The ICEP is the Indicator for Coastal Eutrophication Potential. This indicator was developed by Billen and Garnier (2007); it reflects the production of non-siliceous phytoplankton (e.g., potentially harmful algae), which are sustained in coastal waters by the N and P discharged by rivers, while accounting for the nutrient requirements for growth (represented by the Redfield ratios). Changes in the nutrient ratios may alter the phytoplankton population, for example, shifting from siliceous phytoplankton to non-
33 siliceous (harmful) phytoplankton. Specifically, excess N and P in the waters relative to Si stimulate the growth of harmful algae that generally occurs with eutrophication. The ICEP is estimated based on the Redfield ratio of C:N:P:Si=106:16:1:20. Either N-ICEP or P-ICEP (kg C-eq. km-2 day-1) is estimated depending upon which nutrient is limiting.
We followed the ICEP approach by Garnier et al. (2010), as implemented in Global NEWS-2 (Strokal & Kroeze 2013):
N-ICEP = [TNflx (1 ∙ 1 ) – DSiflx ( ∙ )] ∙ 1 ∙ 1 N:P<16 (N is limiting) (2.2)
P-ICEP = [TPflx / 31 – DSiflx ( ∙ )] ∙ 1 ∙ 1 N:P>16 (P is limiting) (2.3)
TNflx, TNflx and DSiflx are the fluxes of total N (sum of DIN, DON, and PN), total P (sum of
DIP, DOP, and PP) and dissolved Si, respectively (kg km-2 year-1). Positive ICEP values
indicate rivers with the “potential” for coastal eutrophication because they export N and P to coastal waters in excess relative to Si. Negative values indicate a low risk for coastal eutrophication, but they should not be interpreted as zero risk for harmful algal blooms because ICEP reflects the average annual basin values. This indicator does not account for sub-basin and seasonal variations.
Another indicator for potential coastal eutrophication is silica deficiency. A silica deficiency in combination with increased amounts of N and P is a favorable condition for the growth of harmful algae (Billen & Garnier 2007; Garnier et al. 2010). Therefore, we analyzed the ratios between N, P and Si following Turner et al. (2003). We estimated the ratios of the total N to total P (TN:TP), the dissolved Si to total N (DSi:TN) and the dissolved Si to total P (DSi:TP) for the 16 selected rivers using their fluxes at the river mouth (kg km-2 year-1) and their atomic weights. The TN fluxes were calculated as the
sum of the DIN, DON and PN. The TP fluxes are the sum of the DIP, DOP and PP. The nutrient fluxes were taken from the Global NEWS-2 model (Section 2.2.2). We compared the TN:TP ratios with the DSi:TN and DSi:TP ratios (see Figure 2.5 in Section 2.3.3). P- limited rivers are identified when their TN:TP ratios exceed a Redfield value of 16 (N:P = 16:1, vertical lines in Figure 2.5), while N-limited rivers are identified when these ratios are below 16. A silica deficiency is then identified based on the TN and TP fluxes. Rivers with a DSi deficiency relative to the TN fluxes are calculated when their DSi:TN ratios are below the Redfield value 1.25 (SI:N = 20:16, horizontal lines in Figure 2.5). A Si deficiency relative to the TP fluxes is calculated when the DSi:TP ratios are below a Redfield value of 20 (Si:P = 20:1, horizontal lines in Figure 2.5).
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We calculated the three indicators for the dissolved inorganic N and P because most of the total N and P entering the Chinese seas are in dissolved inorganic forms (see also Figure 2.2) (Qu & Kroeze 2010). Another important reason is the scarcity of the data for the other forms of nutrients. We compared the measured and modeled total annual DIN and DIP exports (yields, kg km-2 year-1) by the Yellow, Liao (covering approximately
80% of the Bohai Gulf drainage basin), Yangtze, Fuchunjiang (covering approximately 80% of the Yellow Sea drainage basin) and Pearl (covering 85% of the South China Sea drainage basin). The measured annual yields of DIN for Yangtze and Zhujiang, and of DIP for Yangtze, Yellow, Zhujiang, Liao, and Fuchunjiang were provided by Mayorga et al. (2010). The measured annual DIN yields for the Yellow were taken from Dumont et al. (2005); the values for the Fuchunjiang and Liao were obtained from the GEMS/GLORI database (Meybeck & Ragu 1995) in terms of the nitrate and ammonium concentrations (N-NO3; N-NH4 in mg L-1). The modeled DIN and DIP yields for the selected river basins
were derived from the Global NEWS-2 model and refer to the year 2000 (Mayorga et al. 2010).
The Global NEWS-2 model for DIN and DIP performed well based on the three indicators. We calculated an RP2 of 0.96, indicating that 96% of the variance in the measured DIN
and DIP yields is explained by the Global NEWS-2 model for the selected rivers. Our RNSE2 value lies within 0-1 (0.42), indicating an acceptable performance by the Global
NEWS-2 model. The calculated ME for the DIN and DIP export by the rivers is 18%, indicating good performance.
These three indicators have been widely accepted to evaluate model performance, and they have different weaknesses and strengths. For instance, RNSE2 is sensitive to
extremely high values due to the squared differences (Moriasi et al. 2007). A drawback of RP2 th t n th p n nt (Krause et al. 2005), rendering it
insensitive toward the additive and proportional differences between the measured and modeled values (Legates & McCabe 1999). Our combination of these three indicators builds trust in the model performance for the DIN and DIP. We argue that these results, when combined with the previous validations for the other N and P forms, support the performance of the Global NEWS-2 model for Chinese rivers.
2.2.3 ICEP: An Indicator for Coastal Eutrophication Potential
The ICEP is the Indicator for Coastal Eutrophication Potential. This indicator was developed by Billen and Garnier (2007); it reflects the production of non-siliceous phytoplankton (e.g., potentially harmful algae), which are sustained in coastal waters by the N and P discharged by rivers, while accounting for the nutrient requirements for growth (represented by the Redfield ratios). Changes in the nutrient ratios may alter the phytoplankton population, for example, shifting from siliceous phytoplankton to non-
33 siliceous (harmful) phytoplankton. Specifically, excess N and P in the waters relative to Si stimulate the growth of harmful algae that generally occurs with eutrophication. The ICEP is estimated based on the Redfield ratio of C:N:P:Si=106:16:1:20. Either N-ICEP or P-ICEP (kg C-eq. km-2 day-1) is estimated depending upon which nutrient is limiting.
We followed the ICEP approach by Garnier et al. (2010), as implemented in Global NEWS-2 (Strokal & Kroeze 2013):
N-ICEP = [TNflx (1 ∙ 1 ) – DSiflx ( ∙ )] ∙ 1 ∙ 1 N:P<16 (N is limiting) (2.2)
P-ICEP = [TPflx / 31 – DSiflx ( ∙ )] ∙ 1 ∙ 1 N:P>16 (P is limiting) (2.3)
TNflx, TNflx and DSiflx are the fluxes of total N (sum of DIN, DON, and PN), total P (sum of
DIP, DOP, and PP) and dissolved Si, respectively (kg km-2 year-1). Positive ICEP values
indicate rivers with the “potential” for coastal eutrophication because they export N and P to coastal waters in excess relative to Si. Negative values indicate a low risk for coastal eutrophication, but they should not be interpreted as zero risk for harmful algal blooms because ICEP reflects the average annual basin values. This indicator does not account for sub-basin and seasonal variations.
Another indicator for potential coastal eutrophication is silica deficiency. A silica deficiency in combination with increased amounts of N and P is a favorable condition for the growth of harmful algae (Billen & Garnier 2007; Garnier et al. 2010). Therefore, we analyzed the ratios between N, P and Si following Turner et al. (2003). We estimated the ratios of the total N to total P (TN:TP), the dissolved Si to total N (DSi:TN) and the dissolved Si to total P (DSi:TP) for the 16 selected rivers using their fluxes at the river mouth (kg km-2 year-1) and their atomic weights. The TN fluxes were calculated as the
sum of the DIN, DON and PN. The TP fluxes are the sum of the DIP, DOP and PP. The nutrient fluxes were taken from the Global NEWS-2 model (Section 2.2.2). We compared the TN:TP ratios with the DSi:TN and DSi:TP ratios (see Figure 2.5 in Section 2.3.3). P- limited rivers are identified when their TN:TP ratios exceed a Redfield value of 16 (N:P = 16:1, vertical lines in Figure 2.5), while N-limited rivers are identified when these ratios are below 16. A silica deficiency is then identified based on the TN and TP fluxes. Rivers with a DSi deficiency relative to the TN fluxes are calculated when their DSi:TN ratios are below the Redfield value 1.25 (SI:N = 20:16, horizontal lines in Figure 2.5). A Si deficiency relative to the TP fluxes is calculated when the DSi:TP ratios are below a Redfield value of 20 (Si:P = 20:1, horizontal lines in Figure 2.5).
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