 for the 23 years in each depth range from 516 to 1006 m, 1006 to 1618 m, 1618 to 2250 m, and 2250 m to the bottom, respectively.
3.6.3. Spatial pattern of physical and biological contributions to DO