As long as all required nutrients and energy are available to the organism in fixed relative amounts, it can buffer temporal variations in abundance using a single reserve. This situation is approximated in organisms that eat other organisms, as discussed in the previous chapters. If energy and various nutrients are taken up independently, however, a reserve is required for each of them to buffer variations in abundance. The surface layers of seas are poor in nutrients and rich in light, while the reverse holds for the bottom of the photozone. Algal cells, which commute between these two environments on the wind-induced currents, can barely grow and survive, unless they use intracellular energy and nutrient reserves.

The purpose of this chapter is to show how the univariate deb models can be extended to include several substrates, reserves and structural masses, in a way that reduces to the one-reserve, one-structure case if just one nutrient (or light) is limiting, or if nutrient abundances covary, and the reserve turnover times are identical. The concept of the Synthesising Unit, see {101}, will be used to show that a nutrient becomes almost non-limiting as soon as its availability exceeds that of the limiting nutrient, only by a small amount, relative to its needs. Simultaneous limitations of growth by nutrients and light only occur incidentally, and usually during a short period. This is why the simple one-reserve deb theory can be applied so widely.