Oceanic carbon uptake depends on the productivity of the marine ecosystem. Here we study the dependence of primary productivity on the spatial and temporal variability of the nutrient flux and the functional form used to parameterize it. We show that primary productivity is significantly affected by the form of the nutrient input. For a restoring flux, used to parameterize nutrient input by upwelling, primary productivity depends on the size and/or temporal duration of the upwelling events. For a fixed-flux nutrient input, we show that high-nutrient, low-chlorophyll (HNLC) regions can easily appear, without necessarily implying the lack of some micronutrient. These results have interesting implications on the interpretation of primary productivity estimates from observational data and ocean circulation models, and indicate a way to obtain upper and lower bounds to primary productivity in coarse-resolution models.