A Century of Legacy Phosphorus Dynamics in a Large Drainage Basin

Abstract

There is growing evidence that the release of phosphorus (P) from "legacy" stores can frustrate efforts to reduce P loading to surface water from sources such as agriculture and human sewage. Less is known, however, about the magnitude and residence times of these legacy pools. Here we constructed a budget of net anthropogenic P inputs to the Baltic Sea drainage basin and developed a three-parameter, two-box model to describe the movement of anthropogenic P though temporary (mobile) and long-term (stable) storage pools. Phosphorus entered the sea as direct coastal effluent discharge and via rapid transport and slow, legacy pathways. The model reproduced past waterborne P loads and suggested an~30-year residence time in the mobile pool. Between 1900 and 2013, 17 and 27 Mt P has accumulated in the mobile and stable pools, respectively. Phosphorus inputs to the sea have halved since the 1980s due to improvements in coastal sewage treatment and reductions associated with the rapid transport pathway. After decades of accumulation, the system appears to have shifted to a depletion phase; absent further reductions in net anthropogenic P input, future waterborne loads could decrease. Presently, losses from the mobile pool contribute nearly half of P loads, suggesting that it will be difficult to achieve substantial near-term reductions. However, there is still potential to make progress toward eutrophication management goals by addressing rapid transport pathways, such as overland flow, as well as mobile stores, such as cropland with large soil-P reserves.Plain Language Summary All life depends on phosphorus (P), which is why it is an important crop fertilizer. Humans generally consume more P than needed and the excess ends up in sewage systems. Past management of P in fertilizer and human sewage has led to the accumulation of P in soils and sediments of lakes and streams. This accumulation is called "legacy" P because it can leak for decades to downstream lakes and coastal areas where it contributes to environmental problems. We developed a model to understand P dynamics for the entire drainage basin of the Baltic Sea since 1900. This model included a rapid transport pathway that represented sources such as runoff from cropland and a slow pathway that represented leakage from mobile legacy sources. The model suggests that loss from the mobile pool contributes about half of current waterborne inputs to the sea; as a result, it could be difficult to make substantial near-term reductions. However, there are opportunities to meet environmental goals by slowing the accumulation of P in the landscape and by implementing measures that address the rapid transport pathway, such runoff from cropland, and the mobile stores, such as cropland with large soil-P reserves.

Barriers and Bridges in Abating Coastal Eutrophication

“…The difficulty in achieving reductions is compounded by the legacy storage of nutrients in soils and waterways, for phosphorus, and groundwater, for nitrogen. McCrackin et al (2018) estimated the residence time for phosphorus in Baltic catchments of about 30 years, but concluded that the system has shifted from accumulation to depletion of legacy storage. While it remains difficult to achieve near-term goals for load reductions to the sea, addressing pathways of rapid transport such as overland flow, and mobile stores, such as cropland with large soil-phosphorus reserves, would accelerate load reductions.…”

“…It is critical to ascertain the influence of lag times in the catchment through more diagnostic groundwater monitoring and modeling (e.g., Hansen et al, 2017), not only to avoid imposing changes to intervention measures due to impatience (Vero et al, 2018), but also to guide when and how to make adjustments if they are not effective. Progress can be accelerated by addressing rapid transport pathways, such as surface runoff of phosphorus and the large soil phosphorus reserves in croplands (McCrackin et al, 2018) and large nitrogen losses from subsurface drainage systems. About half of the agricultural nitrate load in the Mississippi River catchment comes from such tile drainage systems (McCrackin et al, 2017) and can be substantially reduced by use of controlled drainage, especially if the drainage then flows through wood-chip bioreactors or constructed wetlands (Christianson et al, 2018).…”

Where Have All the Nutrients Gone? Long‐Term Decoupling of Inputs and Outputs in the Willamette River Watershed, Oregon, United States

“…That is, if there is a lag between interventions and water quality outcomes, it may be particularly important to set targets that are realistic in order to ensure that (1) decision-makers and their political constituencies are not discouraged from their efforts, and (2) that support for interventions does not cease midway through because water quality does not improve quickly. Multi-year, and even multi-decade, legacy effects must be taken into consideration (Chen et al, 2018;McCrackin et al, 2018). Although there has been some success in improving water quality across the U.S., in particular related to point sources in urban watersheds, agricultural watersheds remain a 10.1029/2020JG005792 Journal of Geophysical Research: Biogeosciences challenge for both N and P management (Stets et al, 2020).…”

Shared socio-economic pathways extended for the Baltic Sea: exploring long-term environmental problems

“…Despite the advancements, the current nutrient loads are still far too large: for example, N and P loads to the major basin Baltic Proper need to be reduced by 13% and 50%, respectively, in order to reach the Baltic Sea Action Plan targets jointly agreed by the HELCOM countries (Svendsen et al 2015). Especially for P, significant legacy effects in the catchment is expected between measures and effect on diffuse loads to the sea (Mccrackin et al 2018). In addition to delays in the catchment, the Baltic Sea is a slow coastal system with i.a.…”