Impacts of a deep reactive layer on sedimentary phosphorus dynamics in a boreal lake recovering from eutrophication

Using biogeochemical analyses of sediments and porewaters, we investigate the legacy of a brief, intense period of eutrophication on sedimentary phosphorus (P) cycling in a boreal lake (Enonselkä basin, Lake Vesijärvi, Finland). Point-source sewage inputs in the twentieth century caused deoxygenation of the lake and accelerated the focusing of iron (Fe) and manganese (Mn) oxides into deeper areas. Early diagenesis under Fe–Mn-rich conditions now favors rapid burial of P in these areas, likely as a combination of both oxide-bound P phases and authigenic manganous vivianite. A new P budget for Enonselkä basin shows that P burial causes an annual drawdown of 1.2% (± 0.2%) of the surface sediment P inventory, supporting a long-term trend towards recovery since the construction of a wastewater treatment plant in the mid-1970s. However, remineralization of organic matter and associated dissolution of Fe–Mn oxides continues to regenerate P from a deep reactive layer (20–60 cm depth) deposited at the height of past eutrophication, leading to an upwards diffusive flux of dissolved phosphate towards the surface sediments. The magnitude of this flux is similar to that of external P loading to the lake. The combined incoming fluxes of P are likely to retard the complete recovery from eutrophication by decades, despite ongoing restoration actions.

The sixth R: Revitalizing the natural phosphorus pump

Humans and natural systems face three pressing concerns: the loss of large animal biodiversity, eutrophication of many aquatic systems, and the need to better recycle phosphorus. Here we propose a mechanism to help alleviate these problems. Some have hypothesized that we are approaching “peak phosphorus,” where phosphorus may become more expensive as it becomes rarer, thus endangering the green agricultural revolution and the ability to feed ourselves. Animals play a key role in the recycling of phosphorus (P) from the ocean depths to the continental interiors, but this movement has declined by >90% over the past 10,000 years. Prior to this decline, animals played a critical role in the global P budget and the pre-Anthropocene P budget was in steady state only after accounting for animal P inputs. Recently a 5R strategy was developed by Withers et al (2015) to Realign P inputs, Reduce P losses, Recycle P in bio-resources, Recover P in wastes, and Redefine P in food systems. Here we suggest a sixth R, to Revitalize the natural phosphorus pump. Countries are starting to mandate P recycling, and here we propose a P-trading scheme based on REDD+, where a country could partially achieve its recycling goals through revitalizing the natural P pump. Accrued money from this scheme could be used to restore or conserve wild animal populations while increasing natural phosphorus recycling.