Twentieth century water quality trends in Minnesota lakes compared with presettlement variability

Abstract

A diatom-based transfer function was used to reconstruct water chemistry before European settlement in 55 Minnesota lakes. The lakes span three natural ecoregions, which differ in their history of land use, as well as in surficial geology, climate, and vegetation. Postsettlement trends were compared with water chemistry change reconstructed from two presettlement core sections (circa 1750 and 1800) as a measure of natural variability. Presettlement water quality changes were generally small and nondirectional in all three ecoregions. In contrast, half of the urban lakes showed a statistically significant increase in chloride, whereas 30% of urban and 30% of agricultural region lakes record a statistically significant increase in total phosphorus between 1800 and the present. These changes, which are attributed to road salt and nutrient runoff, are strongly correlated with the percentage of watershed area that is developed (residential or urban) in the case of chloride increases and the percentage of developed (metropolitan areas) or agricultural (agricultural areas) land in the case of nutrient increases. Water quality has changed little since 1800 for lakes in the forested regions of northeastern Minnesota. The few changes that are seen in this region are likely related to natural variations in climate or catchment soils.Résumé : L'utilisation d'une fonction de transfert basée sur les diatomées nous a permis de reconstituer la chimie des eaux dans cinquante-cinq lacs du Minnesota avant l'établissement des européens. Les lacs se répartissent dans trois écorégions naturelles qui différent par l'utilisation que l'on a fait des terres dans le passé, ainsi que par la géologie de surface, le climat et la végétation. Nous avons comparé les tendances qui sont apparues après la colonisation avec les changements estimés à partir de deux sections de carottes datant d'avant la colonisation (vers 1750 et 1800) qui repré-sentent la variabilité naturelle. Avant la colonisation, les changements de la qualité de l'eau dans les trois écorégions étaient généralement faibles et sans tendance marquée particulière. En revanche, entre 1800 et l'époque actuelle, la moitié des lacs urbains affichent une augmentation statistiquement significative des chlorures et 30 % des lacs urbains et 30 % des lacs des régions agricoles subissent une augmentation statistiquement significative du phosphore total. Il existe une forte corrélation entre ces changements, qui sont attribuables à l'apport du sel routier et de nutriments par ruissellement, avec le pourcentage des terres du bassin versant qui sont urbanisés (résidentielles ou urbaines) dans le cas de l'augmentation des chlorures ou alors développées (régions métropolitaines) ou exploitées pour l'agriculture (zones agricoles) dans le cas de l'augmentation des nutriments. Dans les régions forestières du nord-est du Minnesota, la qualité des eaux des lacs a peu varié depuis 1800. Les quelques changements observés dans cette région sont vraisemblablement reliés aux variations naturelles...

Possible Sediment Mixing and the Disparity between Field Measurements and Paleolimnological Inferences in Shallow Iowa Lakes in the Midwestern United States

“…This showed an increase in total phosphorus of 33% ( Figure 6). The diatom-inferred increases in total phosphorus in the Iowa lakes are also comparable to those found by Ramstack et al [36] for lakes in agricultural areas of Minnesota. These authors examined sediment cores for 5 lakes in the Western Corn Belt Plains ecoregion and 9 in North Central Hardwood Forests ecoregion and found that diatom-inferred TP concentrations increased from an average of 35 µg/L in 1800 to 43 µg/L in 1995 for an increase of 23%.…”

“…We found the precision and accuracy of the transfer functions used by Heathcote et al [9] were similar to those used in other regional paleolimnological studies (Table 4). While the bootstrapped root mean squared error of predication of 0.30 log units was greater than that used in a study of Minnesota lakes of 0.25 log units [36], it was smaller than that (0.34 log units) for the transfer functions used in a study of the lakes of the northeastern United States [37] and the natural lakes (0.36 log units) of the United States in the National Lakes Assessment of 2007 [38]. We used the Heathcote et al [11] results in part because Saulnier-Talbot [39] noted that while there are problems with diatom transfer functions, great strides have been made using this approach and that their use at present can hardly be discarded.…”

Application and Tests of the Canadian Water Quality Index for Assessing Changes in Water Quality in Lakes and Rivers of Central North America

“…While few observations, and even fewer quantifiable data exist prior to settlement, there are several methods for setting site specific objectives based on the presumed natural condition. These methods include: the use of appropriate and comparable reference sites that more closely reflect the natural state because of a lack of human influence (background approach); establishing objectives using predictive relationships from geographically similar regions, including land use, geology, climate, and vegetation (geographical approach); accepting that current conditions are acceptable, with the idea of protecting the current condition from changing (ambient approach); and inferring past conditions through use of paleolimnological techniques (paleolimnology approach; US EPA 2000, Smol 2002, Ramstack et al 2004, CCME 2005a. If one accepts that water quality objectives can be set from the above list, then a WQI for aquatic life should evaluate the current state relative to those objectives.…”

Historical Changes in Abundance of Floating‐Leaf and Emergent Vegetation in Minnesota Lakes

“…Ramstack et al (2004), by reconstructing water chemistry for 55 Minnesota lakes from sedimentary diatom assemblages, increased awareness of baseline conditions in the lakes and provided managers with advice on restoration goals. Studies on historical changes to lake vegetation could serve the same role for aquatic macrophytes.…”