Internal P. Loading in Shallow

Internal P. Loading in Shallow Lakes

Internal Phosphorus Loading in Shallow Lakes

Improving water quality in shallow lakes has been an issue of concern for many years. It used to be assumed that a majority of the phosphorous loading in shallow lakes stemmed from external source. External sources include wastewater and industrial wastes. The old paradigm was that if one could reduce the external loading of phosphorous, then the lake should clear.

However, it soon became clear that in some cases lake water quality did not improve as expected, even when externally loaded phosphorous had all but been completely eliminated. This led aquatic scientists to explore other mechanisms for phosphorous loading.

Once the lake sediment is loaded with P, the potential for internal loading is increased. Natural sources of P. contribute to the whole. These sources used to be considered inconsequential. However, literature suggests that natural mechanisms play an important role in the ability to reduce P. levels in a shallow lake, more so then in the past. They can enhance or limit remediation methods from dredging to chemical elimination of P. In the water. The primary concern in using artificial methods to clear a lake is that it may cause more harm to the ecological system of the lake in the long-term. This study revealed that reducing the amount of P. entered into a lake is the most effective means to control it. After that there are methods that can help speed the natural processes of lowering P. levels in lakes, but these methods still take many years to reduce P. levels to a healthy level.

Introduction

Internal loading occurs when phosphorous attaches itself to organic matter in the lake, and unstable iron sediments. Phosphorous accumulates when phosphorous levels are high. They are then released into the water as the substrate deteriorates, resulting in a steady loading of phosphorous in the lake. The internal loading potential of the lake is highly dependent on the external loading history of the lake (Sondergaard et al., 2001). This means that the internal phosphorous loading potential of every body of water is unique. The biological structure of the lake is an important factor in its phosphorous loading ability (Sondergaard et al., 2001).

Cutting the amount of phosphorous externally loaded is an important factor in the ability to reduce total available phosphorous in the lake. In order to set reasonable limit on the amount of phosphorous externally loaded into a waterway, the U.S. EPA has set a Total Maximum Daily Load (TMDL), which is the maximum amount of pollutant that a waterway can receive and still meet water quality standards (U.S. EPA, 2007). TMDL's are determined by individual states, territories and tribes. They are determined by the usage of the waterway, including seasonal usage (U.S. EPA, 2007). The TMDL reflects the sum of the loads of a single pollutant from all sources in the waterway. The EPA includes a margin of safety to ensure that the waterway is useful for its intended purpose (U.S. EPA, 2007). The establishment of TMDL's is a part of the Clean Water Act, section 303.

In order to understand the internal loading mechanism of shallow lake, one must first gain an understanding of the P. fractionation profile of the body of water (Farmer et al., 2006). Phosphorous occurs in many different forms within a waterway. For instance, in Loch Leven a core sample revealed that phosphorous occurred in loosely bound, reductant-soluble, oxide-absorbed, organic, apatite-bound and residual (Farmer et al., 2006). All of some of these may be present in any given waterways. Fractionation refers to the percentages of these various forms of phosphorous, as they relate to the total phosphorous in the lake. This will be unique for every body of water and may vary within that body of water if the body is large enough.

When phosphorous enters a waterway it will settle to the bottom. When it reaches the bottom, it begins to undergo chemical and biological changes that eventually lead to the dissolution of the P. into the waterway (Wassman & Ollie, 2004). There are several factors that influence the exchange of P. between sediment and water. These factors include:

Molecular diffusion

Temperature

Water Turbulence

Gas ebullition

Bioturbation

In stagnant waters molecular diffusion is considered to be the primary mechanism for P. release (Wassman & Ollie, 2004). There is usually a marked difference in P. concentrations between the overlying water column and surface pore water. The degree of differences in concentrations are directly proportional to the rate of diffusion of P. into the water (Wassman & Ollie, 2004). Temperature has a major effect on the release rate of P. from sediment to water. This is largely due to the effects of temperature on microbial action (Wassman & Ollie, 2004). Increases in temperature increase the metabolic rate of bacteria, resulting in a higher rate of mineralization of organic matter (Wassman & Ollie, 2004).

Another factor that influences the rate of diffusion of P. into surrounding waters is the rate of physical water movement (Wassman & Ollie, 2004). This is a much faster P. transport system than diffusion. There are many factors that influence the rate of waterflow. They include the amout of turbulence at the sediment-water interface, gas convection, seepage, and organisms (Wassman & Ollie, 2004). Gases can be the result of geological forces, or the result of the processes of microbial reduction. Movement of water caused by gases produced by microbial reduction are referred to as bioturbation and can have an effect on physical water movement. This can also effect the ability of P. To bind in sediments (Wassman & Ollie, 2004).

Shallow lakes are often inhabited by a variety of creatures. When one discusses how they effect chemical processes, microbial action is most likely the mechanism being mentioned. However, macro-invertebrates such as crawfish, shrimp, mollusks, and crabs can have a significant impact on P. dissolution into the surrounding water. When they are present in higher population, they can have a dramatic effect of P. dissolution into surrounding water. These vertebrates influence P. release by creating turbulence in the sediment. They dig, burrow, and kick up the sediment as they move along the bottom of the lake. They increase the contact between the interstitial water and the overlying water. There is also a chemical reaction by their respiration, feeding and waste elimination processes (Wassman & Ollie, 2004).

Management and restoration of the waterway hinders on the ability to limit available P. from external loading. While internal loading plays a role in maintaining higher P. levels, internal loading is limited by the amount of P. made available through external loading. A small fraction of internal loading of P. can be fully attributed to organic sources. Many of the organic processes produce little P, as compared to the dumping of industrial wastes. The biggest factor in reducing high levels of P. In lakes is to limit the available of P. from externally loaded sources.

Phosphorous forms in Sediment

Phosphorus is the key element that limits natural cycles in streams and wetlands (Reddy et al., 1999). Mass reactions and balance are useful in estimating long-term reactions, but are limited in their ability to categorize short-term effects (Reddy et al., 1999). This is largely due to changes in the disposition of various fractions within the Total P. There are many types of fractionated forms of P. In lake systems including loosely sorbed P, reductant soluble P, Ca-P complexes, Apatite P. And many other organic and inorganic P. compounds. The goal of remediation is to reduce the total amount of P. available in the system. However, each lake has its own characteristic combination of fractionated P. compounds.

Understanding the fractionated P. profile of a lake is the first step to reducing the total P. Of the lake system. For instance, if the highest concentration of P. is in loosely sorbed P. within the sediment layer, then physically removing a portion of the lake floor may help to lower the total P. In the waterway. If a majority is in Ca-P then chemical methods may be most effect in removing excess P. from the water column.

The magnitude of P. released into the waterway depends on a number of variables. The process by which P. enters the water column for removal is not one-way. When external P. falls into the sediment the process begins to reintegrate this P. into the water column. This occurs via a number of chemical and biological processes. However, sometimes it may reintegrate with the sediment again, or sometimes many times over before finally entering the water column for removal.

Effective release of P. from sediment depends on a number of variables. Some of these processes are biological. Others are geological and chemical. Samples from two lakes were incubated in controlled oxygenated and anoxic conditions. These samples were then tested for all fractions of P. release. Similar release rates for NH4Cl-RP and NaOHCl - RP were found in both lakes. However differences in Calcium bound P, BD-RP, and HCl - RP were found between the two samples. This study demonstrates that different total P. fraction releases may differ between two bodies of water under similar oxygen conditions (Kisand & Noges, 2003). This study is important in that it highlights the complexity of understanding P. fractions in any given body of water. There are a multitude of potential reactions in any body of water. Oxygen plays a role in the reactions of any individual lake, but one cannot make predictions based on oxygen level alone.

Shallow lakes differ from stratified lakes in many ways. A stratified lake typically reaches equilibria in such a manner that it becomes divided into regions. This is not the case with shallow lakes. With a shallow lake, the entire lake may change from clear water to macrophyte dominated to algae dominated, each phase has its own state of equilibrium (Dokulil & Teubner, 2003). Total chlorophyll to phosphorus ratios are different in these various states of equilibria. Light levels tended to contribute significantly to the chlorophyll to phosphorus ratio. Lakes that were heavy in macrophytes tended to be higher in chlorophyll, as opposed to phosphorus. However, in lakes where light was restricted by algae, levels of phosphorus tended to be higher (Dokulil & Teubner, 2003). This demonstrates plant stocking levels influence water light levels, which in-turn has an effect on chlorophyll to phosphorus levels. Light levels can effect P. fractions in shallow lakes, more so than in stratified lakes.

Phosphorus Release Mechanisms number of mechanisms affect the total P. In a lake. Chemical processes may account for soluble P. In the water column. Some of the processes are biological and some are geological. The total P. available is a limiting factor in all of these mechanisms. This is a key concern when establishing load limits for a lake. Limiting the external P. load of the lake has a direct impact on the ability of the lake to internally load

Chemical Processes.

Shallow lakes may contain high concentrations of oxygen due to greater contact with wind turbulence. As oxygen diffuses into the bottom sediments, a highly oxygenated layer is created. This layer can provide fuel for a variety of chemical reactions, all of which represent different fractionated contributions to P. loading of the waterway. Underneath the oxygenated layer is a redox layer. This layer was the topic of experiments by Olila & Reddy (1997) conducted in a natural setting using two different lake systems.

This experiment found that redox potential had little effect on the stability of NaOH - P and Ca/Mg/P fractions in the lake. However, increases in ortho-P and NH4Cl-P were observed. Loosely bound P. And labile organic P. were found to be highly reduced. P uptake by bottom sediments at elevated P. concentrations in the water column was found to be due to the formation of Ca-P (Olila & Reddy, 1997). Conditions and mechanisms of uptake and resortion of various P. compounds was found to be different in the two lakes studies. This suggests that each site must be studied and understood under its own merit. One cannot generalize about mechanisms of P. release. It was once thought that calcareous systems could over-ride the presence of Fe or Al in regards to P. uptake. However, this experiment demonstrated that even in a calcareous system, the presence of Fe and Al significantly impact the regulation of P. uptake and geochemistry of the water body.

Physical Processes.

In at laboratory setting, the effects of varying concentrations of P. In bodies of water was examined. It was found that P. concentrations seek to establish equilibrium. When two different bodies of water are mixed, the P. will flow from the water of higher concentration to that of lower concentration (Koski-Vahala & Hartikainen, 2001). The experiment also demonstrated that increased pH increased the mobility of the P. between bodies of water. This laboratory experiment helps add to the body of knowledge regarding how P. suspends and re-suspends in a body of water. This concept is yet to be tested in a field study. However, it does add to our understanding of the mechanisms of internal P. loading.

High percentages of organic matter have been found to increase P. mobilization from sediments. Seasonal fluctuations were found, which supported this hypothesis (Burger et al., 2007). This hypothesis is supported by field and laboratory results from a number of studies. Organic matter can be a result of plant infestation or animals in the waterway. P can be suspended and re-suspended many times in a cyclical manner. This makes it difficult to lower P. levels in lakes. P contained in sediment can be churned, bringing a larger surface of the sediment in contact with an anaerobic layer on the bottom of the lake. This process can increase internal loading of P. within the water system.

Biological Processes.

Community organization within a lake can have a dramatic impact on the ecology of the lake (Persson & Svenson, 2006). I an experiment using field enclosures, various communities of fish were introduced to ponds that previously did not contain any fish. Sediments were observed and analyzed after introduction of the fish species. Analyses were conducted to examine sediment composition, water column composition and exchange between the sediment and water column. It was found that the introduction of fish had a significant impact on the nitrogen and phosphorus levels in the different environments. It was found that fish had a direct impact on their environment due to resuspension of compounds due to excrement (Persson & Svenson, 2006)

Plants have long been thought to clean waterways. However, as efforts increased to significantly decrease sources of externally loaded waterways, there were concerns that in certain waterways P. levels remained high, despite efforts to reduce external sources. Core samples were used from different sites to test the hypothesis that macrophytes increase P. load during the growing season. This experiment failed to yield the results expected. It was found that site specific conditions influenced the results obtained. However, it was found that when macrophytes do have a significant effect on P. levels, it is to increase, rather than to decrease them (Stephen et al., 1997).

Plants effect their environment in many ways. They contribute to rapid build up of sediment containing high organic matter. Oxygenation processes of their roots help to produce conditions that mobilize P. However, there are several factors that limit a plant's ability to release P. into the waterway. For example, the presence of FE (III) was found to increase P. levels when reducing conditions are present. As FE (III) is reduced to FE (II), P is mobilized into the waterway (Stephen et al., 1997). However, under oxidizing conditions with FE (III), P is immobilizesd in the sediment layer (Stephen et al., 1997). It was once thought that this mechanism explained P. release into waterways. However, this process is considered to be one of many processes that release P. into the water column. High concentrations of NO3 have also been found to reduce P. release (Stephen et al., 1997).

Internal phosphorus loading in Danish shallow lakes was found to be 2-4 times higher during the summer months than during the winter months (Sondergaard et al., 1999). No conclusive explanation was provided for this phenomenon. There are several factors that could have influenced these results. The first is that temperature may have had an impact on the number of reactions between P. fractions. Another potential explanation may lie in the differences between plant growth in the summer and winter months. This study concluded that internal P. loading can have a significant impact on the Total P. Of a shallow lake.

Management and Remediation

Excess P. levels are one of the greatest threats to the ability of the lake to establish equilibrium. Finding effective management techniques and remediation of lakes that have high total P. levels is a priority for government agencies. High P. levels threaten water quality and the ability of humans to use this water to their benefit. There are many theories and thoughts regarding what constitutes best practices in the remediation of high P. levels in lakes. Theories have developed into three major categories of thought. The first is the use of physical means such as dredging and flushing to remove sediments containing high concentrations of P. Another method is to use agents such as Aluminum Sulfate or gypsum to bind P. In the water system. Yet, other methods have focused on long-term reduction of external loading as the ultimate solution to the problem.

Management of Phosphorus concentrations in shallow lakes has concentrated on the reduction of P. In the lake. However, due to the effects of internal loading, this may not be enough in many circumstance (Spears et al., 2006a). A combination of primary and secondary treatments may be beneficial in producing the desired result. The ultimate success of primary strategies depends on the conditions that exist between sediment and the water column (Spears et al., 2006a). Primary strategies involve reduction of P. release into the lake.

There are a number of secondary strategies that have been proposed. For instance, there has been as suggestion that flushing the water column until it is clean may pose one solution to speeding the clearing of the water column (Spears et al., 2006a). Flush rates vary from lake to lake. Thos with a low flush rate are likely to maintain their internal P. loading longer than those with a higher flesh rate (Spears et al., 2006a). Managing flush rates Iakes may help to clear excess P. from the lake at a faster rate.

There are many factors that may reduce or enhance the effectiveness of managing flush rates in lakes. For instance, algae effect oxygenation levels in lakes, thus increasing the available chlorophys. Variations in reduction-absorbed P. was found to have a direct correlation to Chlorophyll in the sediment layer (Spears et al., 2006b). Managing flush rates in lakes that contain a high level of algae many be less effective than in lakes that are clear or have a high concentration of macrophytes or fish.

Wetland, Flush Rates and Dredging

Wetlands can function as natural sources or sinks for nutrients such as N. And P. Some have proposed using wetlands as a potential sink for P. To reduce load on the waterway (Fisher & Reddy, 2001). Flooding causes shifts in P. concentration within a wetland and can result in a release episode of P. into the water column (Novak et al., 2004). Wetland soils characteristically have low turnover rates for organic matter. Before wetlands can be used, the direction and magnitude of the flow must be determined. There are many variables with this method that cannot be controlled. However, in some cases this may prove to be a feasible method for removal of P. from adjacent waterways.

Lake Ringsjon is a case where the effects of years of P. loading by humans has been difficult to remediate using any means. The sediment in the bottom of this lake contains a high level of P. In many fractionated forms, ready to be released into the water column. High levels of fish and plant life add large amounts of organic matter to the mix. It may be many years before P. levels are reduced significantly in this waterway (Graneli, 1999). This is a prime example that demonstrates the difficulty in correcting the problems that we have created in the past. It also emphasizes the importance of discontinuing practices that raise P. levels in lakes. It is much easier to prevent the problem, than to correct it once it occurs.

There may be processes within the lake that render methods of remediation ineffective. In the case of lake Ringsjon, using natural wetlands or changing the source/sink status of the lake would be ineffective in reducing the P. loading. Changing flush rates could churn up the high-phosphorus containing sediment and make the problem worse, not better. Lake Ringsjon is an extreme case that suffered from years of high levels of external P. loading. When the sediment of the lake contains such as high concentration of P, there is little that can be done to reduce it but wait for it to reduce naturally and this may take decades.

Internal loading can continue for many years, even decades after external sources of P. have been eliminated. It has been suggested that dredging out P. containing sediments can significantly reduce the time of internal leading and help the waterway return to lower levels of P. more quickly (Sondergaard et al., 2001). Changing the source/sink status of the site may also help to remove accumulated. In Lake Erken, the cyanobacteria Gloeotrichia was found to increase Summer internal P. loading (Pettersson, 1998).

Chemical Removal

Some have suggest the addition of iron or alum to elevate the sorption capacity of sediments. However, an important prerequisite for achieving long-term benefits to water quality is a sufficient reduction of the external P. loading. (Sondergaard et al., 2001).In one study involving a shallow lake, aluminum sulphate was added in an attempt to reduce Soluble reactive phosphorus. However, the aluminum sulphate was not added in sufficient quantities to reduce the amount of P. In the lake. There was a short-term reduction, but internal loading soon returned levels to their former levels (Hullebusch et al., 2002). One of the factors that influenced the result was that this experiment had a history of high P. levels before the treatment. Aluminum suphate has no known toxic effects on wildlife. Therefore it is considered safe for an aquatic environment. This conclusion was arrived using modeling only, and has not been tested in a natural environment.

Aluminum Sulphate added to anaerobic and aerobic lakes with high externally and internally loaded P. demonstrated that chemical treatments vary according to water conditions. It was also found that the addition of gypsum responded differently according to water conditions within the lake (Haggard et al., 2005). This study found that anaerobic water responded almost 4 times better than aerobic water.

The use of chemical methods for the remediation of P. In lakes is questionable for many reasons. This method is expensive and may not be effective, based on the natural chemical characteristics of the lake. There are many natural factors that may limit its effectiveness. Those that support this method of removal indicate that it has no harmful effect on local fish and plant populations. However, these assumptions are based on toxicological data, rather than field observations. It is difficult to take action based on the theory that a practice poses no harm, when in fact we have no way of knowing the effect that it may have.

Even if harm does not come from direct exposure to the agent, it may have an effect on wildlife due to changes in water pH, or nutrients in the water. The actions of chemical agents differ according to the water characteristics of the water. These methods may cause harm in one waterway, but not in another. Extensive studies of the waterway will have to be conducted in order to determine the exact potential effects on any particular waterway for which this method is proposed..

Summary

Key research findings agree that the key to reducing P. In water lies in reducing the amount of externally loaded P. from man-made sources. It was once thought that if we could effectively reduce the amount of P. being dumped into waterways, then the problem would quickly clear up. However, even after successful efforts to reduce the amount of P. dumped into waterways, it was found that levels remained high, and in some cases continued to climb. This led to the investigation of internal methods for P. loading in shallow lakes.

It is now known that the same principles that apply to stratified lakes do not necessarily apply to shallow lakes. Shallow lakes behave differently due to the effects of wind and the degree of contact, creating a more highly oxygenated aquatic atmosphere. Every shallow lake is unique. Field research conducted on shallow lakes presents a problem in terms of policy and recommendations. The experimental results only apply to that particular lake. As we saw in some research, the results of the experiment were highly dependent on individual conditions in the lake itself. This makes it difficult to generalize the information obtained from the experiment.

The individual environment of the lake makes it difficult to draw inferences and make predictions as to what a particular result will be in a different lake. However, these experiments do add to the body of knowledge about shallow lakes by demonstrating that a factor must be considered in the evaluation of other lakes. For instance, studies that varied due to temperature or pH demonstrated that this factor needs to be considered in the design of remediation methods.

One of the most difficult aspects to aquatic research is that the researcher has little control over variables that may effect the outcome of the experiment. Laboratory experiments offer the researcher the ability to control variables and to more precisely isolate the dependent variable. However, laboratory experiments may not apply to field conditions where a number of unforeseeable factors can influence the outcome. This was one of the key observations that arose through conduct of this literature review.

One of the most important questions that has arisen from a study of internal loading of P. is how significant it is to the entire picture as a whole. A study conducted on the St. John's River Estuary compared aerobic and anaerobic core samples from the sediment layer of the lake in an attempt to estimate how much internal loading contributed to P. levels in the waterway. The results of this study found that natural sources of P. can contribute a significant portion of the load and should be considered when setting TMDL limits (Malecki et al., 2004).

This study was a significant contribution to the body of knowledge regarding load limits in waterways. Although the results of this Study are limited to the waterway used in the study, they concept can be applied to remediation of waterways in general. It was suggested that as the P. level in the waterway decreased, anaerobic P. loading should decrease (Malecki et al., 2004). Loads from natural sources have often been discounted as insignificant to the reduction of P. In waterways. However, this study demonstrates that we must consider natural sources of P. In the total load when planning remediation strategies for any waterway

Analyzing the Unknowns

The most important factor that was discovered through the course of this research is that there are significant number of variables that could affect the outcome of out efforts. Some of these variables can be controlled to a certain degree. For instance, we can control the amount of externally loaded P. that we release into out waterways. We can then institute secondary methods such as controlling the flush rate of the waterway. We can use additives in the water to reduce the amount of P. present. This most difficult factor in the implementation of these measures is that there are many processes that we cannot control, but that can effect the outcome.

We cannot control the geological processes, or biological processes that occur within the lake. We are limited by these processes in our ability to affect a positive outcome. If we attempt to take any drastic measures to control P. levels, we run the risk of changing the ecology of the lake in such as way that it could have devastating effects on the wildlife. In some cases this could dramatically affect the usefulness of the lake to the humans that use it.

The only factor that we know for certain is that any measure that we take will have some effect on the ecology of the lake. From this perspective, using the most natural means possible is the best solution. However, if we begin to use wetlands as a filtration method, then we will contaminate the wetlands. Both the wetlands and the lake would have to be monitored for adverse changes in their chemistry or ecology. In some cases this may work and prove to be a sustainable method for long-term reduction of P. However, the sustainability of this approach is highly dependent on the chemical, biological, and geological forces active within the lake. The feasibility of this method would have to be assessed for each proposed site.

Conclusions/Recommendations majority of the research agrees that external loading of P. poses much more of a risk to our watersupply than does internal loading mechanisms. We are the greatest contributor to high levels of P. In the waterway. That is also the one factor over which we have the most control. There are many factors that we cannot control, which makes those that we can control even more important.

The EPA sets TMDLs conservatively in an effort to reduce P. To the greatest extent possible. However, once the TMDl is set, the EPA does little to monitor the waterway. One of the most obvious gaps in the EPA plan for setting TMDLs is that there is little provision for monitoring and enforcement once the levels have been established. There are many reasons for this, one of which may be a lack of staff and budgeting for such activities.

Once TMDLs are established, it cannot be assumed that these levels will remain the optimum minimal levels of pollutants. There are many variables that could cause the biological load of the lake to increase or decrease. There may be times where changes necessitate the lowering of TMDLs to accommodate natural changes in the lake. However, at the current time, the EPA has a weak review process for monitoring the recommended TMDLs in waterways