News » Increases in phosphorus losses to and retention in freshwaters

Beusen et al. (2016) calculate changes in global phosphorus (N) and nitrogen (N) losses to freshwaters (rivers and streams), retention in sediments and final transfer to oceans. This is based on the IMAGE-GNM model which combines information on land use with the global hydrological model PCR-GLOBWB. They conclude that over the twentieth century worldwide nutrient losses to freshwaters increased from 34 to 64 million tonnes N per year (mtN/y) and from 5 to 9 million tonnes of P (mtP/y).

Accumulation of N in groundwater worldwide is estimated at 376 mtN (total over 100 years). Agriculture?s contribution to losses to freshwater over the century rose from 19% to 51% for N, and 35% to 56% for P. At the same time, nutrient retention in freshwaters increased considerably, from 14 to 27 mtN/y and from 2.6 to 5 mtP/y. 54% of this N retention occurred in rivers, and 63% of P retention, the remainder in lakes and reservoirs. The authors consider that the development of dams and reservoirs today accounts for a significant part of this nutrient retention (24% and 22% of global N and P retention in freshwater systems).

The increased retention did not balance the increases in inputs, so that final discharges to oceans also increased, from 19 to 37 mtN/y for N and from 2 to 4 mtP/y for P, leading to coastal eutrophication. Human activities have also led to a general increase in the N:P molar ratio in freshwaters, and an increase in the N:P ratio in rivers draining into most oceans (Pacific, Indian, Mediterranean and Black Sea, but not the Atlantic).

For more information visit: ?Global riverine N and P transport to ocean increased during the 20th century despite increased retention along the aquatic continuum?, A. Beusen et al., Biogeosciences, 13, 2441?2451, 2016 https://doi.org/10.5194/bg-13-2441-2016