Modeling the long-term variability of phytoplankton functional groups and primary productivity in the South China Sea

Abstract

Primary productivity (PP) and phytoplankton structure play an important role in regulating oceanic carbon cycle. The unique seasonal circulation and upwelling pattern of the South China Sea (SCS) provide an ideal natural laboratory to study the response of nutrients and phytoplankton dynamics to climate variation. In this study, we used a three-dimensional (3D) physical–biogeochemical coupled model to simulate nutrients, phytoplankton biomass, PP, and functional groups in the SCS from 1958 to 2009. The modeled results showed that the annual mean carbon composition of small phytoplankton, diatoms, and coccolithophores was 33.7, 52.7, and 13.6 %, respectively. Diatoms showed a higher seasonal variability than small phytoplankton and coccolithophores. Diatoms were abundant during winter in most areas of the SCS except for the offshore of southeastern Vietnam, where diatom blooms occurred in both summer and winter. Higher values of small phytoplankton and coccolithophores occurred mostly in summer. Our modeled results indicated that the seasonal variability of PP was driven by the East Asian Monsoon. The northeast winter monsoon results in more nutrients in the offshore area of the northwestern Luzon Island and the Sunda Shelf, while the southwest summer monsoon drives coastal upwelling to bring sufficient nutrients to the offshore area of southeastern Vietnam. The modeled PP was correlated with El Niño/Southern Oscillation (ENSO) at the interannual scale. The positive phase of ENSO (El Niño conditions) corresponded to lower PP and the negative phase of ENSO (La Niña conditions) corresponded to higher PP.

Appendix

The model equations of each compartment can be expressed as:

$$ \frac{{\partial C_{i} }}{\partial t} = {\text{Physics}}(C_{i} ) + {\text{Biology}}(C_{i} ), $$

(A1)

where C _i (i = 1, 2, …, 13) denotes each of the 13 compartments listed in Sect. 2. The first term on the right hand side represents physical processes which include vertical/horizontal advection and eddy diffusion. The second term on the right hand side represents biological processes. Fujii et al. (2007) gave detailed equations for small phytoplankton and diatoms. Here, we only show equations of the nitrogen (A2), carbon (A3) and chlorophyll (A4) concentration that represent dynamics of coccolithophores. The short description and corresponding value of parameters are listed in Table 2.

$$ {\text{Biology}}\left( {{\text{S3}}\left( z \right)} \right) \left[ {{\text{mmol N m}}^{ - 3} {\text{day}}^{ - 1} } \right] = {\text{NP3}}\left( z \right) + {\text{RP3}}(z) - {\text{GS3}}(z) - \gamma_{10} \times {\text{S3}}(z) - \frac{\partial }{\partial z}\left( {{\text{W3}} \times {\text{S3}}(z)} \right), $$

(A2)

$$ \begin{gathered} {\text{Biology}}\left( {{\text{C3}}\left( z \right)} \right) \left[ {{\text{mmol C m}}^{ - 3} {\text{day}}^{ - 1} } \right] = (P^{\text{C3}} \left( z \right) - \lambda_{\text{S3}} \times \hbox{max} \left( {\frac{{{\text{NP3}}\left( z \right)}}{{{\text{NP3}}\left( z \right) + {\text{RP3}}\left( z \right)}}, 0.5} \right) \times ({\text{NP3}}(z) + {\text{RP3}}\left( z \right))/(1 - {\text{ES3}})) \\ \; \times (1 - {\text{ES3}}) - {\text{GC3}}(z) - \gamma_{10} \times {\text{C3}}(z) - \frac{\partial }{\partial z}\left( {{\text{W3}} \times {\text{C3}}(z)} \right), \\ \end{gathered} $$

(A3)

$$ {\text{Biology}}\left( {{\text{Chl3}}\left( z \right)} \right) \left[ {{\text{mg Chl m}}^{ - 3} {\text{day}}^{ - 1} } \right] = (P^{\text{Chl3}} \left( z \right) - {\text{GChl3}}(z) - \gamma_{10} \times {\text{Chl3}}(z) - \frac{\partial }{\partial z}\left( {{\text{W3}} \times {\text{Chl3}}(z)} \right), $$

(A4)

Table 2 The model parameters and values

where NP3 and RP3 represent new production and regenerated production, respectively. GS3, GC3, and GChl3 denote the grazing of P3 by Z2 in nitrogen, carbon, and chlorophyll unit, respectively. γ ₁₀ is the mortality coefficient of P3. The last term in equations A2–A4 represents vertical sinking of P3, and W3 is the sinking velocity. The detailed expressions of biological process are as follows.

1.1 New production

$$ \begin{gathered} {\text{NP3}}\left( z \right)\left[ {{\text{mmol}}\, {\text{N m}}^{ - 3} {\text{day}}^{ - 1} } \right] = V_{\text{N refS3}}^{\text{C}} \times e^{{ - \Uppsi {\text{NH}}_{4} }} \times \frac{{{\text{NO}}_{3} \left( z \right)}}{{K_{{{\text{NO}}_{3} }} + {\text{NO}}_{3} \left( z \right)}} \times T_{\text{func}} \left( z \right) \times \frac{{1 - f_{\text{nitp3}} \left( z \right)}}{{1.015 - f_{\text{nitp3}} \left( z \right)}} \\ \; \times {\text{C3}}\left( z \right) \times \left( {1 - {\text{ES3}}} \right) \times \left( {1 - e^{{ - \frac{{\alpha^{\text{Chl3}} \theta^{\text{C3}} (z){\text{PAR}}(z)}}{{P_{\text{ref}}^{\text{C3}} \times f_{\text{nitp3}} \left( z \right) \times T_{\text{func}} \left( z \right)}}}} } \right), \\ \end{gathered} $$

(A5)

where PAR is photosynthetically active radiation,

$$ V_{\text{N refS3}}^{\text{C}} = P_{\text{ref}}^{\text{C3}} \times Q_{\hbox{max} } , $$

(A6)

$$ T_{\text{func}} \left( z \right) = \left\{ {\begin{array}{*{20}c} {e^{{ - 4000(\frac{1}{278.15} - \frac{1}{303.15})}} , {\text{Temp}}\left( z \right) < 278.15} \\ {e^{{ - 4000(\frac{1}{298.15} - \frac{1}{303.15})}} , {\text{Temp}}\left( z \right) > 298.15} \\ {e^{{ - 4000(\frac{1}{{{\text{Temp}}(z)}} - \frac{1}{303.15})}} , 278.15 \le {\text{Temp}}\left( z \right) \le 298.15} \\ \end{array} } \right., $$

(A7)

$$ f_{\text{nitp3}} \left( z \right) = \frac{{\frac{{{\text{S3}}\left( z \right)}}{{{\text{C3}}\left( z \right)}} - Q_{\hbox{min} } }}{{Q_{\hbox{max} } - Q_{\hbox{min} } }}, $$

(A8)

$$ \theta^{\text{C3}} \left( z \right) = {\text{Chl3}}(z)/{\text{C3}}(z), $$

(A9)

1.2 Regenerated production

If $\frac{{{\text{S3}}\left( z \right)}}{{{\text{C3}}\left( z \right)}} \le Q_{\hbox{min} } ,$

$$ \begin{gathered} {\text{RP3}}\left( z \right)\left[ {{\text{mmol N m}}^{ - 3} {\text{day}}^{ - 1} } \right] = V_{\text{N refS3}}^{\text{C}} \times \frac{{{\text{NH}}_{4} \left( z \right)}}{{K_{{{\text{NH}}_{4} }} + {\text{NH}}_{4} \left( z \right)}} \times T_{\text{func}} \left( z \right) \\ \; \times \frac{{1 - f_{\text{nitp3}} \left( z \right)}}{{1.015 - f_{\text{nitp3}} \left( z \right)}} \times {\text{C3}}\left( z \right) \times \left( {1 - {\text{ES3}}} \right), \\ \end{gathered} $$

(A10)

If $\frac{{{\text{S3}}\left( z \right)}}{{{\text{C3}}\left( z \right)}} > Q_{\hbox{min} } ,$

$$ \begin{gathered} {\text{RP3}}\left( z \right)\left[ {{\text{mmol N m}}^{ - 3} {\text{day}}^{ - 1} } \right] = V_{\text{N refS3}}^{\text{C}} \times \frac{{{\text{NH}}_{4} \left( z \right)}}{{K_{{{\text{NH}}_{4} }} + {\text{NH}}_{4} \left( z \right)}} \times T_{\text{func}} \left( z \right) \times \frac{{1 - f_{\text{nitp3}} \left( z \right)}}{{1.015 - f_{\text{nitp3}} \left( z \right)}} \\ \; \times C3\left( z \right) \times \left( {1 - {\text{ES3}}} \right) \times \left( {1 - e^{{ - \frac{{\alpha^{\text{Chl3}} \theta^{\text{C3}} \left( z \right){\text{PAR}}\left( z \right)}}{{P_{\text{ref}}^{\text{C3}} \times f_{\text{nitp3}} \left( z \right) \times T_{\text{func}} \left( z \right)}}}} } \right) , \\ \end{gathered} $$

(A11)

1.3 Grazing by Z2

$$ {\text{GS3}}\left( z \right)\left[ {{\text{mmol N m}}^{ - 3} {\text{day}}^{ - 1} } \right] = \beta 2 \times \zeta_{10} \times {\text{S3}}\left( z \right) \times \frac{{{\text{Z2}}\left( z \right)}}{{K_{\text{Z2}} + \zeta_{8} + \zeta_{9} }} \times {\text{S3}}(z), $$

(A12)

$$ {\text{GC3}}\left( z \right)\left[ {{\text{mmol C m}}^{ - 3} {\text{day}}^{ - 1} } \right] = \beta 2 \times \zeta_{10} \times {\text{S3}}\left( z \right) \times \frac{{{\text{Z2}}\left( z \right)}}{{K_{\text{Z2}} + \zeta_{8} + \zeta_{9} }} \times {\text{C3}}\left( z \right), $$

(A13)

$$ {\text{GChl3}}\left( z \right)\left[ {{\text{mg Chl m}}^{ - 3} {\text{day}}^{ - 1} } \right] = \beta 2 \times \zeta_{10} \times {\text{S3}}\left( z \right) \times \frac{{{\text{Z2}}\left( z \right)}}{{K_{\text{Z2}} + \zeta_{8} + \zeta_{9} }} \times {\text{Chl3}}(z), $$

(A14)

where

$$ \zeta_{8} = \zeta_{5} \times {\text{S2}}\left( z \right) + \zeta_{6} \times {\text{Z2}}\left( z \right) + \zeta_{7} \times {\text{DN}}\left( z \right) + \zeta_{10} \times {\text{S3}}\left( z \right), $$

(A15)

$$ \zeta_{9} = \zeta_{5} \times {\text{S2}}^{2} \left( z \right) + \zeta_{6} \times {\text{Z2}}^{2} \left( z \right) + \zeta_{7} \times {\text{DN}}^{2} \left( z \right) + \zeta_{10} \times {\text{S3}}^{2} \left( z \right), $$

(A16)

1.4 Carbon uptake

$$ P^{\text{C3}} \left( z \right)\left[ {{\text{mmol C m}}^{ - 3} {\text{day}}^{ - 1} } \right] = P_{\text{ref}}^{\text{C3}} \times f_{\text{nitp3}} \left( z \right) \times T_{\text{func}} \left( z \right) \times \left( {1 - e^{{ - \frac{{\alpha^{\text{Chl3}} \theta^{\text{C3}} \left( z \right){\text{PAR}}\left( z \right)}}{{P_{\text{ref}}^{\text{C3}} \times f_{\text{nitp3}} \left( z \right) \times T_{\text{func}} \left( z \right)}}}} } \right) \times {\text{C3}}(z), $$

(A17)

1.5 Chlorophyll uptake

$$ P^{\text{Chl3}} \left( z \right)\left[ {{\text{mg Chl m}}^{ - 3} {\text{day}}^{ - 1} } \right] = \theta_{\text{Nmax}}^{\text{Chl3}} \times \frac{{P_{\text{ref}}^{\text{C3}} \times f_{\text{nitp3}} \left( z \right) \times T_{\text{func}} \left( z \right) \times \left( {1 - e^{{ - \frac{{\alpha^{\text{Chl3}} \theta^{\text{C3}} \left( z \right){\text{PAR}}\left( z \right)}}{{P_{\text{ref}}^{\text{C3}} \times f_{\text{nitp3}} \left( z \right) \times T_{\text{func}} \left( z \right)}}}} } \right)}}{{\alpha^{\text{Chl3}} \theta^{\text{C3}} \left( z \right){\text{PAR}}\left( z \right)}} \times ({\text{NP3}}\left( z \right) + {\text{RP3}}\left( z \right)). $$

(A18)