Characteristics of subsurface chlorophyll maxima during the boreal summer in the South China Sea with respect to environmental properties

Abstract

Characterizing the vertical distribution of chlorophyll a in the water column in each oceanic region is crucial for accurate assessment of depth-integrated phytoplankton biomass. In this study, the characteristics of the subsurface chlorophyll maximum (SCM) in the South China Sea (SCS) during the boreal summer were investigated by using bio-optical and hydrological data collected during four cruises from 2008 to 2015. During the boreal summer, a well-developed SCM layer was found to be a prominent feature in the SCS, with the thickness, depth, and magnitude of the SCM exhibiting large spatial variability. The vertical position of the SCM varied between 11 and 99 m, with an average value of 53 m. Light attenuation played a fundamental role in determining the depth (ZSCM) and magnitude of the SCM (Chlmax), as reflected by their relationships with the euphotic zone depth (Zeu). However, because significant positive correlations were found between ZSCM and the depth of potential density at 23 kg m−3 (Zσ=23), physical processes were inferred to be more important in modulating the fluctuation of ZSCM, especially in open-ocean areas. Anticyclonic eddies, which act to deepen the nutricline by means of isopyncnal displacement, may play a role in aggravating nutrient limitation in the SCM layer, which leads to deepening of ZSCM, weakening of Chlmax, and a fall in the total integrated chlorophyll a within the euphotic layer (ChlintZeu). Knowledge of ocean physical conditions and the variability of ZSCM should be taken into consideration to improve the accuracy of ChlintZeu estimates based on the surface chlorophyll a concentration (Chlsurf), especially when Chlsurf is very low. Our assessment of SCM parameters provides the basis for a better understanding and quantification of their role in primary production estimation within the SCS.