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Environmental implications of stratification and turbulent mixing in a shallow lake basin

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Abstract:

The extent of stratification and vertical mixing in the water column (7–11 m deep) was investigated over an offshore reef in the western basin of Lake Erie. Measurements reveal that the vertical transport of oxygen and heat is controlled by the complex interaction of several physical mechanisms. Generally, when the wind speed (W) was >7 m·s–1 and the air was cooler than the water (Tair< Tw), the water column was well mixed due to turbulent mixing. However, when W < 7 m·s–1 (~65% of the summer), turbulence was too weak to overcome the stratification and mix the water column. An analysis of 25 years of meteorological data revealed that a period of 4.5 ± 1.9 days of calm, warm weather (W < 7 m·s–1 and Tair > Tw) occurs every year. Results indicate that there is strong probability of hypoxia due to stratification (i.e., when diffusivities < 10–6 m2·s–1) and sediment oxygen demand (i.e., 0.1–1.0 g·m–2·day–1) during these periods. The environmental implications of stratification to water quality and its effects on benthic organisms, such as the burrowing mayfly (Hexagenia spp.), require further considerations in large temperate lakes and basins that are sufficiently shallow that there is no permanent seasonal stratification.

Nous avons étudié l'importance de la stratification et du brassage vertical dans la colonne d'eau (7–11 m de profondeur) sur un récif du large dans le bassin occidental du lac Érié. Nos mesures indiquent que le transport vertical de l'oxygène et de la chaleur est contrôlé par l'interaction complexe de plusieurs mécanismes physiques. Généralement, lorsque la vitesse du vent (W) est >7 m·s–1 et que l'air est plus frais que l'eau (Tair < Tw), la colonne d'eau est bien brassée à cause de la turbulence. Cependant, lorsque W < 7 m·s–1 (~65 % de l'été), la turbulence est trop faible pour vaincre la stratification et brasser la colonne d'eau. Une analyse de 25 années de données météorologiques montre qu'une période de 4,5 ± 1,9 jours de chaleur et de calme (W < 7 m·s–1 et Tair > Tw) se produit chaque année. Nos résultats indiquent qu'il y a une forte probabilité d'hypoxie à cause de la stratification (par ex., lorsque les diffusions < 10–6 m2·s–1) et de la demande en oxygène des sédiments (par ex., 0,1–1,0 g·m–2·jour–1) durant ces périodes. Les conséquences environnementales de la stratification sur la qualité de l'eau et ses effets sur les organismes benthiques, tels que les éphémères fouisseurs (Hexagenia spp.), nécessitent des études supplémentaires dans les grands lacs et bassins qui sont assez peu profonds pour qu'il n'y ait pas de stratification saisonnière.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: 2007-01-01

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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