Department of Physics, Chemistry and Biology (IFM)

3.1 Description of Site

Lake Hällerstadsjön is eutrophic lake located between Östra Ryd and Västra Husby. It is located in the county of Östergotland in Sweden. The lake is found on latitude 58^o 27’ (58.45 ^o) north and longitude 16^o 8^l (16.1333^o) east. The average elevation is 78 meters (256 feet).  The lake has an average depth of about 4 m and an area of 1.47 Km²

The lake is surrounded by farmlands, forestry, animal husbandry, wetlands and houses.  

Methodology

Sediment samples were collected from the lake with the aid of a sampling core. This was to enhance the attainment of two different depths that is 0-5 cm and 5-10 cm from 20 different points. These sampling points were randomly distributed across the lake. The samples were stored in a cold room until day of analysis.

Samples were analyzed for Organic matter (Loss on Ignition, LOI), TP, and different fraction of P following a fractionation scheme as proposed by Paludan and Jensen (1995), except Humic acid bound P (HA-P) and organic P procedures. Each respective fraction was acidified by a known volume of sulphuric acid and stored in the fridge until day of analysis. Both total phosphorus (TP) and the dissolved reactive phosphorus (DRP) were determined by the ascorbic acid method as shown in the Swedish Industrial Standard (SIS, 1996). The organic P was obtained by subtracting all inorganic fractions from the TP, thus resultant being the Residual-P. Residual-P constitutes both Organic and Refractory-P. Loosely-P, Fe-P, Al-P and Ca-P constitutes the inorganic P forms. 

 

A subsample of each sediment sample was dried to constant mass with the corresponding moisture content recorded. Dried sediment was grind and sieved with 2mm mesh size. 0.5 g of the sieved sediment was put in a furnace at a temperature of 550^o for 2 hours. This was done to estimate the LOI (organic matter). TP was determined by digestion with 1 M HCl according to Andersen (1976) and Svendsen et al., (1995).

To ascertain spatial distribution and the variation of TP and the P forms, a regression analysis was conducted for the respective P form as against the latitude (North coordinates). This analysis was carried out using the lm code in R studio 0.98.1103. 

The sediment samples were stored for about 28 weeks where the P fractionation step was repeated for sediments with 0-5 cm depth. This was done to check for consistency in the P concentration and also used the sediment for the sediment P flux experiment. A paired T.test was conducted using R studio 0.98.1103 to quantify the significance of difference if there be. 

Sediment Incubation

Samples from the 0-5 cm depth were used in a subsequent experiment to determine the release rate of P during oxic and anoxic conditions, this was done by incubating the sediment in both anoxic (N₂ gas treatment) and oxic (aeration treatment) conditions. Mass of not less than 27 g was weighed from each sediment into respective two different jars for respective treatment. A known volume of pre filtered water from the lake was used to fill each jar containing a pre weighed sediment. A 100 µm mesh size net was used for the filtration to prevent the introduction of zooplanktons into the setup. All samples were aerated for about 5 days after which half of the samples were treated with N₂ gas (anoxic treatment) whiles the other half continued the oxic treatment. Air or nitrogen gas were bubbled through the upper 2 cm of the water column, 1 mm inner diameter silicon tubing. Bubbling speed was adjusted to not disturb the sediment. Counting began on this day and continued to the 20^th day. The experiment was carried out in a dark chamber for all 20 days. 

Sediment P flux 

An aliquot of 25 ml water was drawn from all 20 in both conditions of treatment. This portion was used for the TP determination. Another 25 ml was drawn from all the 20 in both treatment conditions and filtered with 0.45µm pore size, this was done to determine the Soluble Reactive Phosphorus (SRP). Both TP and SRP was determined using the ascorbic acid method as proposed by the Swedish Industrial Standard (SIS, 1996). The P concentrations were also determined using Sciencetec spec at a wavelength of 880nm. The water used for each analysis was replaced immediately with the filtered lake water after samples were taken, thus 2-3 days before the next analysis. Analysis was carried out in concurrent intervals of days. The pH, DO and Turbidity were measured on the last day of incubation. pH and turbidity were measured with portable HANNA instruments; whiles DO was measured with YSI pro 2030 DO probe.  The resultant sediment P flux was obtained by plotting the mass of P (µg) against day. The gradient (µgd^-1) was divided by the area of the glass jar (m²). The sediment density was used to estimate the release per m². 

Data was statistically treated using R studios 0.98.1103 and Microsoft Excel 2016 was used for plotting graphs. Paired t-test and linear regression were studied. A p value < 0.05 was considered as significant. The code used in R for linear regression was lm.

 

3.3 Data Analysis 

A linear regression model (lm) was used to model the relationship between the dependent variable (TP and P forms) and the independent variable (latitude). Latitude was used as the independent variable since the longitude did not show any strong relationship. This was done by using R studios 0.98.1103.

Paired T-Test was used to compare the means of P concentrations in the 0-5 cm depth and 5-10 cm depth. Paired T-Test was also used for comparing sediment P flux in both anoxic and oxic treatments.  R studios 0.98.1103 was used for this analysis.