Inland waters under threat

Enhanced protection is needed for freshwater bodies in the United States — in particular impermanent streams and wetlands outside floodplains — according to an assessment of their value and vulnerability.

Many of the world's saline lakes have been shrinking due to consumptive water use. The Great Salt Lake, USA, provides an example for how the health of and ecosystem services provided by saline lakes can be sustained.

Lakes are sources of the greenhouse gas methane. A synthesis of measurements of methane emissions reveals that lakes and ponds above 50 °N emit 16.5 Tg methane annually, and emissions may increase by 20 to 50% with longer ice-free seasons.

Glaciers and polar ice sheets store and release a small but important pool of organic carbon. The changing climate is making glaciers an increasingly important driver of carbon dynamics in aquatic ecosystems.

A substantial amount of atmospheric carbon taken up on land is transported laterally from upland terrestrial ecosystems to the ocean. A synthesis of the available literature suggests that human activities have significantly increased soil carbon inputs to inland waters, but have only slightly affected carbon delivery to the open ocean.

Rivers may be efficient environments for metabolizing terrestrial organic carbon that was previously thought to be recalcitrant, owing to pockets that provide geophysical opportunities by retaining material for longer, and to the adaptation of microbial communities, which has enabled them to exploit the energy that escapes upstream ecosystems.

The production and consumption of organic carbon in inland waters varies with water residence time and biotic processes, suggest analyses of dissolved organic carbon from Northern Hemisphere water bodies. Inland waters mediate carbon transport between land and ocean.

Approximately 8% of the fluvial suspended sediment exported to the world’s oceans comes from rivers draining the Greenland ice sheet, according to an analysis of satellite imagery. Furthermore, the export is dominated by areas where subglacial erosion is high.

Dissolved inorganic carbon is buried in dryland basins that do not drain to the sea. Based on measurements of sediment chemistry in twelve of these sites, closed basins are estimated to store 0.15 Pg of dissolved inorganic carbon annually.

Organic carbon decays as it travels through inland waters from soils to the sea. Analysis of data from across the continuum of inland and marine aquatic systems reveals that the rate of organic carbon decay depends on water retention time.

Very small ponds have been omitted from greenhouse gas budgets. Estimates of CO₂ and CH₄ emissions from 427 lakes and ponds show that very small ponds account for 15% of CO₂ and 40% of diffusive CH₄ emissions, but 8.6% of lake and pond area.

Lakes are a large source of CO₂. An analysis of chemical and physical data from 5,118 boreal lakes reveals that a majority emit CO₂ originating primarily from terrestrial sources rather than CO₂ produced within the lakes.

Streamflow is a mixture of precipitation of various ages. Oxygen isotope data suggests that a third of global river discharge is sourced from rainfall within the past few months, which accounts for less than 0.1% of global groundwater.

Rivers and streams are key sources of CO₂. Estimated emissions and aquatic productivity from across the US show that small streams predominantly emit CO₂ produced in soils, but the contribution from aquatic metabolism increases with river size.

Inland waters are important sources of greenhouse gases. Measurements over eight years suggest that African inland waters are a substantial source of greenhouse gases, equivalent to a quarter of the global land and ocean carbon sink.

Carbon dioxide emissions from lakes contribute to the continental carbon balance. Water chemistry analyses of reservoirs in Spain suggest that carbonate weathering causes CO₂ supersaturation in lakes above a threshold alkalinity.

Aquatic ecosystems are important sources of the greenhouse gas nitrous oxide. Measurements of nitrous oxide concentrations from 321 rivers, lakes and ponds in Canada reveal that some boreal aquatic systems can act as net nitrous oxide sinks.

Current estimates of carbon dioxide evasion from inland waters are based on incomplete spatial coverage. Streams and rivers in the United States release 97 Tg of carbon to the atmosphere each year, according to an analysis of chemical and morphological data.

Wastewater can make up a large fraction of stream flow. An analysis of over 14,000 US streams shows that under severe low-flow conditions, wastewater containing endocrine disruptors is poorly diluted, and many streams exceed safety thresholds.

Many lakes in China are subject to eutrophication. Water quality analyses on 862 Chinese lakes reveal that better sanitation has reduced phosphorus inputs in the most populated areas, but aquaculture and livestock offset improvements elsewhere.

The polygonal patterns in permafrost regions are caused by the formation of ice wedges. Observations of polygon evolution reveal that rapid ice-wedge melting has occurred across the Arctic since 1950, altering tundra hydrology.

Warming thaws permafrost, releasing carbon that can cause more warming. Radiocarbon, soil carbon, and remote sensing data suggest that 0.2–2.5 Pg of carbon has been emitted from permafrost as CO₂ and CH₄ around Arctic lakes since the 1950s.

Most dissolved organic carbon in rivers originates from young carbon in soils and vegetation. A global radiocarbon data set suggests that human disturbance is also introducing aged carbon to rivers and to active carbon cycling.

Reservoirs emit significant amounts of greenhouse gases. An analysis of data from 85 globally distributed hydroelectric reservoirs indicates that about 48 Tg carbon is emitted as carbon dioxide and 3 Tg carbon as methane, and that carbon emissions are correlated with reservoir age and latitude.

The world's inland waters are under siege. A system-level view of watersheds is needed to inform both our scientific understanding and management decisions for these precious resources.

Debate rages over which water bodies in the US are protected under federal law by the Clean Water Act. Science shows that isolated wetlands and headwater systems provide essential downstream services, but convincing politicians is another matter.

Quality requirements for water differ by intended use. Sustainable management of water resources for different uses will not only need to account for demand in water quantity, but also for water temperature and salinity, nutrient levels and other pollutants.

Satellite measurements indicate that Greenland's meltwater rivers are exporting one billion tons of sediment annually, a process that is controlled by the sliding rate of glaciers. This rate is nearly 10% of the fluvial sediment discharge to the ocean.

Phosphorus loading can cause eutrophication of lakes. Analyses of lake chemistry in China reveal that policies have led to lower phosphorus levels overall, but increasing trends in some lakes suggest that expanded policies may be needed.

The terrestrial water cycle is often assessed annually at catchment scale. But water stored in catchments is poorly mixed, and at timescales often well beyond the calculation of annual water balance.

Climate change is causing widespread permafrost thaw in the Arctic. Measurements at 33 Arctic lakes show that old carbon from thawing permafrost is being emitted as methane, though emission rates have not changed during the past 60 years.