Phytoplankton, light, and nutrients in a gradient of mixing depths: Theory

The depth of the well-mixed surface layer of lakes and oceans fundamentally affects phytoplankton populations. Specific nutrient supply, specific algal production, and specific sinking losses all decrease with increasing mixing depth. I use a dynamical model to investigate how phytoplankton biomass, light availability, and the distribution Of nutrients among various pools vary along a mixing depth gradient, and how the relationships of these variables to mixing depth depend on algal sinking velocity, abiotic light absorbents, nutrient enrichment, and the mode of nutrient supply (closed system with recycling vs. open system with external input). If phytoplankton is dominated by sinking algae, the primary causes of biomass limitation shift with increasing mixing depth from sinking loss limitation to nutrient limitation to light limitation. Consequently, algal biomass in the mixed layer (expressed per volume or area) and sedimented nutrients are unimodally related to mixing depth, whereas dissolved inorganic and total water column nutrients show the inverse pattern. Compared to closed systems, the maximum in the biomass concentration-mixing depth relationship occurs at much shallower depths in open systems without recycling of sedimented nutrients (such as mixed surface layers on top of stratified water columns). With increased algal sinking velocity, algal biomass decreases, and light penetration and dissolved nutrients both increase, whereas sedimented and total water column nutrients may increase or decrease. Increased abiotic turbidity reduces light penetration, algal biomass, and sedimented nutrients but increases dissolved and total water column nutrients. Finally, with nutrient enrichment, algal biomass and all nutrient compartments increase, whereas light penetration decreases. I use a graphical isocline approach to show that increasing external light supply, decreasing abiotic turbidity, and decreasing mixing depth represent three conceptually different forms of enrichment with light. Of those, decreasing abiotic turbidity is conceptually similar to enrichment with a mineral nutrient.