Date Published: February 15, 2018
Publisher: Public Library of Science
Author(s): Paul McKinney, Kathy S. Tokos, Katsumi Matsumoto, Maite deCastro.
Lake Superior′s ecosystem includes distinct nearshore and offshore food webs linked by hydrodynamic processes that transport water and tracers along and across shore. The scales over which these processes occur and their sensitivity to increasing summer surface temperatures are not well understood. This study investigated horizontal mixing between nearshore and offshore areas of Lake Superior over the 10-year period from 2003 to 2012 using a realistically forced three-dimensional numerical model and virtual tracers. An age tracer was used to characterize the time scales of horizontal mixing between nearshore areas of the lake where water depth is less than 100 m and deeper areas. The age of water in nearshore areas increased and decreased in an annual cycle corresponding to the lake′s dimictic cycle of vertical mixing and stratification. Interannual variability of mixing in the isothermal period was significantly correlated to average springtime wind speed, whereas variability during the stratified season was correlated to the average summer surface temperature. Dispersal of a passive tracer released from nine locations around the model lake’s perimeter was more extensive in late summer when stratification was established lakewide than in early summer. The distribution of eddies resolved in the model reflected differences between the early and late summer dispersal patterns. In the eastern part of the lake dispersal was primarily alongshore, reflecting counterclockwise coastal circulation. In the western part of the lake, cross-shore mixing was enhanced by cross-basin currents.
The nearshore areas of large lakes are valuable water resources that provide water to surrounding communities and habitat for a variety of species. They also are vulnerable to degradation by runoff from the surrounding watershed. Water quality in these critical areas is controlled by horizontal mixing processes that disperse pollutants, nutrients and sediment along shore and across shore, thus predicting the effects of runoff on nearshore areas requires an understanding of the scales over which these processes occur. In this study, we evaluate the variability of horizontal mixing (mixing, hereafter) in Lake Superior using a realistically forced three dimensional model of lake hydrodynamics and virtual tracers.
The nearshore areas of large lakes are valuable water resources and habitat where water quality is affected by inputs from the surrounding watershed as well as horizontal mixing processes that disperse pollutants, nutrients and sediment along shore and across shore. In this study, horizontal mixing in Lake Superior was investigated using a realistically forced 3 dimensional numerical model and virtual tracers. Two sets of model runs were completed. The first was a simulation of lake conditions over the ten year period 2003–2012 which focused on mixing between nearshore areas where water depth is less than 100 m and deeper offshore areas. The concentration of an aging tracer was used to measure residency time of water in the nearshore areas. Mixing, indicated by changes in tracer concentration, was weaker during winter and spring when the lake was isothermal or weakly stratified than in summer after seasonal density stratification was established.