The effect of food on size at a given stage is hyperbolic in form (Fig. 7.15),
becoming asymptotic at about 4 to 6 ppm of phytoplankton by volume, equivalent to
160 μg C liter−1. Clearly, growth does have a required food level, a nutritional
threshold. This minimum doesn’t change much with temperature but is surprisingly
low, perhaps 50 μg C liter−1. With full nutrition (above about 160 μg C liter−1), there
is a very strong effect of temperature on growth rate: it is faster in warmer water. The
effect of temperature on size at a given stage is progressively less for older stages, but
colder water mostly produces somewhat larger adult sizes. That is commonly but not
always observed for plankton in the field. Higher temperatures allow a considerably
faster start, especially for the change from 8° to 12°C. However, the effect of higher
temperature on progress through the stages (development rate) is even greater, so that
final adult sizes are smaller at warmer temperatures.
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