Phosphorus is one of the most biologically important elements for life on Earth and is thus ubiquitous in many crop fertilizers. Following heavy rain, particulate phosphorus often runs off field crops into nearby bodies of water, where it can dramatically alter the health of aquatic ecosystems over time.
Environmental regulations have reduced phosphorus discharges to some aquatic ecosystems to improve water quality, but despite the reductions, many systems have not experienced water quality improvement. This may be because of the internal release of accumulated, or legacy, phosphorus in sediments, which can drive eutrophic conditions—with low oxygen levels and excess nutrients—that are often hazardous to native aquatic animals.
To better understand how and in what forms phosphorus accumulates in lakes, scientists have conducted studies in which they artificially add the nutrient to a controlled experimental lake. In one such lake in Ontario, Canada, known as Lake 227, in the International Institute for Sustainable Development Experimental Lakes Area, researchers have used phosphorus to transition the aquatic environment from oligotrophic, or relatively oxygen rich and low in nutrients, to eutrophic.
In a new study, O’Connell et al. use a combination of sediment cores and laboratory analyses to track changes in sedimentary phosphorus forms during and after the transition. The sediment cores represent a natural history of the lake’s relationship with phosphorus, showing the rates at which the element has been buried in the sediment as well as the elements with which it has formed complexes.
Prior to eutrophication of Lake 227, most of the phosphorus existed in an organic form in the lake sediment. As the lake transitioned to eutrophic conditions, the rate of phosphorus burial increased overall, with much of this increase attributable to phosphorus bound to humus, the dark-colored soil fraction that’s rich in organic matter. The researchers found that the humic phosphorus was usually bound in iron-humic complexes, especially with ferric iron (Fe3+). They noted that the large quantity of Fe3+ found in the sediment suggests that the organic carbon–rich sediment stabilizes this oxidation state of iron, preventing it from being reduced to other forms even in highly reducing conditions.
The researchers also observed that much of the phosphorus added to Lake 227 is stored in reactive forms, meaning that the lake could remain eutrophic for years even without external phosphorus loading. They conclude that eutrophication may thus have far-reaching consequences in the future because of legacy phosphorus. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2020JG005713, 2020)
—David Shultz, Science Writer