Scaling the seas
Primary producers in aquatic environments cover a range of living biomasses from 10 -13 to 10 4 g dw. Benthic plants at the high end of this range contribute ≤ 5 Gt C year -1 to global primary productivity. Plankton at the lower end (up to 10 -4 g dw) contributes about 30 Gt C year -1 . While many problems of interpretation remain, in general terms the size of the organisms which dominate particular habitats can be related to the physics of water movement and its interaction with the availability of light and nutrients, the generation time of the organism, and the attentions of grazers. A second scaling problem is that of methods of studying the global energy flow and nutrient cycling roles of aquatic primary producers. Problems with scaling up from small-scale and mesoscale to regional or global scale, and the prospects of more direct estimates of large-scale productivity, are discussed.