External Phosphorus Loading in New Lakes

Abstract

The water quality of lakes is highly dependent on external phosphorus (P) loading. The vast external loadings from sewage and other wastewater discharge that European lakes have historically received have been dramatically reduced today by improved wastewater treatment. Gaining knowledge of the catchment characteristics that influence external P-loading should enable predictions of the achievable water quality of lakes. In this study, we tested this proposition. Data from 90 new Danish lakes show no apparent relationship between the mean summer P-concentration and the size or land use of the catchments. The external P-loading and resulting annual P-concentration were further investigated on a representative subset of 12 of the new lakes, using six methods. Three of the methods used empirical estimates of P-transport from catchments, based on the national average P-transport, runoff-dependent P-transport, and crop-dependent P-transport, and the other three methods used different empirical models tested on the lakes. External P-loading was reliably predicted by several of the methods. The predictions of the annual P-concentration were highly dependent on the inclusion of annual runoff. However, the predicted P-concentrations were generally overestimated, most pronounced for the nutrient-poor and most recently established lakes. In these lakes, internal P-loading was found to be the most important factor in predicting achievable water quality.

Groundwater–surface water interaction in Denmark

“…In the past 25 years, due to greater concern about environmental issues and a desire to increase biodiversity, about 50 large (>10 ha) Danish lakes have been re‐established in former lake basins (B. Hansen et al, 2012). Comprehensive studies of a few of these lakes (Kragh et al, 2022; Sø et al, 2020) show that rewetting of drained low‐lying organic‐rich agricultural areas to wetlands and lakes improves biodiversity and water quality, and changes greenhouse gas emission at the same time (Baastrup‐Spohr et al, 2016). Despite the economic impact of losing agricultural land, lake restorations have generated other values such as natural areas for recreation, improved conditions for biodiversity, and reduced nutrient transport to downstream coastal waters (Hoffmann & Baattrup‐Pedersen, 2007), which might also be converted into economic benefits.…”