Quantification of Reactive Phosphorus in Lake Mendota Sediments

Transcription

1 Quantification of Reactive Phosphorus in Lake Mendota Sediments By: Jeremy, Advisor Dr. Mike Penn Most of the phosphorus (P) that enters lakes from external inputs is attached to soil or sediment. This sediment settles to the bottom of the lake where it accumulates. Over time, the P attached to this sediment may be released into the lake through various chemical and biological reactions. Simply reducing external inputs of P (runoff, fertilizer, manure) may not be enough to immediately lower P levels within the lake. In some cases, lake P concentrations remain high even after external inputs are decreased. This is caused by internal inputs of P that occur within the lakes. These internal inputs, specifically inputs from reactive sediment P release, are the subject of my research. Reactive P is a fraction of the total P bound to the sediment that can be recycled into the lake. The release of recycled P from sediments increases the internal loading of P in the lake and therefore increases P concentration throughout the entire lake. Therefore, it is necessary to identify the fractional amount of reactive P compared to total P found in the sediment. The purpose of my research is to determine what percentage of this total P is bio-available (potentially contributing to algal growth). Introduction Phosphorus (P), a naturally occurring nutrient, is essential for all life forms. As the agricultural industry continues to expand, supplemental additions of P (beyond naturally occurring levels) have occurred in order to increase production in crop and livestock enterprises (Sturgul, 2002). Supplemental additions of P have also occurred from urban stormwater runoff due to increased development. The increasing level of P usage causes concern due to the impact the nutrient has on surface water resources. Phosphorus is the limiting nutrient in freshwater environments (Cooke, 1993), thus it has the greatest influence on the amount of biological growth, such as algae and other aquatic vegetation, which occurs in a given body of water. The growth of this aquatic vegetation is often limited by the amount of P present. Consequently, the more P that is present, the more biological growth will occur. The Big M, Vol IV,

2 The increase in biological production in turn accelerates the eutrophication process of a water body. Eutrophication is defined as the natural aging of lakes or streams (Sturgul, 2002). High levels of eutrophication often lead to significant decreases in water quality. For example, the increase in biological growth means a considerable increase in the amount of organic material present. This organic material eventually decays, and as decomposition occurs, the bacteria breaking down the organic material use dissolved oxygen found in the water. As levels of dissolved oxygen decrease, fish and other aquatic organisms are killed or impaired. In addition, certain types of blue-green algae have been found to form potent toxins that can potentially cause taste and odor problems, interfere with drinking water treatment, or even pose as a health hazard to humans and livestock. High levels of eutrophication can also have substantial economic impacts on tourism and recreation (Sturgul, 2002). Lake Mendota in Madison, WI is one such eutrophic lake. In order to solve this problem, the Lake Mendota Priority Watershed project was created to substantially decrease the concentration of P in the lake. The project is budgeted at just under $18 million (DNR, 2000). When spending this much money to decrease nutrients such as P from entering Lake Mendota, it is necessary to understand the P cycle once the P enters the lake. Simply reducing external inputs of P (runoff, fertilizer, etc.) may not be enough to immediately lower P levels within the lake. In some cases, P concentrations remained high after external inputs were decreased (Cooke, 1993). This was caused by internal inputs of P that occur within the lakes (Cooke, 1993). These internal inputs, specifically inputs from sediment P release, are the subject of my research. The Big M, Vol IV,

3 Purpose of Research As P enters the lake from external inputs, it often attaches to soil or sediment. This sediment often settles to the bottom of the lake where it accumulates (see Figure 1a). Over time, the P attached to this sediment may be released into the lake through certain chemical and biological reactions (Sturgul, 2002). The release of the attached P is shown in Figure 1b. This release of sediment P into lakes has been found to contribute up to 60-80% of the total P concentration within the lake (Korienk, 1995). Since sediment P is a large potential contributor, sediment cores from the bottom of Lake Mendota were collected to study the accumulation of P over time. The depth of the layers of sediment from the lake floor profile can be correlated to a time of deposition. Figure 1: Accumulation of sediment (a) and release of reactive P into lake (b). The purpose of my research was to study these sediment profiles in order to better understand the impact the release of sediment P has on the lake. As stated before, numerous sediment cores were previously collected from the bottom of the lake. These profiles were approximately 65 cm in depth. Total P levels were measured at incremental depths and recorded. There are many chemical forms of P and the sum of these is measured by an aggressive digestion procedure to determine total P levels (Penn, 1995). However, not all of the P forms are necessarily harmful to the lake. The purpose of my research was to determine what The Big M, Vol IV,

4 percentage of this total P is potentially contributing to eutrophication, and what percentage poses no immediate threat to the lake. The type of P that is potentially harmful to the lake is known as bio-available P (Sturgul, 2002). This P is often readily available to be consumed by plants or algae. Non-available P is not an immediate threat to the lake and is often incorporated in inorganic material. It is necessary to identify the two P fractions to quantify the bio-available P that is potentially released into the lake. This release increases the internal loading of P in the lake and therefore increases P concentration throughout the entire lake. As a result, to meet water quality goals, external P loadings must be decreased further in order to compensate for this impact. This could have a potential impact on the levels of external inputs of P that are set by the Lake Mendota Priority Watershed project as well as the financial assistance required to reach those levels. Methodology The experimental method utilized to distinguish between the two types of P involved the use of a 0.1 N sodium hydroxide (NaOH) solution as an extractant. The sodium hydroxide solution is one method to extract (estimate) the portion of sediment that can be recycled out of the sediment (i.e. bio-available) and result in internal loadings to the lake. Sodium hydroxide was chosen because it is a fairly weak extractant and was therefore thought to be comparable to chemical or biological reactions that could occur within the lake and cause the release of P. The total amount of P attached to the sediment consists of four different kinds of P. Figure 2 shows the components of total sediment P. The NaOH extract removes most of the adsorbed inorganic P (1) and readily degradable organic P (3) from the total sediment P. The amount of adsorbed inorganic P removed is called NaOH reactive P, or NaOH-rp. The amount The Big M, Vol IV,

5 of readily degradable organic P removed is known as NaOH non-reactive P, or NaOH-nrp. The amount of NaOH-nrp is found by taking the total amount of P extracted from the sediment (NaOH-TP) and subtracting the NaOH-rp. The NaOH-nrp requires digestion in acid and an oxidizer in order to make it reactive. To determine NaOH-rp, approximately 0.05 grams of each sediment sample was placed in 50 milliliters (ml) of 0.1 N NaOH solution for 16 hours (see Figure 3). Figure 2: Types of P present on sediment. Figure 3: Sediment samples place in NaOH extract. After sitting in the extract for 16 hours, the sediment was filtered and a reagent was added to the remaining solution (containing the extracted reactive P). The added reagent reacted with the reactive P in the solution and turned the solution different shades of blue depending on the concentration of the reactive P in solution. The samples with higher P concentrations turned a darker blue while samples with a low P concentration were lighter blue. Figure 4 displays different concentrations of P in solution. The Big M, Vol IV,

6 Figure 4: Samples with different concentrations of P. After the reagent was added to each solution and the solutions had turned different shades of blue, a spectrophotometer was used to determine the concentration of each sample. The sample readings obtained from the spectrophotometer were compared to values from standards created using dilutions of a stock solution of P. Comparing the samples to the standards, the concentration of P from the sediment samples could be determined. Concentrations of the sedmient P were computed in millgrams (mg) of P per kilogram (kg) of sediment. Results To determine the fraction of reactive P compared to total P, the concentrations of the NaOH extracted sediments, which are NaOH-rp (reactive), were compared to the concentrations for total sediment P. Total P concentrations used were obtained from previously measured values calculated by UWP students. To justify the use of the previously calculated values, several total P values were determined experimentally and found to be comparable to the values obtained by the students. Figure 5 shows the plot comparing the NaOH extracted reactive P to the total sedmient P. Figure 5 represents samples from the University Bay sampling site (one of four sites sampled throughout Lake Mendota). The Big M, Vol IV,

7 Figure 5: Plot of P concentration versus depth for NaOH extracted P and total P. To better display the percentage or fraction of total P that the reactive P represents, the concentrations were shaded in (see Figure 6). The Big M, Vol IV,

8 Figure 6: Fractional amount of NaOH extracted P (reactive) compared to total P. According to Figure 6, the reactive P (NaOH extracted) comprises approximately percent of the total sediment P. The results also show that this percentage is largely variable with depth (age). This variance with depth could be attributed to a number of factors, such as an increase in external loading, which could affect the amount of reactive P released. After determining the percentage of reactive P to total P in the lake, the values were also compared to reactive P and total P found in streams feeding into the lake. Because insufficient samples were available for the first 10 cm of depth (see Figure 6), sediment samples from contributing streams were analyzed. Assuming that the stream samples would be comparable to the uppermost layer of sediment in the lake, the reactive P and total P concentrations from the stream sediments were plotted on the same graph as the lake sediment values. Figure 7 shows the comparison of stream sediment P concentrations to lake sediment P concentrations. The reactive P represents about 80 percent of the total P attached to the sediment in the streams. Average Stream Sample The Big M, Vol IV,

9 Figure 7: Lake sediment P concentrations compared to stream sediment P concentrations. Conclusions Based on the results of this research, conclusions could be made regarding the impact the reactive P has on the total P concentration within the lake. As shown in Figure 7, the stream sediments had a significantly higher amount of reactive P still attached to the sediment. With the total sediment P concentration being similar for the lake and stream sediments, the difference in reactive P concentrations shows that P must be being released into the lake. Otherwise, the concentrations should have been similar in the stream sediments and lake sediments (assuming that the stream sediment sample is a good representation of the top layer of the lake sediment). Because the reactive P concentration is so much higher in the streams than in the lakes, something must be happening to the P in the top 10 cm of the lake. Therefore, the conclusion was made that a large amount of the reactive P is being released into the lake. The conclusion made supports the theory that P attached to sediment can in fact be released back up into the lake. The released P increases the internal loading to the lake which in turn increases the overall P concentration of the lake; leading to problems with algal growth and eutrophication. Further studies will need to be conducted to better understand what exactly is happening in the top 10 cm of the lake. In addition, studies will need to be performed to determine the percentage of bioavailable P (for algae and plants in lakes) that may be NaOH-nrp (non-reactive). While bioavailable P is assumed to contain only NaOH-rp (reactive), some studies have shown that it may also contain NaOH-nrp. Therefore, studying the percentage of bio-available P that is NaOH-nrp in Lake Mendota may prove to be one of the valuable steps in determining the impact the internal loadings of P and how they can be controlled and eventually reduced. The Big M, Vol IV,

10 Bibliography Cooke, Dennis G., Eugene B. Welch, Spencer A. Peterson, and Peter R. Newroth. Restoration and Management of Lakes and Reservoirs. Second Edition. Boca Raton: Lewis, Holtan, H., L. Kamp-Nielsen, and A.O. Stuanes. Phosphorus in soil, water and sediment: an Overview. Hydrobiologia. 170 (1988): Lake Mendota Priority Watershed Project: Project Summary. Wisconsin Department of Natural Resources. April, Penn, Michael R., Martin T. Auer, Eric L. Van Orman, and John J. Korienk. Phosphorus Diagenesis in Lake Sediments: Investigations using Fractionation Techniques. Mar. Freshwater Res. 46 (1995): Sturgul, Scott J. Understanding Soil Phosphorus. Madison: Cooperative Extension, The Big M, Vol IV,