Figure 2: Comparison of observations and model output of inorganic phosphate uptake during ³³P-phosphate pulse and ³²P-phosphate chase experiments.

Observations (black circles) and model output (blue lines) of traced inorganic phosphate (³³P_i and ³²P_i) in cells during six pulse–chase experiments (top row of plots) at two contrasting sites: least productive waters of the gyre centre and more productive tropical waters. Seawater samples with bioassayed ambient ³¹P_i were pulsed with 0.8 or 2.4 nmol l^{−1 31}P_i labelled with 0.05 nmol l^{−1 33}P_i and chased after 1 h incubation with 80 or 800 nmol l⁻¹ and 240 or 2,400 nmol l⁻¹ of ³¹P_i labelled with 0.01 nmol l^{−1 32}P_i. Note that observations and model output values are normalized to the maximum uptake measure (counts per minute per ml) in a time course in each experiment to lie between 0 and 100%. dil, dilution.