Sediment Mass and Nutrient Accumulation Rates in Lake Erie Using Geographic Information System

Introduction

Due to the continuous release of contaminants, including phosphorus, from agricultural development in the Maumee River drainage basin of Ohio (Logan, 1987), nutrients and contaminant loadings have increased in Lake Erie continuously since the 19^th century (Matisoff, 1999), leading to cultural eutrophication. Although eutrophication is a natural process, it is easily seen in Lake Erie because it is accelerated by human activities (Bentley, 2000). In addition, phosphorous is a primary element in eutrophication, a process that naturally occurs underwater. Phosphorous is a significant mineral required by photosynthesis that, in excess, can promote algae growth.

In the Western Basin of Lake Erie, algal blooms are a major environmental problem since the Maumee River discharges high concentrations of these contaminants. In the Central Basin of Lake Erie, a major water quality problem are dead zones, areas that have low oxygen concentrations in the hypolimnion during summer thermal stratification. The temperature and density of water during winter is the same from the surface to the bottom of the lake, but it changes during summer since the sun heats the lake's surface. Thus, warmer surface water overlies the colder, deeper water (Ullyott & Holmes, 1936). In contrast to the Western and Central Basins, the Eastern Basin is not affected by oxygen depletion since it has a large volume of water below the summer thermocline relative to the bottom surface area. However, since the water flows from west to east, it is necessary to focus on the origin of water flow-the Western Basin.

Beginning in the 1960s, the increasing flux of nutrients into the lake resulted in larger and more frequent algal blooms. During this time, the Western Basin suffered damage due to increased algal growth and industrial waste that came from the Maumee River and Detroit River (Bertram et al., 2009). For instance, phosphorus dissolved in water fertilizes algae, which grow and expand on the surface of water. But when algal blooms die, they sink to the bottom of the lake and decompose. Fungi and bacteria require dissolved oxygen during decomposition, so they use all the nearby oxygen ("Lakewide Management Plans", 2002). Oxygen is a necessary factor of survival for most organisms, but large algal blooms require vast quantities of oxygen during decomposition, leading to a dead zone-an area with insufficient oxygen for aquatic organisms' survival.

However, before solving the issues of this environmental problem, it is imperative to understand the current state of the lake. To do so, the distribution of the sedimentation rates, water depths, and chemical parameters must be analyzed. To understand sediment deposition and focusing, it is important to know the distribution of these parameters in the surface sediment of Lake Erie.

Until now, many researchers have gathered new data on sedimentation rates for Lake Erie. This particular study aims to provide updated sediment mass accumulation rates and other chemical deposition distributions using data from previously conducted research and newly compiled data sets. Most sedimentation rates and chemical deposition data for Lake Erie were obtained from Klump et al. (2005) and augmented with data for sedimentation rate, bulk density, organic matter, and phosphorous in the Western Basin (Matisoff, unpublished data). The lake data from this research is crucial to determine various locations and magnitudes of sediment and chemical deposition and depositional fluxes.

The main reasons for separating maps of the whole Lake Erie from the Western Basin are both scientific and practical. From a scientific perspective, Lake Erie is a major lake in central North America that, especially in its Western Basin, is heavily impacted by agricultural and industrial development. Therefore, further analysis of the basins' current conditions is required in order to seek out solutions to better their conditions.

Methods

This study focuses on using the geological sedimentation rate map, created with geographic information system (GIS) to visualize, analyze, and understand the sediment deposition and nutrient distribution in Lake Erie. In this research project, GIS technology was used to create a map and compare the locations and patterns of sediment and other chemical elements between Lake Erie and its Western Basin. This GIS map will provide a rigorous visualization tool for noting similarities and differences of sedimentation rates depending on many conditions, such as water depth and water flow.

To begin mapping for the whole Lake Erie, sedimentation rates determined from radionuclide and pollen dating of sediments, nitrogen, nitrogen flux, carbon, and carbon flux data were collected from Klump et al. (2005), unpublished data (Online Tables 2-5), and data for sedimentation rates from Matisoff (unpublished, Online Table 6).

According to previous research, sedimentation rates at 40 stations (Online Table 1) were determined through coring (Klump et al. 2005).

Sedimentation rates in cores were determined from 137Cs and 210Pb radio-chronometry using high-purity germanium (HPGE) spectroscopy (Mahmood & Yii, 2013). Yet, since the sedimentation rate data used in this research was taken more than two decades ago, it is crucial to compile recent data before observing the changes that occurred through the years.

Figure 2 shows ranges of sedimentation rates taken in each station marked using different colors. Darker colors indicate higher sedimentation rates while lighter shades indicate lower sedimentation rates. Since the map of Lake Erie has to be in the same scale of the original map that in Klump et al. (2005) (unpublished data), it was made using a Digitizer program that helps to produce the same scale of the map. Using thousands of data points on the original map with the Digitizer, the GIS program connected them to make an analyzable coordinate map, which was used to locate and identify the coring stations.

Along with sedimentation rates from Klump et al. (2005) (Online Table 2) and Matisoff (Online Table 6), several types of chemical and pollen data from Kemp et al. (1974) as reported in Klump et al. (2005) (Online Tables 3-5) were plotted into the GIS program. This was done for sedimentation rate and chemical element coring stations to produce contour lines for each map of the whole Lake Erie. For the complete Lake Erie map, two types of maps were created using the GIS application. First, natural neighbor plots showed contouring of sedimentation rates and chemical elements with different color shades to represent high or low rates of sedimentation or chemical concentrations at the surface (1 cm). The other map type was graphed using bathymetry plotting, as shown in Figure 2 (bathymetry of Lake Erie and Lake Saint Clair, 1998).

Figure 3 represents an overlay of sediment accumulation rates (color) and bathymetry (contours). This overlap provides a visualization of the potential significance that bathymetry is a major control on sediment focusing. For example, high sedimentation rates occurring at deeper locations are an indication of sediment remobilization and focusing, whereas high sedimentation rates at river mouths is an indication of little post-depositional remobilization.

As mentioned in the contouring method above, there are two types of contouring methods in GIS. One is called natural neighbor, which interpolates a raster surface from points using at most 15 million data points. An alternative method, called inverse distance weighting (IDW), uses at most 45 million points, though it requires higher point density. The results and procedure of generating these contours is modeled in the online supplement. After various maps of Lake Erie were made, Western Basin maps were produced separately. However, the Western Basin dataset consisted only of bulk density, organic matter, and phosphorous data from 13 different locations. This data was collected from Matisoff (Online Table 7). For the Western Basin map, the same Digitizer method was performed to create a base map of the Western Basin of Lake Erie. Then, the points of outline were connected through GIS application and the sample locations were plotted and graphed.

After bulk density, organic matter, and phosphorous datasets were plotted onto the map, the same natural neighbor contouring technique was used. A value of 'O' on the shoreline was used for the whole Lake Erie to force the contouring to follow the shape of the lake. This helps provide focus on how the sediments settle down and spread out in the lake.

Results

The data for each core is given in the online tables and is plotted in the figures below. All the maps of sediment accumulation rates and other chemical elements for Lake Erie and for the Western Basin are created using two different types of contouring options in GIS program. The Lake Erie maps came from using natural neighbor contouring technique, while both natural neighbor and IDW methods were used for the Western Basin.

Figure 3 shows the distribution of sedimentation rates in Lake Erie, based on Klump et al. (2005) sedimentation rate data with augmented data from Dr. Matisoff (Online Table 6). The map shows that high sedimentation rates occur in the Eastern Basin (0.04~1.09 g/cm²/yr), low sedimentation rates occur in the Western Basin (0.03~0.31 g/cm²/yr), and very low rates occur in the Central Basin (0.03~0.14 g/cm²/yr). Figure 3 also shows the sedimentation rate in color superimposed on the contour lines of bathymetry. The background color indicates sedimentation rates and the black lines are bathymetry contours. It is apparent that areas of high sedimentation rate occur in deeper water, indicating the movement of sediment from shallow depths to deeper locations in the lake, a process called sediment focusing.

Sedimentation rates determined by pollen are not based on concentrations. Instead, the change in pollen provides a time marker (around 1850) for when the forests were cleared for farming. Figure 4 shows the map of the sedimentation rates in Lake Erie based on the pollen data (Kemp et al., 1874, as reported in Klump et al., 2005; Online Table 3). Although there are some differences, the sedimentation rates obtained from the pollen data shown in Figure 4 are similar to those obtained by the radionuclide data shown in Figure 3. The high accumulation rates in the Western Basin are 0.431, 0.459, and 0.645 g/cm²/yr. According to Figure 4, the highest sedimentation rates occur in the Western Basin where the water depth is shallow, while lower accumulation rates occur in the Central Basin. However, higher accumulation rates are also seen in deep water in the Eastern Basin.

In Figure 5a, most high concentrations of carbon deposition are observed in the Central Basin (2.16~3.14 mmol g^-1 ), and moderate deposition is observed in the Western basin and Eastern Basins (1.72~2.06 mmol g^-1 and 0.55~2.06 mmol g^-1). The concentration of carbon deposition decreases as distance to the shoreline decreases. Notable carbon deposition is present in the Central Basin at a depth of 25 meters.

Figure 5b shows high concentrations of the carbon flux in the Eastern Basin (1.2~9.8 m^-2 yr^-1 ), while concentrations in the Western Basin and in the Central Basin are lower (0.62~3.07 m^-2 yr^-1 and 1.54~3.98 m^-2 yr^-1, respectively). The highest carbon flux occurs in the Eastern Basin of Lake Erie. Moderate carbon flux occurs in the Western Basin, where many nutrients are delivered by watershed river flow.

The nitrogen map in Figure 6a displays very similar contours as those in the carbon map. It shows high concentrations in the Central Basin (0.29~0.39 mmol g^-1) but somewhat lower concentrations in the Western and Eastern Basins (0.24~0.27 mmol g^-1 and 0.06~0.28 mmol g^-1, respectively). This figure further shows that most nitrogen deposits occur in approximately 25 meter deep of the Central Basin.

The nitrogen flux map in Figure 6b is very similar to the carbon flux map. The contouring of nitrogen flux shows high fluxes in the Eastern Basin (0.06~1.48 m^-2 yr^-1) and moderate fluxes in the Western and Central Basins (0.06~0.74 m^-2 yr^-1 and 0.19~0.51 m^-2 yr^-1, respectively). This figure shows that most nitrogen flux occurs in deep waters of the Eastern Basin. Less flux occurs in shallow waters of both the Western and Central Basins.

Bulk densities are high near the western shoreline and the Lake Erie islands, as shown in Figure 7a. Intermediate bulk densities are found north of the islands and in the middle of the Western Basin, which spreads toward the shoreline. The highest bulk densities are found on the west side of the Western Basin (0.79~1.55 g/cm³), while moderate bulk densities occur near the islands (0.39~1.18 g/cm³). In Figure 7b, the highest organic matter concentrations are located in the middle of the Western Basin with concentrations as high as 7.469.25%. Organic matter concentration decreases towards the shoreline to a minimum of 1.3%. This figure shows high phosphorous depositions near the Maumee Bay and across the central portion of the Western Basin (0.71~0.98 mg TP/g). The contouring of phosphorous gradually decreases towards the northern shoreline (0.7~0.13 mg TP/g).

Discussion

There are several important points found in the GIS mapping. By creating the Lake Erie map using sedimentation rate, pollen, and other chemistry data, a strong correlation between sediment deposition and bathymetry called sediment focusing was observed. When water is deeper, sedimentation rates increase, as reflected by high concentrations and fluxes in the Eastern Basin and lower sedimentation rates in the shallower Western and Central Basins.

Another finding in the complete Lake Erie map was the similar pattern of contours between the map of sedimentation rates determined by radiochronology and that produced by pollen, nitrogen, and carbon analysis with the natural neighbor contouring method. Sediment accumulation rates determined by radio-chronology and pollen dating showed high accumulation rates in the Eastern Basin and low accumulation rates in the Central Basin.

The maps plotting nitrogen and carbon data also show very similar contour patterns. The highest concentrations were observed in the Central Basin for both maps (0.4~3.13 mmol g^-1). The Eastern and Western basins have lower concentrations compared to the Central Basin. Lastly, in the maps using nitrogen flux and carbon flux, the pattern of contours is nearly the same. These maps represent not only a similar shape of contours, but also high values of nitrogen flux and carbon flux in the Eastern Basin.

Unlike the map of the whole Lake Erie, the patterns of contours using the bulk density, organic matter, phosphorous, and fluxes in the Western

Basin are slightly unique, possibly due to uniform shallowness in the basin. For example, in Figure 7d, the map shows high fluxes of phosphorous in the west side of the Western Basin since most phosphorous comes from Maumee River in west. According to the Great Lakes Water Quality Agreement (1972), the total load limit set for Lake Erie is 11,000 tons/year, with a target reduction of 40% in the 2012 Amendment. However, of the 11,000 tons of load, 7012.366 tons are deposited in the sediment (Horne et al.). The annual depositional flux of phosphorous flux can be calculated from the daily depositional flux and the area of the Western Basin:

Flux (mg/m²/day) * Area of Western Basin (m²) * 365 (day/year) = 7.012366 10¹² mg/y = 7012.366 tons.

Finally, the result of maps using sedimentation rates, pollen, carbon, carbon flux, nitrogen, nitrogen flux, organic matter, bulk density, phosphorous, and phosphorous flux data can be used as an input into computer models to make better predictions for algal blooms, which would help prevent high frequencies of dead zones.

Acknowledgments

I would like to thank Dr. Matisoff in geology department. He provided help for choosing the data, advising, and helping the data calculating and analysis.

Supplementary online tables for this article can be found on our website at http://case.edu/discussions/archives.html

Biography

Jinyu Seo is a fourth-year student studying environmental geology at CWRU. He is involved with CWRU Geology Society and Korean Student Association. He wishes to study more about surface water and problems affecting our water supply in graduate school.

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