Subsurface maxima of phytoplankton and chlorophyll: Steady-state solutions from a simple model

In oligotrophic lakes and oceans, the deep chlorophyll maximum may form independently of a maximum of phytoplankton biomass, because the ratio of chlorophyll to phytoplankton biomass (in units of carbon) increases with acclimation to reduced light and increased nutrient supply at depth. Optical data (beam attenuation as proxy for phytoplankton biomass and chlorophyll fluorescence and absorption as proxies for chlorophyll concentration) and conventional measurements of biovolume, particulate organic carbon, and chlorophyll from two oligotrophic systems (Crater Lake, Oregon, and Sta. ALOHA in the subtropical North Pacific Ocean) are presented and show a vertical separation of the maxima of biomass and chlorophyll by 50–80 m during stratified conditions. We use a simple mathematical framework to describe the vertical structure of phytoplankton biomass, nutrients, and chlorophyll and to explore what processes contribute to the generation of vertical maxima. Consistent with the observations, the model suggests that biomass and chlorophyll maxima in stable environments are generated by fundamentally different mechanisms. Maxima in phytoplankton biomass occur where the growth rate is balanced by losses (respiration and grazing) and the divergence in sinking velocity, whereas the vertical distribution of chlorophyll is strongly determined by photoacclimation. A deep chlorophyll maximum is predicted well below the particle maximum by the model. As an interpretation of these results, we suggest a quantitative criterion for the observed coexistence of vertically distinct phytoplankton assemblages in oligotrophic systems: the vertical position at which a species occurs in highest abundance in the water column is determined by the ‘‘general compensation depth’’— that is, the depth at which specific growth and all loss rates, including the divergence of sinking/swimming and vertical mixing, balance. This prediction can be tested in the environment when the divergence of sinking and swimming is negligible. Subsurface maxima of phytoplankton biomass and/or chlorophyll often occur in clear waters in the ocean and in lakes—for example, the deep chlorophyll maximum at the base of the euphotic zone is a ubiquitous feature (Riley et al. 1949; Steele and Yentsch 1960; Venrick et al. 1973; Cullen and Eppley 1981; Cullen 1982). A basic understanding of the generating mechanisms for these vertical maxima is important for the assessment of the functioning of the pelagic food web and for the application of remote sensing techniques to infer vertically integrated values of biomass and primary productivity. Current textbooks in biological oceanography (e.g., Jumars 1993; Mann and Lazier 1996) do not provide a consistent explanation for the existence of subsurface pigment and/or phytoplankton biomass maxima. In the present article, we provide a mathematical framework that is consistent with observations and revisit explanations 1 Corresponding author ([email protected]).