Aquatic ecology and environmental biology
Radboud University Nijmegen
Nutrient mobilization in peat lake Terra Nova after gradual iron addition
Amie Janssen
Supervisor: Jeroen Geurts
Content
Abstract
Introduction
Material and Methods
Results
Discussion
Conclusions
References
Abstract
Peat lake Terra Nova is located in the Loosdrecht lakes, right next to a water supply area. The last few years, this lake was mostly dominated by algal blooms and cyanobacteria, while macrophytes were hardly occurring. To improve the quality of the lake, in 2010 gradual iron addition was applied in the lake to reduce the phosphate mobilization rate from the sediment. In a closed compartment iron addition has taken place in three months, the addition started in 2009. Another closed compartment was used as the control area, this compartment has never been treated with iron.
The project investigates whether the addition of iron is still effective in 2012 and whether changes have occurred compared to previous years.During the experiment resuspension is taken into account.This results in a bigger contact surface between sediment and water. It was hypothesized that phosphate mobilizes more easily to the water layer after resuspension, which results in a higher phosphate mobilization rate.
Sedimentprofilesof four different sites in Terra Nova were collected in glass cylinders.For 5 weeks water and pore water samples were collected.The results show that the phosphate and phosphorus mobilization rates are both very low at all sampling sites in the lake, even at the control site. At the control site, the phosphate and phosphorus mobilization rates were much higher in 2010 and 2011 compared to mobilization rates of 2012.The low phosphate mobilization at the compartments can be caused by holes in the barrier, which results in water exchange between the compartments and the lake. Another possible explanation is the lower temperatures in the weeks before the samples were collected. A lower temperature causes a lower mobilization rate. The resuspension treatment does not seem to be causing a higher phosphate and phosphorus mobilization rate.
Introduction
Peat lake Terra Nova is located in the Loosdrechtlakes (52⁰13’N, 5⁰02’E) (Geurtset al., 2008), right next to a water supply area. Lake Terra Nova is used as a buffer for the water supply area and therefore a high quality of the lake is important. Between 1950 and the late 1970s, the Loosdrecht lakes became highly eutrophic, ‘’due to discharge of untreated household and the inlet of P rich river water’’ (Gulati et al., 2002). The last few years, Terra Nova was mainly dominated by algal blooms and cyanobacteria, while macrophytes hardly occurred. A major cause was the high phosphate concentration and the relatively low iron concentration in the Lake.
The binding of P to iron oxyhydroxides plays an important role in decreasing P availability in sediments (Xu et al., 2012). Beside iron (Fe), other compounds that bindphosphate are aluminium (Al) and calcium (Ca). Oxidized iron binds phosphate most effectively and therefore it is able to immobilize phosphate in the sediment (Geurts et al., 2008).
In the Netherlands groundwater levels are decreased due to water extraction and water regulation measurements (Smolders et al., 2003). The decreasing groundwater levels prevented the natural supply of iron to Terra Nova. When there is a low concentration of iron available in the lake, only small amounts of phosphate will become immobilize. The excess of phosphate ends up in the water layer, what leads to internal eutrophication (Roelofs, 1991). In 2009 Waternet, NIOO and the Radboud University started a project to reduce the phosphate exchange between sediment and water and to improve the ecosystem quality in Terra Nova. Through iron addition, the iron availability was gradually increased for eighteen months, which should lead to a better phosphate binding in the sediment. In Terra Nova also two closed compartments were examined. One closed compartment was used as a control area, this compartment has never been treated with iron. The other closed compartment was treated with iron addition, this addition has taken place in three months.
The gradual iron addition was monitored in the field and in the lab. The column experiment of 2010 showed that after iron addition, the phosphate mobilization was indeed reduced, but that Fe-P colloids were stillmobilized. From this experiment it has become clear that in the first few years after iron addition the phosphate concentration was reduced and therefore less phosphate is available to the algae. It is unclear whether the algae can absorb the Fe-P colloids directly and still take up a sufficient amount of phosphorus.
Now in 2012, the column experiment will be repeated to show what the mobilization rates of phosphate are in Terra Nova, two years after the iron addition. In this column experiment the following questions will be answered:
- Is the phosphate stillimmobilizedin the lakeand does theP mobilizationstill occur in the compartment with iron addition?
- What effect has resuspension on the mobilization rates of phosphorus, phosphate and nitrogen?
Hypothesis
Question 1
The expectation is that in a few decades, the applied iron addition has to be repeated, because by then all the added iron has bounded tophosphate. After two years there is still enough iron in the sediment of the lake, to bind the excess of phosphate. The hypothesis is that in 2012, the phosphate mobilization rate in the lake is hardly increased and the iron addition is still effective. Based on the results of 2011, the expectation is a further increase of the phosphate mobilization rate in the closed compartment with iron addition.
Fe-P colloids are measured with the phosphorus concentration. These colloids do not bind to the sediment, therefore they can mobilize easily to the water. It is expected that in the closed compartment with iron addition the phosphorus mobilization rate will be much higher than the phosphate mobilization rate.
In the area where no iron addition has taken place, it is expected that for phosphate and phosphorus the mobilization rate is higher than at the areas without iron addition.
Question 2
In the columns,resuspension is simulated by causing swirls in the water layer. Due to this swirl, the sediment detaches and comes in suspension, which results in a bigger contact surface between sediment and water. In this way nitrogen can transfer more easily to the water layer than without resuspension. The hypothesis is that the nitrogen mobilization rate will be higher with resuspension than without resuspension.
By causing the swirls, oxygen can beadded to the water layer. The added oxygen will probably be very low. This way, compared with the columns without the resuspension treatment, the iron-phosphate binding will not increase that much.
The particles form a much larger contact surface with the water. In this way phosphate and phosphorus can mobilize easier to the water layer. Therefore it is expected that the bigger contact surface has a larger influence on the phosphate and phosphorus mobilization rate than the small amount of added oxygen. The hypothesis is that in the columns where resuspension is induced, therewill be a higher phosphate and phosphorus mobilization rate.
Material and Methods
For this experiment four sites in Terra Nova were examined (figure 1). Two sites are located in the middle of the lake (NO and M-NO) and two sites are closed compartments located in Terra Nova. The compartments are closed off by a low barrier, so they are not able to connect to the other compartment or to the rest of the lake.Since 2011 a few holes appeared in the barrier, therefore the closed compartments cannot prevent all exchange.The sites in the open lake were treated for eighteen months with gradual FeCl addition. The FeCl addition started in May 2010 and the treatment was stopped in 2011, which means that the open lake has not received any FeCI addition in the lasthalf year. One closed compartment is treated with a higher amount of FeCl as the lake (85 g/m2 vs. 33 g/m2), and the addition has taken place in only three months, during the summer of 2009.The other closed compartment has never been treated with iron.
Figure 1: Map of peat lake Terra Nova with the adjacentwater Supply Lake. The points represent the locations where the samples were collected. The colors represent the exact sites. NO and M-NO are the same names that were used at the original column experiment in 2010.
Because two locations are in the middle of the lake, samples had to be collected by boat. At each site, eight intact sediment profiles were collected with a six centimeter thick metal sediment corer. Immediately after collecting, the sediment profiles were carefully transferred into a marked glass cylinder. This was doneunder water, to prevent the loss of parts of the sedimentprofiles. The used cylinders have a diameter of 6 centimeter and a height of 50 centimeter. On top of the intact sediment profiles the original water remains, until the actual experiment starts.
At each site a surface water sample, a sediment sample and apore water sample were also collected. The collected sediment samples were stored under anaerobic conditions. The pore water was collected with a ceramic field rhizon and a 60 ml vacuum Syringe.Rhizons allow fast collecting of pore water, with minimum disturbance of the sediment structure (Seeberg-Elverfeldtet al., 2005) All the glass cylinders and samples were taken back to the lab, where the samples were put in a cold environment of 4°C and the glass cylinders were placedin a dark climate room with a temperature of 15°C.
One day after collecting the samples in the field, the surface water samples and pore water samples were used to measure the pH and alkalinity. The pH of the water samples were measured using a combined pH electrode with an Ag/AgCl internal reference (Orion Research, Beverly, Ca, USA), and a TIM800 pH meter. The alkalinity of the water samples was determined by titration to pH 4,2 with 0,01 M HCl using an ABU901 Autoburette (Radiometer, Copenhagen, Denmark) (Geurts et al., 2008).
The rest of the samples were put in ICP tubes and polyethylene vials. To the ICP tubes 100 micro liters of concentrated nitric acid (65 %) was added. The polyethylene vials were stored in a freezer at – 20⁰C and the ICP tubes were put back into the cold environment of 4°C.
The setup and treatment of the column experiment
The eight intact sediment profiles of each location were divided into twoseparate rows of four cylinders (figure 2). After the cylinders were set into the right position, they were left alone for one week so that the sediment particles were able to sink.
The cylinders were divided into two groups, sixteen with and sixteen without resuspension.
In this column experiment,resuspension was simulated by using a coffee mixer. The coffee mixer was kept half way into the cylinder and stayed there only a few seconds until the sediment started to swirl. This treatment was applied three times a week and started immediately after the first measurement (t=0).
Figure 2: The setup of the column experiment. The colors represent the sites where the sediment profiles were taken. The + symbol indicates that the cylinder has had the resuspension treatment.
Sampling of the column experiment
To be able to sample at different depths, the short rhizons were cut through the middle, then both parts were connected again by placing a long tube between them. For each glass cylinder two rhizons were used. Both rhizons were attached to a stick, which held the rhizons at the right position in the glass cylinder. In the stick, two holes were made in which the rhizons fitted perfectly. One of the rhizonswas placed one cm in the sediment and the other rhizonwas placed five cm above the sediment. Both rhizon needles were kept together with a rubber stopper.
One week after collecting the intact sediment profiles, the original water was replaced by water containing a standardized composition of 1.2 mmol/l CaCl2, 2 mmol/l NaHCO3 and 0.25 mmol/l MgCl2*6H2O. This was done very carefully, to keep the sediment profile intact. The standardized water was added up to 20 cm above the sediment. After the water was replaced, therhizons were added in the first centimeter of the sediment and five centimeter above the sediment.Figure 3 shows the construction of the glass cylinder, with the rhizons at the exact places into the sediment and water. The stick and rhizons were attached to the glass cylinder with tape, to limit their movement as much as possible.
Figure 3: a; A schemetic representation of the construction of the glass cylinder. 20 cm of a standardized water solution is added on top of the sediment. One rhizon is placed 1 cm in the sediment and another rhizon is placed in the water, 5 cm above the sediment. Vacuum glass bottles are attached to the rhizon needles, in which the samples were collected. b; A photograph of some of the glass cylinders from the experiment.
pH and alkalinity measurements
The first measurement started two days after replacing the original water.One day before measuring, marked vacuum glass bottles were attached to each rhizon. The next day the water in the bottleswas used formeasuring the pH and the alkalinity. The rest of the samples were put in ICP tubes and vials. To the ICP tubes 100 micro liters of concentrated nitric acid (65%) was added. The vials were stored in a freezer (-20°C) and the ICP tubes were stored in a cold environment of 4°C. During the experiment, the collected bottles were replaced by a rubber stopper. After the first measurements were performed, the resuspension treatment was started immediately in the sixteen left-hand cylinders. The same measurements were repeated each week, during a period of 5 weeks.
Chemical analysis
When all the samples were collected, measurements were done on the auto analyzer and the ICP Spectrometer. The auto-analyzer measures the concentrations of PO4, NH4, NO3, K, Na and Cl. This was done with an Auto Analyzer 3 system (Bran+Luebbe, Norderstedt, Germany, using ammonium molybdate, hydrazine sulphate, salicylate and ferriammonium sulphate (Geurts et al., 2008).
An ICP spectrometer (IRIS Intrepid II, Thermo Electron Corporation, Milford, Ma, USA) was used to measure the concentrations of Al, Ca, Fe, K, Mg, Mn, Na, P, S, Si and Zn (Geurtset al., 2008).
Statistical analysis
The statistical analysis were made with SSPS 15,0 for windows. When the test of homogeneity of variances had a significance level higher than 0.05, the oneway ANOVA test was used. When this was not the case, the Welch test was used.
When for more than two groups significance was tested, a post-hoc test was used to show the results. For the oneway ANOVA test, a Tukey post-hoc test was used and for the Welch test, a Games-Howell post-hoc test was used.
The phosphate, phosphorus and nitrogen mobilization rates were calculated in mmol/m2/year, by linear regression of surface water concentrations between t=2 and t=30 days. (Geurts et al., 2010).To get the total N mobilization rate, the average mobilization rates of NH4 and NO3 are added together.
Results
During the entire experiment the pH remained stable. The average pH remained between 7.3 and 7.6 (with a maximum standard error of 0,0498).
Phosphate mobilization rate
Figures 4 and 5 show the average phosphate mobilization rate in the water layer of the columns. In figure 4 the PO4 mobilization rates during the past three years are compared. Figure 4 shows no significant differences between 2010 and 2012, but a trend is found between the data at site M-NO (p=0,083; Welch test). At the closed compartment with iron addition, there is a significant difference in the PO4 mobilization rate between 2011 and 2012.
Figure 5 shows the PO4 mobilization rate with and without resuspension treatment. Figure 5 shows no significant differences, but a trend is found between the treatments at site NO(p=0,090; oneway ANOVA).For the resuspension treatment, a trend is also found between the closed compartment with iron addition and site NO (p=0,059; oneway ANOVA), and between site M-NO and site NO (p=0,071; oneway ANOVA).
Figure 4: The PO4 mobilization rates for the past three years at each location in peat lake Terra Nova (without resuspension). The PO4 mobilization rate from 2012 is equal to the PO4 mobilization rate without resuspension (control treatment) shown in figure 5. Theerrorbars represent thestandard error. Significant differences between 2011 and 2012 are indicated with a star (* p < 0,05). Significance is tested with the oneway ANOVA test.No differences are found between 2010 and 2012 (trends are indicated with a t). (n=4; N=3 for closed compartment + Fe in 2010)
Figure 5: The PO4 mobilization rate with resuspension treatment and without resuspension treatment at each location. No differences are found between the treatment (trends are indicated with a t) and no differences are found between locations. Theerrorbars represent thestandard error. (n=4)
Phosphate concentration in the pore water
Figures 6 and 7 show the average phosphate concentration in the pore water of the columns (1cm deep).In figure 6 the average PO4 concentrations during the past three years are compared. Figure 6 shows no significant differences between 2010 and 2012. At the closed compartment without iron addition, there is a significantly higher average PO4 concentration in 2011 than in 2012. At site M-NO, a trend is found between 2011 and 2012 (p=0,075; oneway ANOVA).