Steady-state Solutions for Subsurface Chlorophyll Maximum in Stratified Water Columns with a Bell-shape Vertical Profile of Chlorophyll

Abstract:

A bell-shape vertical profile of chlorophyll a (Chl a) concentration, conventionally referred as Subsurface Chlorophyll Maximum (SCM) phenomenon, has frequently been observed in stratified oceans and lakes. This profile is assumed to be a general Gaussian distribution in this study. By substituting the general Gaussian function into ecosystem dynamical equations, the steady-state solutions for SCM characteristics (i.e. SCM layer depth, thickness, and intensity) in various scenarios are derived. These solutions indicate that: 1) The maximum in Chl a concentrations occurs at or below the depth with the maximum in growth rates of phytoplankton locating at the transition from nutrient limitation to light limitation, and the depth of SCM layer deepens logarithmically with an increase in surface light intensity; 2) The shape of SCM layer (thickness and intensity) is mainly influenced by nutrient supply, but independent of surface light intensity; 3) The intensity of SCM layer is proportional to the diffusive flux of nutrient from below, getting stronger as a result of this layer being shrank by a higher light attenuation coefficient or a larger sinking velocity of phytoplankton. 4) The depth of SCM layer is approximately linear with the nutricine depth, which is inversely proportional to light attenuation coefficient in regions with a low subsurface turbulent diffusivity. The analytical solutions can be useful to estimate environmental parameters which are difficult to observe on site.