Marine groundwater discharge of N by coastal type for the Arctic, Atlantic and Indian Oceans, Mediterranean Sea and Black Sea, and the Pacific Ocean for the year 2000, (a) in teragram of nitrogen per year; (b) in kilogram of nitrogen per kilometer of shoreline per year

Figure 4. Marine groundwater discharge of N by coastal type for the Arctic, Atlantic and Indian Oceans, Mediterranean Sea and Black Sea, and the Pacific Ocean for the year 2000, (a) in teragram of nitrogen per year; (b) in kilogram of nitrogen per kilometer of shoreline per year.

Abstract

The role of submarine groundwater discharge (SGD), the leakage of groundwater from aquifers into coastal waters, in coastal eutrophication has been demonstrated mostly for the North American and European coastlines, but poorly quantified in other regions. Here, we present the first spatially explicit global estimates of N inputs via SGD to coastal waters and show that it has increased from about 1.0 to 1.4 Tg of nitrate (NO₃-N) per year over the second half of the 20th century. Since this increase is not accompanied by an equivalent increase of groundwater phosphorus (P) and silicon (Si), SGD transport of nitrate is an important factor for the development of harmful algal blooms in coastal waters. Groundwater fluxes of N are linked to areas with high runoff and intensive anthropogenic activity on land, with Southeast Asia, parts of North and Central America, and Europe being hot spots.