Reconstructing Eutrophication and Phosphorus Loading for Lake Volney, Minnesota: Combining Lake Sediments and Land-Use History to Establish ‘Natural’ Baselines for Management and Restoration

ABSTRACT

Establishing management targets for lake nutrient inputs remains a major challenge to limnologists and resource managers concerned with the cultural eutrophication of lakes. In this study we used multiple sediment cores to reconstruct pre-Euroamerican (1650–1850) phosphorus (P) loading to Lake Volney, a hypereutrophic lake in Le Sueur County, Minnesota, and compared changes in P inputs to inferred changes in lake productivity based on biogenic silica (bSi). Euroamerican changes in land-use, as abstracted from census and tax records, were also compared with sedimentary proxies for soil erosion – loss-on-ignition and environmental magnetism. Whole-basin P accumulation in Lake Volney ranged from 0.31 – 0.39 g m⁻² yr⁻¹ prior to the arrival of Euroamerican agriculture in the 1850s. There-after P accumulation rose nearly three-fold largely due to increased fluxes of organic and non-apatite inorganic P. P inputs were highest (3.9 g m⁻² yr⁻¹) when organic, apatite and non-apatite P all peaked. Modern P accumulation rates of 1.3 g m⁻² yr⁻¹ match closely P-loading estimates based on monitoring data and mass-balance calculations. Lake productivity showed little change until the early 1900s when bSi accumulation rose 5–10x over pre-1900 values. The initial increase in P loading observed in the 1850s corresponds closely to the arrival of Euroamericans in the Lake Volney watershed. Causes for the second increase in P inputs (1910–1930) seem related to an overall increase in large animal numbers (swine and cattle) that began in the 1910s. The methods combined in this study provide site-specific reconstructions of trophic change and its causes that can be used to guide lake management and restoration.

Key Words:

• eutrophication
• Lake Volney
• LOI
• magnetics
• paleolimnology
• phosphorus
• sediment
• watershed