Laboratory-determined Phosphorus Flux from Lake Sediments as a Measure of Internal Phosphorus Loading

The overall goal of this procedure is to conduct measurements of the phosphorus release from lake sediments for estimation of the internal phosphorus load. This is accomplished by first collecting intact sediment cores in the field. In the second step, the sediment cores are exposed to redox treatments and incubated in an environmental growth chamber.

Next, the water overlying the sediment cores is sampled for phosphorus concentrations at regular intervals throughout the incubation period. Finally, the phosphorus release rates are calculated and the internal phosphorus load can be estimated based on the changes in phosphorus concentration over time. Ultimately, the laboratory based sediment incubations are used to quantify internal phosphorus loading to lakes, identifying the role of the sediments in lake unification.

The main advantage of this technique over other methods for estimating internal phosphorus load, such as phosphorus, mass balance, and in situ changes in phosphorus concentrations over time, is that this method can be adjusted to answer a variety of lake management and research questions. For example, with this method, the relative importance of internal versus external loads to lake can be analyzed to inform lake management in atrophic lakes, as well as effectiveness of treatment efforts to reduce internal loading, Conduct the water quality sampling first at a minimum measure water depth and vertical profiles of water temperature and dissolved oxygen, collect any other water quality data and samples that are desired to fulfill the specific goals of the study at this time as well. At each sampling location, use an akin bottle to fill a 10 liter carboy with water collected one meter above the sediment surface.

Then to collect the sediment cores, assemble the corring device by first threading the swivel clip end of the plastic coated piston cable through the top of the PPC attachment assembly. Then orient a graduated 0.6 meter long polycarbonate seven centimeter diameter core tube with the bolt holes facing upward and extend the cable through the length of the tube. Clip the piston cable to the eye bolt of the piston stopper, and then use a wire lock hitch pin to attach the core tube to the PVC attachment assembly.

Pull the piston cable to advance the piston 20 centimeters from the bottom of the core tube to maintain a water layer on top of the sediment surface during the core collection. Then use another wire lock hitch pin to attach an aluminum drive rod to the other end of the PVC attachment assembly and lower the coring device vertically into the water, adding additional sections of aluminum drive rod as needed. Now, position the core vertically at the sediment water interface and pull the piston cable tau.

Once the core is in place at the sediment water interface, then attach vice grips to the cable. Step on the cable to the inside of the vice grips and push downward. Bring the core to the surface and seal it with a rubber stopper prior to breaking the water surface, securing the bottom stopper with duct tape.

Then both the piston to the top of the core tube to keep its stationary during transit. Place the core tube in a vertical rack and maintain it at ambient near bottom lake temperature using ice as necessary. Upon return from the field, adjust the cores to contain the desired depth of sediment and overlying water column Excess sediment can be carefully let out of the bottom of the core tube by removing the bottom stopper.

Add water from the carboy collected at the corresponding site if needed. Place the sediment core tubes into a darkened environmental growth chamber with the temperature maintained to match the ambient bottom water temperatures measured in the field. Next, expose the cores to redox treatments for the oxic treatment bubble.

The water column of three cores per site with air for the anoxic treatment bubble. The water column of the remaining three cores per site with nitrogen containing approximately 350 PPM CO2 to buffer the pH for both treatments. Adjust gas flow to ensure a slow and consistent bubble rate that is non-disruptive to the sediment surface.

Position the end of the gas tubing at approximately the middle of the water column. The next day, filter each 10 liter car containing the near bottom water collected from each site in the field with a peristaltic pump and filter cartridge housing. Filter the water first through a one micron filter followed by a 0.2 micron filter.

Store the filtered water at four degrees Celsius for the duration of the core incubation to sample the cores for the phospho release rate over the duration of the incubation period. Use the syringe to remove a 40 milliliter water sample through the sampling port of each sediment core at the desired intervals immediately after removal, dispense a 20 milliliter subsample into a scintillation vial and refrigerate the sample for analysis of the total phosphorus. Filter the other 20 milliliter subsample through a 0.45 micron membrane filter into a different scintillation vial and freeze a second vial.

For analysis of the soluble reactive phosphorus, replace the 40 milliliter subsample with an equal volume of filtered water from the corresponding site. Calculate phosphorus flux based on the change in water column total phosphorus or soluble reactive phosphorus using this equation where P sub RR is the net P release for positive values or net P retention for negative values per unit surface area of sediment. C sub T is the total phosphorus or soluble reactive phosphorus concentration in the water column at time.

TC sub zero is the total phosphorus or soluble reactive phosphorus concentration at time. Zero V is the volume of water in the water column of the core tube, and A is the planer surface area of the sediment cores then calculate the P release rate using the linear portion of the concentration versus time curve to give the maximum apparent release rate. In this representative experiment.

The phosphorus release was measured from sediment cores collected in Mona Lake, Michigan to identify the relative contribution of the internal versus the external phosphorus loads. Four sites were sampled over three seasons to estimate the annual internal phosphorus load accounting for the spatial variation in the phosphorus flux. The total phosphorus concentrations were highest during the summer in the anoxic treatments and spatial variability in the total phosphorus release was evident during all seasons.

The mean internal total phosphorus flux was less than 1.4 milligrams phosphorus per meter squared per day in all IC cores with the negative flux values at three of the four sites during the fall, indicating that the IC sediments were acting as a sink rather than a source of phosphorus during that season. The total phosphorus release rates were considerably higher in the anoxic cores with the flux as high as 15.56 milligrams phosphorus per meter squared per day in the summer, and as low as 0.80 milligrams phosphorus per meter squared per day in the spring. These flux values were then used to calculate the seasonal internal phosphorus flux based on the dissolved oxygen conditions measured at the time of the sediment co collection.

The annual internal phosphorus load was estimated to be 3.4 metric tons with the majority of the load occurring during the summer. Comparing these results with the concurrent external phosphorus load estimates coming from tributaries flowing into Mona Lake, it was estimated that the sediments in Mona Lake contribute between nine to 82%of the total annual phosphorus load. Another series of experiments was conducted in Spring Lake, Michigan as demonstrated in these next two figures to determine the potential effectiveness of an aluminum sulfate or treatment in reducing internal phosphorus loading and the efficacy of an NC two alum treatment laboratory experiments simulating a lake wide application of alum demonstrated a dramatic decline in internal phosphorus release with treatment in anoxic cores without alum treatment, simulating the natural summer conditions in spring Lake sediments.

The mean total phosphorus concentrations in the overlying water column reached more than 1.2 milligrams per liter. In contrast, the anoxic cores dosed with alum had virtually no phosphorus release, and concentrations were not different from either of the IC treatments. A sediment core incubation conducted one year following the lake wide application of aluminum sulfate in Spring Lake revealed that the treatment was highly effective at reducing sediment phosphorus release with release rates similar between the anoxic and IC treatments.

When the experiment was repeated five years following the aluminum sulfate treatment, the total phosphorus release remains substantially lower than pre-treatment, but was greater than that measured one year following the treatment, suggesting a slight decline in the alum efficacy over time. In conjunction with this procedure, additional analysis like sediment, total phosphorus, poor water soluble reactive phosphorus, sequential phosphorus, fractionation, and metals can be performed to help interpret the sediment phosphorus release results. After watching this video, you should have a good understanding of how to conduct laboratory experiments to determine phosphorus release rates from lake sediments, including sediment core collection, incubation, and calculations.