Water quality describes the characteristics of a water body that allow it to support the organisms that live in it as well as the uses that people expect of it. Before the Duluth-Superior Port was developed a high diversity of organisms existed in the river, harbor and watershed that had coevolved over long periods of time to create a rich, diverse ecosystem. While the ecosystem certainly varied between seasons and years, European settlement caused dramatic changes to the estuary and nearshore waters of Lake Superior. In the past 150 years we’ve harvested much of the original forest, flushed our sewage and industrial wastes into the river, removed much of the original shoreline wetland vegetation, and dredged to allow passage of big ships. Fortunately, in the past 40 years there has been a concerted effort to restore it for both the natural ecosystem, as well as for people to use it for its social, cultural, and economic benefits – recreation, shipping, industry, urban and rural living, aesthetic and spiritual enjoyment, drinking water, and other uses. The stressor map provides a tool for predicting how the ecosystem is likely to change if we change the shoreline or the inland watershed in different ways. The better we are able to predict the results of our decisions on the land and water, the better we can avoid problems that could be irreversible or extremely expensive to fix in the future.
Here are several data sets that are encouraging us to continue our research. The data were collected near the river, but just up from the shoreline above the mouths of feeder streams
This set of graphs indicates that an increase in the index of stress in the watershed correlates with increases in two different water quality indicators. The first one shows nitrate, a form of the essential nutrient, nitrogen, that is readily available to plants in the water. Too much nitrate running off the landscape can degrade water quality by stimulating too much algal growth and perhaps also by affecting wetland plant communities.
The element chloride (see figure) appears to be even more sensitive to the stress indicator. Like nitrate it also dissolves easily in water and, during snowmelt runoff, may reflect a winter’s worth of roadsalt. At other times of year it can be associated with sediment, grit, and grime washing off roads, parking lots, and lawns. The stressor scores do a good job of predicting nitrate and chloride dissolved in the nearshore waters of the river, even during the summer when flows are greatly reduced compared to springtime (see below).
As the results of this and future studies are analyzed and interpreted, the full story is likely to be even more complicated. However, much like crime scene investigators, we can examine additional sources of data in more detail, looking for “forensic evidence” of activities on land that degrade water quality.
Excess sediment is one of the major stresses to aquatic organisms and is one that often results from human activities in the watershed. . The excess sediment that results in muddy water in streams and rivers is considered to be the biggest source of water quality problems in the United States. Turbidity is a relatively easy measurement that reflects the loss of water clarity that occurs when sediment levels increase. In our stressor gradient study, for all of our sampling sites at different times of year, we either found no association with “environmental stress” or an inverse relationship. However, if you look at the satellite image below you can see that there are a number of sites along Wisconsin’s south shore where the landscape is mostly forested with few homes and roads, but where the streams are clearly very muddy. These are areas where the clay soils are naturally very erodible and the streams have many steep banks.
To sort out whether clay-influenced watersheds in Wisconsin affected the relationship between turbidity and our stressor index, we removed these sites from the analysis. The graph to the right shows increasing turbidity with increasing stress, which makes more sense and suggests that the natural erodibility of soils needs to be included in an assessment of environmental stress. In fact, the most recent information available (early 2012) on the “erodibility” of soils in the region (captured in the variable called “K-factor” in the graph below) further confirms this hypothesis by showing the strong association of TSS (total suspended sediment) with soil K values.