Dead, zero-oxygen areas in the sea are reported to have quadrupled in size since 1950, with the number of very low oxygen sites near coasts multiplied tenfold. For example, the coastal waters of the Baltic Sea have been starved of oxygen to a level unseen in at least 1,500 years, largely due to waterborne nutrient loads from land. A recent review in WIREs Water addresses the role of subsurface legacy sources for such nutrient loads, with concrete exemplification of catchment cases in Sweden, from where runoff largely discharges into the Baltic.
Subsurface legacy sources may accumulate in soil, groundwater and sediment zones due to continuous partial retention there of nutrient (and possible other pollutant) inputs from sources at the land surface (see Figure). These diffuse subsurface legacies can then release and contribute nutrients (and other pollutants) from earlier land-surface inputs to the nutrient (pollutant) loads currently carried by streams to downstream inland and coastal waters, in addition to the load contributions from current land-surface sources. This study develops and exemplifies the application of a diagnostic test to distinguish and detect dominant load contributions from such subsurface legacy sources, which are otherwise inaccessible for direct observation.
Dominant discharge contributions from subsurface legacy sources are in practice untreatable within the commonly short time frames given for compliance with environmental regulation.
The authors therefore argue that, for considerable water quality improvements to be achieved within such short time frames, mitigation measures need to be spatially differentiated and directed to (sub)catchments without major legacy sources. The diagnostic test presented in the study can be used to distinguish between (sub)catchments with and without dominant load contributions from such sources.
 The Guardian, 4 January 2018, https://www.theguardian.com/environment/2018/jan/04/oceans-suffocating-dead-zones-oxygen-starved
 The Guardian, 5 July 2018, https://www.theguardian.com/environment/2018/jul/05/baltic-sea-oxygen-levels-at-1500-year-low-due-to-human-activity?
Kindly contributed by Georgia Destouni and Jerker Jarsjö, Department of Physical Geography, Stockholm University, Sweden