Internal phosphorus loading mechanisms in shallow lakes

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.

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

This research mentioned using secondary methods in addition to adhering to established TMDLs an examination of the literature regarding these methods revealed that philosophies regarding these methods could be divided into two groups the first group proposes using chemical and other artificial means, such as the addition of gypsum in order to reduce P. In the water. The second groups proposes to enhance natural mechanisms in order to allow them to process higher amounts of P. from the main waterway. One of these proposed methods involves using wetlands to filter excess P. from the water. However, the question that determines which of these philosophies is correct depends on the sustainability of the methods used.

Those in the first camp propose clean-up by chemical means. However, there are many variables that can have an effect on the outcome of these efforts. One of the key considerations is whether these methods will have a negative impact on the ecosystem of the lake. Employing such methods may have the short-term effect of dramatically reducing the P. levels in the lake.

Dredging of Lake Grosser Muggelsee has been proposed as a means to reduce the total P. present in lake sediment. However, high costs of dredging have proven cost prohibitive. It has been proposed that even without dredging the lake should return to lower P. levels in 11-16 years (Kleeberg & Korserski, 1997). Remediation methods such as dredging, changing flush rates and changing the source/sink status are major construction projects, regardless of the size of the lake. Their costs often precludes their usefulness as a solution to reducing P. concentrations in sediment. In addition, there are many site-specific factors that could influence the effectiveness of these measures. They offer no guarantee that the P. loading will be reduced in the lake, as opposed to letting the lake restore natural levels over time.

However, if they harm the ecology of the lake, the net long-term effect could be that of eliminating natural methods for reducing P. levels in the future. This means that the lake would have a lower capacity for eliminating P. This would have several effects. The TMDL of the lake would have to be lowered to accommodate the decreased ability to eliminate P. There may have to be repeated treatments in the future. Future treatments would be an extra-added cost and if funds were not available when they were needed, the ecology of the lake would suffer further. This could create a viscous cycle that would be difficult to break once it were started.

The second group of researcher proposes that we use the natural means available to use in order to enhance pollution reductions. This groups proposes to use natural means, such as wetlands to filter P. from a waterway. This works in waterways where there is a wetland readily available. This limits the number of lakes where this method can be used.

Proponents of using natural methods as a means of P. removal claim that this method is more sustainable than the above mentioned chemical methods of P. reduction. However, there are several ways to view this perspective. Wetlands may be a more sustainable method for removing P. from waterways as they maintain themselves. However, what this group of researchers failed to consider is that in the process of using this ecosystem for our own benefit, we may harm that ecosystem in the process. We may place loads on it that are beyond its handling capacity. If we damage the ecosystem by overloading it with excess, we could present a larger problem than was present in the beginning.

Using wetlands and other natural means for clearing a waterway are sustainable as long as the wetland retains the ability to handle the extra pollutants. However, where the ecosystem cannot handle the excess load, this method will cause strain and could reduce the lake's capacity for filtering P. In the future. The sustainability of this method depends on the ability of the lake to process the extra

It is clear by now that there is no perfect solution that is best for every situation and in every case. The need to reduce external sources of P. was highlighted in the literature by a number of authors. The best method for curbing the long-term increase of P. In waterways is to reduce our inputs into the system. Prevention is the best method for reducing P. In waterways in the future.

However in many cases, this is not enough and secondary methods must be explored. The most significant point that was brought out by the literature is that there is no cure that will work and be sustainable in every situation., the site specific conditions of the lake will determine what the best method will be. In terms of preference, using the natural ecosystem, such as existing wetlands poses the solution that will cause the least amount of harm to the local ecology., However, this is only the case if these structures can support the increased P. loads without damage.

Chemical methods for removal of P. In lakes should only be used if the lake is in a dire condition. There is little chance of long-term sustainability with these methods, even if they are effective initially. They may be used to enhance the ecosystem's ability to clear excess P, but this should only be used if natural methods are not available, or would not be beneficial in a particular place. Using this type of remediation strategy is risky due to long-term damage to the ecosystem that could reduce its natural capacity to process P. In the future. Even if no apparent damage is obvious in the short-term, there is not way to predict what the long-term effects of such methods will be.

In conclusion, we have established three lines of defense in the removal of P. from shallow lakes. The first, and most effective long-term solution is to reduce the amount of P. that we dump into lakes. A long-term review plan of TMDLs needs to be developed. Enforcement methods need to be increased in order to make certain that industry is complying with the mandates. Natural methods are preferred over artificial methods for reducing P. In shallow lakes. However, these methods are not always sustainable and may not always be effective.

Chemical removal of P. In lakes should be used with extreme caution and only where other methods have failed, or where dire circumstances exist. The bottom line is that the particular circumstance for each waterway must be analyzed and considered in order to devise the best long-term solution. Internal loading has an impact on the ability to the lake to eliminate P, but in the end, human actions will help the lakes to recover, more than ever in the past.

We now know that we cannot simply clean up the mess that we make. Lessons learned from the internal loading mechanisms of P. In shallow lakes demonstrates that even when we take immediate corrective action, it may still take years or decades to see the results of our clean up efforts. Excess P. In shallow lakes changes the equilibrium of the lakes and established a new norm. This norm does not reestablish overnight, but takes years for all of the necessary changes to take place. Perhaps the greatest lesson that we can learn from external loading and internal loading of P. is that prevention is by far the better solution to remediation.