There’s something in the water: an overabundance of nutrients. Eutrophication caused by excess nutrients can lead to algal blooms, ocean acidification, and dead zones. Phosphorus, much of it supplied in runoff from human activities, is one of the key nutrients responsible. Reducing such pollution can lower phosphorus levels in waterways, but phosphorus that settles into lakes and river sediments can remain there for years—and when the sediment is disturbed, the phosphorus reemerges.
Scientists already know that lake beds hold persistent phosphorus. But streams experience constant movement and turbulence that can potentially stir up phosphorus from the sediment. Lannergård et al. examined phosphorus forms and concentrations in streams and lakes in a catchment in Sweden, looking to pin down how big a problem stored “legacy” phosphorus is and how easily it can reenter water.
Using sediment cores, the researchers analyzed phosphorus concentrations in the drainage basin. They found that most stream sediments had higher phosphorus concentrations than lake sediments did, especially as they moved downstream. The lakes still accounted for more total phosphorus than the streams, however, because of their larger areas.
The researchers also found that forested headwaters had more organic phosphorus and more iron-bound phosphorus, likely because of their low water volume and accumulation of forest debris. Agricultural streams amid clay soils had high phosphorus concentrations and a larger proportion of calcium-bound phosphorus. The researchers noted that agricultural streams and ditches are often flushed out, meaning they can become transient sources of mobilized phosphorus to downstream waterways.
The type of phosphorus plays a role in how likely it is to reenter the water from sediment and continue downstream. For example, disturbing sediment generally releases organic phosphorus, whereas iron-bound phosphorus is sensitive to oxygen levels and can be released even in still conditions, such as when a lake is stratified, if oxygen levels drop. Streambeds, which are typically well oxygenated, tend to lock this type of phosphorus in, although conditions can arise that release it.
The scientists estimate that 2.1 metric tons of phosphorus is stored in the streams in this catchment alone. Because many streams and rivers in Sweden flow into the Baltic Sea, a hot spot of eutrophication, the researchers suggest that additional research and tighter management of such catchment systems are needed. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2020JG005763, 2020)
—Elizabeth Thompson, Science Writer