303
11.3 Hypothesis that Human-Impacted SCEs Act As a Net
CO 2 Sink
Strong human impacts can result in changes to the carbon cycle (McIntyre et al.
2000). In particular, nutrient load, wastewater treatment, and freshwater use will
increase with increasing human and livestock populations and farmland area. As a
result of human impacts, the cycling of green carbon and blue carbon related to
climate change mitigation, such as CO 2 exchange between the atmosphere and
water and carbon storage in SCEs, is affected (Table 11.1).
Kuwae et al. (2016) hypothesized that some characteristics of SCEs subject to
human impacts actually strengthen the carbon cycling structure that supports the net
uptake of atmospheric CO 2 (net CO 2 sink; Fig. 11.5). This idea is likely to be controversial, because urban coastal waters are seen as places where eutrophication
progresses and a large amount of CO 2 is emitted by decomposition of organic matter. In this section, we explain why a human-impacted SCE functions as a sink of
atmospheric CO 2 from a mechanistic perspective and provide empirical evidence
from previous studies.
11.3.1 Wastewater Treatment
Urban and agricultural nutrient loading and wastewater treatment have a major
influence on a SCE’s biogeochemical cycling (Grimm et al. 2008; Kaushal and Belt
2012). The following two points are particularly relevant to air–seawater CO 2 gas
exchanges and wastewater treatment. First, in the most common wastewater treatment method (i.e., the activated sludge method), carbon is removed more efficiently
than nitrogen and phosphorus from wastewater (e.g., Sedlak 1991). Hence, the
treated water has relatively less carbon than nitrogen and phosphorous. When such
treated water flows into a SCE, primary production is promoted due to the abundant
nutrients, while decomposition and mineralization are suppressed by less abundant
organic carbon. This means that wastewater treatment suppresses the rise in CO 2
concentration in the water column of a SCE.
The second important point is that organic matter in the treated water is refractory (Kubo et al. 2015), because labile organic matter has already been decomposed
and removed during wastewater treatment. Therefore, further decomposition and
mineralization of the organic matter contained in the treated water is slow, resulting
in suppression of the rise in CO 2 concentration.
Through these two mechanisms, CO 2 concentration in seawater is lowered and
uptake of CO 2 from the atmosphere is promoted. That is, both nutrient loads derived
11 CO 2 Uptake in the Shallow Coastal Ecosystems Affected…
11.3 Hypothesis that Human-Impacted SCEs Act As a Net
CO 2 Sink
Strong human impacts can result in changes to the carbon cycle (McIntyre et al.
2000). In particular, nutrient load, wastewater treatment, and freshwater use will
increase with increasing human and livestock populations and farmland area. As a
result of human impacts, the cycling of green carbon and blue carbon related to
climate change mitigation, such as CO 2 exchange between the atmosphere and
water and carbon storage in SCEs, is affected (Table 11.1).
Kuwae et al. (2016) hypothesized that some characteristics of SCEs subject to
human impacts actually strengthen the carbon cycling structure that supports the net
uptake of atmospheric CO 2 (net CO 2 sink; Fig. 11.5). This idea is likely to be controversial, because urban coastal waters are seen as places where eutrophication
progresses and a large amount of CO 2 is emitted by decomposition of organic matter. In this section, we explain why a human-impacted SCE functions as a sink of
atmospheric CO 2 from a mechanistic perspective and provide empirical evidence
from previous studies.
11.3.1 Wastewater Treatment
Urban and agricultural nutrient loading and wastewater treatment have a major
influence on a SCE’s biogeochemical cycling (Grimm et al. 2008; Kaushal and Belt
2012). The following two points are particularly relevant to air–seawater CO 2 gas
exchanges and wastewater treatment. First, in the most common wastewater treatment method (i.e., the activated sludge method), carbon is removed more efficiently
than nitrogen and phosphorus from wastewater (e.g., Sedlak 1991). Hence, the
treated water has relatively less carbon than nitrogen and phosphorous. When such
treated water flows into a SCE, primary production is promoted due to the abundant
nutrients, while decomposition and mineralization are suppressed by less abundant
organic carbon. This means that wastewater treatment suppresses the rise in CO 2
concentration in the water column of a SCE.
The second important point is that organic matter in the treated water is refractory (Kubo et al. 2015), because labile organic matter has already been decomposed
and removed during wastewater treatment. Therefore, further decomposition and
mineralization of the organic matter contained in the treated water is slow, resulting
in suppression of the rise in CO 2 concentration.
Through these two mechanisms, CO 2 concentration in seawater is lowered and
uptake of CO 2 from the atmosphere is promoted. That is, both nutrient loads derived
11 CO 2 Uptake in the Shallow Coastal Ecosystems Affected…