Regulation of phytoplankton carbon to chlorophyll ratio by light, nutrients and temperature in the equatorial Pacific Ocean
Xiujun
Wendy
Wang, University of Maryland, College Park, MD 20742 USA, wwang@essic.umd.edu
(Presenting)
Raghu
Murtugudde, University of Maryland, College Park, MD 20742 USA, ragu@essic.umd.edu
Michael
Behrenfeld, Oregon State University, Corvallis, OR 97331-2902, behrenfm@science.oregonstate.edu
Chlorophyll, as a phytoplankton biomass index, has been used to estimate oceanic primary productivity. However, carbon to chlorophyll ratio in phytoplankton cells varies over a large range in response to changes in light, nutrients, and temperature in the euphotic zone. Field measurements along 180° show that the phytoplankton C:Chl ratio varies greatly near the sea surface, but is relatively constant near the bottom of the euphotic zone. In the surface waters, there is a linear relationship between the C:Chl ratio and the growth rate. Below the surface, the phytoplankton C:Chl ratio linearly decreases with depth. Here, we employ a fully coupled 3-dimensional physical-biogeochemical model to address the variable C:Chl ratio in the equatorial Pacific. Our approach includes the use of in situ data for derivation and parameterization of the phytoplankton C:Chl ratio, and for model calibration and validation.
The basin scale model reproduces many observed features in chlorophyll dynamics, including shoaling deep chlorophyll maximum (DCM) from the western warm pool to the eastern equatorial Pacific, and a decreasing C:Chl ratio with depth. Model simulations show large spatial and temporal variations of the C:Chl ratio, in relation to changes of light, nutrients, and temperature. Particularly, the model produces strong interannual variability of the C:Chl ratio that is associated with the El Niño/ Southern Oscillation (ENSO) events. The warm pool has strong anomalies during the cold phase of the ENSO with reduced C:Chl ratio and a shallower DCM. However, the central and eastern equatorial Pacific reveal strong anomalies during the warm phase of the ENSO, which include an increased (decreased) C:Chl ratio in the upper (lower) euphotic zone and a deepened DCM. This can induce warm to cold ENSO asymmetries in biogeochemical responses which are not considered in most previous studies.
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