Persistent and widespread long-term phosphorus declines in Boreal lakes in Sweden

- Substantial, long-term declines in TP were detected in boreal lakes across Sweden.
- Lake trophic status has changed in many lakes to ultra-oligotrophic.
- Increased reliance on forest biomass and hydropower will likely exacerbate P declines.
- Legislation is poorly equipped to handle impairments caused by oligotrophication.

We present an analysis of long-term (1988-2013; 26 years) total phosphorus (TP) concentration trends in 81 Swedish boreal lakes subject to minimal anthropogenic disturbance. Near universal increases in dissolved organic carbon (DOC) concentrations and a widespread but hitherto unexplained decline in TP were observed. Over 50% of the lakes (n = 42) had significant declining TP trends over the past quarter century (Sen's slope = 2.5% y− 1). These declines were linked to catchment processes related to changes in climate, recovery from acidification, and catchment soil properties, but were unrelated to trends in P deposition. Increasing DOC concentrations appear to be masking in-lake TP declines. When the effect of increasing DOC was removed, the small number of positive TP trends (N = 5) turned negative and the average decline in TP increased to 3.9% y− 1. The greatest relative TP declines occurred in already nutrient poor, oligotrophic systems and TP concentrations have reached the analytical detection limit (1 μg L− 1) in some lakes. In addition, ongoing oligotrophication may be exacerbated by increased reliance on renewable energy from forest biomass and hydropower. It is a cause of significant concern that potential impairments to lake ecosystem functioning associated with oligotrophication are not well handled by a management paradigm focused exclusively on the negative consequences of increasing phosphorus concentrations.

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