Figure 20.11 Increases in net primary productivity when nutrients are added to aquatic ecosystems. Using data from 928 experiments conducted around the world, researchers determined how NPP responds to different nutrient additions. NPP was examined as a ratio between productivity when nutrients were added and a control treatment in which nutrients were not added. In (a) freshwater and (b) marine ecosystems, adding nitrogen or phosphorus caused an increase in NPP, although nitrogen has a larger effect than phosphorus in the soft-bottom-ocean ecosystems and the ocean-water ecosystems. Error bars are standard errors.

Data from J. J. Elser et al., Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine, and terrestrial ecosystems, Ecology Letters 10 (2007): 1135–1142.