Comparison of Primary Production Using in situ and Satellite-Derived Values at the SEATS Station in the South China Sea

Satellite-based observations of primary production (PP) are broadly used to assess carbon fixation rate of phytoplankton in the global ocean with small spatiotemporal limitations. However, the remote sensing can only reach the ocean surface, the assumption of a PP vertically exponential decrease with increasing depth from the surface to the bottom of euphotic zone may cause a substantial and potential discrepancy between in situ measurements and satellite-based observations of PP. This study compared euphotic zone integrated PP derived from measurements based on ship-based in situ incubation (i.e., PPin situ) and those derived from the satellite-based vertically generalized production model (VGPM; PPVGPM) for the period 2003 similar to 2016 at the South East Asian Time-series Study (SEATS) station. PP values obtained during the NE-monsoon (NEM: Nov similar to Mar; PPin situ = 323 +/- 134; PPVGPM = 443 +/- 142 mg-C m(-2) d(-1)) were similar to 2-fold higher than those recorded during the SW-monsoon (SWM: Apr similar to Oct; PPin situ = 159 +/- 58; PPVGPM = 250 +/- 36 mg-C m(-2) d(-1)), regardless of the method used for derivation. The main reason for the higher PP values during the NEM appears to have been a greater abundance of inorganic nutrients were made available by vertical advection. Note that on average, PPin situ estimates were similar to 50% lower than PPVGPM estimates, regardless of the monsoon. These discrepancies can be mainly attributed to differences from the euphotic zone depth between satellite-based and in situ measurements. The significantly negative relationship between PP measurements obtained in situ and sea surface temperatures observed throughout this study demonstrates that both methods are effective indicators in estimating PP. Overall, our PPin situ analysis indicates that a warming climate is unfavorable for primary production in low-latitude open ocean ecosystems.

Automated summary

Satellite-based observations of primary production are widely used to assess phytoplankton carbon fixation rate in the global ocean. However, discrepancies between in situ measurements and satellite-based observations may occur due to differences in the assumption of primary productivity vertical distribution and the depth of the euphotic zone. This study compared PP values obtained from in situ measurements and satellite-based observations at the SEATS station, finding that PP values during the NE-monsoon were higher than those during the SW-monsoon, potentially due to vertical advection of inorganic nutrients. The analysis showed that in situ measurements were generally lower than satellite-based observations by about 50%, primarily due to differences in euphotic zone depth.