Analytical solution of the nitracline with the evolution of subsurface chlorophyll maximum in stratified water columns

Xiang Gong,Huiwang Gao,Linhui Wang,Wensheng Jiang,Emmanuel Boss,Jie Shi,Xiaohong Yao,Shuh-Ji Kao

doi:10.5194/bg-14-2371-2017

Xiang Gong, Huiwang Gao + Show 6 more

Open Access

https://doi.org/10.5194/bg-14-2371-2017

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Abstract

Abstract. In a stratified water column, the nitracline is a layer where the nitrate concentration increases below the nutrient-depleted upper layer, exhibiting a strong vertical gradient in the euphotic zone. The subsurface chlorophyll maximum layer (SCML) forms near the bottom of the euphotic zone, acting as a trap to diminish the upward nutrient supply. Depth and steepness of the nitracline are important measurable parameters related to the vertical transport of nitrate into the euphotic zone. The correlation between the SCML and the nitracline has been widely reported in the literature, but the analytic solution for the relationship between them is not well established. By incorporating a piecewise function for the approximate Gaussian vertical profile of chlorophyll, we derive analytical solutions of a specified nutrient–phytoplankton model. The model is well suited to explain basic dependencies between a nitracline and an SCML. The analytical solution shows that the nitracline depth is deeper than the depth of the SCML, shoaling with an increase in the light attenuation coefficient and with a decrease in surface light intensity. The inverse proportional relationship between the light level at the nitracline depth and the maximum rate of new primary production is derived. Analytic solutions also show that a thinner SCML corresponds to a steeper nitracline. The nitracline steepness is positively related to the light attenuation coefficient but independent of surface light intensity. The derived equations of the nitracline in relation to the SCML provide further insight into the important role of the nitracline in marine pelagic ecosystems.

Highlights

Nitrogen availability, especially the nitrate upward supply to the euphotic zone where light intensity is sufficient to support net photosynthesis, limits the primary productivity in a stratified water column (Falkowski et al, 1998)
Neglecting terms including self-shading of phytoplankton (Kc) in Eqs. (22–24), both the analytical solutions of the depth and thickness of the subsurface chlorophyll maximum (SCM) layer (SCML) are the same as the results presented in Gong et al (2015)
We have presented a theoretical framework to investigate the interaction of phytoplankton and nutrient in a stratified water column

Summary

Introduction

Especially the nitrate upward supply to the euphotic zone where light intensity is sufficient to support net photosynthesis, limits the primary productivity in a stratified water column (Falkowski et al, 1998). The nitrate supply from below and the light attenuated from above with the depth collaboratively affect the growth of phytoplankton and lead to the subsurface chlorophyll maximum (SCM) (Riley et al, 1949; Steele and Yentsch, 1960; Herbland and Voituriez, 1979; Cullen, 1982). The SCM layer (SCML) has attracted much attention since Riley et al (1949) because the layer contributes significantly to new primary production (NPP) in stratified waters (Probyn et al, 1995; Ross and Sharples, 2007; Fernand et al, 2013). Gong et al.: Analytical solution of the nitracline supply of nitrate to the euphotic zone and affecting NPP (Lewis et al, 1986; Bahamón et al, 2003; Aksnes et al, 2007; Cermeno et al, 2008; Omand and Mahadevan, 2015)

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