306
surface CO 2 concentration, which is exchanged with the atmosphere, are suppressed
(Fig. 11.5). In turn, POC as a source for increasing the surface CO 2 concentration
gradually precipitates due to its own weight. Thus, even if there is stratification, the
SCE’s sedimentation is not disturbed (Kone et al. 2009).
Because SCEs are shallow, POC sinks and reaches the sea bottom in a short time,
leading to less mineralization during sinking in the water column. This also suppresses a rise in the CO 2 concentration in surface water. Furthermore, sediments are
often resuspended due to the effects of wind-driven waves in SCEs, but the resuspension is suppressed when stratification develops. This suppression decreases the
turbidity of the surface water and increases the light intensity available for photosynthesis, and the increased photosynthesis by phytoplankton lowers the CO 2 concentration in the surface water (Chen et al. 2008).
Stratification occurs seasonally: it develops in the summer when the surface
water is heated with strong sunlight. This seasonality also plays an important role in
CO 2 gas exchange. During the summer, upwelling of the bottom layer water containing a high DIC concentration is blocked due to stratification. Seawater is well
mixed vertically in other seasons when stratification does not develop. As a result of
this mixing, the surface CO 2 concentration rises. However, because the water temperature is lower and solubility of CO 2 is higher in seasons other than summer, less
CO 2 is emitted into the atmosphere.
Although there is debate on the topic, the decomposition and mineralization rates
of organic matter are generally considered to be faster when the oxygen concentration is higher (Canfield 1994; Hartnett et al. 1998; Miyajima and Hamaguchi 2018).
In addition, the rate of decomposition of organic matter increases where conditions
fluctuate between aerobic and anaerobic, thus promoting symbiosis between aerobic and anaerobic heterotrophic bacteria (Zonneveld et al. 2010). This suggests that
the presence of diverse chemical and biological environments may promote the
decomposition of more diverse organic matter. For example, if labile organic matter
is first decomposed and mineralized under aerobic conditions in the bottom water
and sedimentary surface layer and then undecomposed organic matter is transported
to the deeper anaerobic environment and further mineralized, the mineralization
rate per unit area may increase as a whole.
Nevertheless, when hypoxic conditions occur in the bottom water during stratification, the aerobic sediment surface layer becomes anaerobic throughout the sediment layers, and the decomposition and mineralization rates of organic matter
decrease. Consequently, organic matter accumulates at the seabed at a faster rate.
Also, because decomposition of organic matter by benthic animals is suppressed
under hypoxic conditions, the presence of hypoxia facilitates the accumulation of
organic matter in sediments (Koho et al. 2013).
Hypoxic water masses are usually seen as purely detrimental, as hypoxia causes
mortality of benthic macrofauna such as fish and shellfish. From the viewpoint of
carbon storage or climate change mitigation, however, hypoxic water masses have
some positive effects, as we have explained. However, hypoxic water masses may
promote the production of other greenhouse gases, such as N 2 O and CH 4 , and further research on the topic is warranted.
T. Kuwae et al.
surface CO 2 concentration, which is exchanged with the atmosphere, are suppressed
(Fig. 11.5). In turn, POC as a source for increasing the surface CO 2 concentration
gradually precipitates due to its own weight. Thus, even if there is stratification, the
SCE’s sedimentation is not disturbed (Kone et al. 2009).
Because SCEs are shallow, POC sinks and reaches the sea bottom in a short time,
leading to less mineralization during sinking in the water column. This also suppresses a rise in the CO 2 concentration in surface water. Furthermore, sediments are
often resuspended due to the effects of wind-driven waves in SCEs, but the resuspension is suppressed when stratification develops. This suppression decreases the
turbidity of the surface water and increases the light intensity available for photosynthesis, and the increased photosynthesis by phytoplankton lowers the CO 2 concentration in the surface water (Chen et al. 2008).
Stratification occurs seasonally: it develops in the summer when the surface
water is heated with strong sunlight. This seasonality also plays an important role in
CO 2 gas exchange. During the summer, upwelling of the bottom layer water containing a high DIC concentration is blocked due to stratification. Seawater is well
mixed vertically in other seasons when stratification does not develop. As a result of
this mixing, the surface CO 2 concentration rises. However, because the water temperature is lower and solubility of CO 2 is higher in seasons other than summer, less
CO 2 is emitted into the atmosphere.
Although there is debate on the topic, the decomposition and mineralization rates
of organic matter are generally considered to be faster when the oxygen concentration is higher (Canfield 1994; Hartnett et al. 1998; Miyajima and Hamaguchi 2018).
In addition, the rate of decomposition of organic matter increases where conditions
fluctuate between aerobic and anaerobic, thus promoting symbiosis between aerobic and anaerobic heterotrophic bacteria (Zonneveld et al. 2010). This suggests that
the presence of diverse chemical and biological environments may promote the
decomposition of more diverse organic matter. For example, if labile organic matter
is first decomposed and mineralized under aerobic conditions in the bottom water
and sedimentary surface layer and then undecomposed organic matter is transported
to the deeper anaerobic environment and further mineralized, the mineralization
rate per unit area may increase as a whole.
Nevertheless, when hypoxic conditions occur in the bottom water during stratification, the aerobic sediment surface layer becomes anaerobic throughout the sediment layers, and the decomposition and mineralization rates of organic matter
decrease. Consequently, organic matter accumulates at the seabed at a faster rate.
Also, because decomposition of organic matter by benthic animals is suppressed
under hypoxic conditions, the presence of hypoxia facilitates the accumulation of
organic matter in sediments (Koho et al. 2013).
Hypoxic water masses are usually seen as purely detrimental, as hypoxia causes
mortality of benthic macrofauna such as fish and shellfish. From the viewpoint of
carbon storage or climate change mitigation, however, hypoxic water masses have
some positive effects, as we have explained. However, hypoxic water masses may
promote the production of other greenhouse gases, such as N 2 O and CH 4 , and further research on the topic is warranted.
T. Kuwae et al.