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...
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…
