Climate Change Effect on the Seasonality of Nutrients in Lake Erken

Climate change effect on the seasonality of nutrients in Lake Erken

Climate change effect on the seasonality of nutrients in Lake Erken /
Evelina Grotuzaitė, Lithuania Līga Lāce, Latvia Supervisor: Kurt Pettersson /

2012.07.05

Uppsala University

Erken Laboratory

Research school

Summer 2012

Table of contents

Abstract...... 3

Introduction...... 4

Materials and methods...... 6

Results ...... 8

Discussion...... 22

Conclusion...... 24

Acknowledgments...... 24

Reference list...... 25

Abstract

The objective of this project was to investigate the seasonality of nutrients in Lake Erken. A nutrient is a substance that provides all living organisms’ with the nourishment needed for growth and metabolism. However, in high concentrations they can be harmful.

In this project we analysed data from 40 years in five year periods (1972-1976, 1977-1981, 1982-1986, 1987-1991, 1992-1996, 1997-2001, 2002-2006, 2007-2011) during four different seasons - spring, summer, autumn and winter. Samples were taken from the deepest point and from five major inlets and outlet of Lake Erken. The latter was done only in order to compare inflow concentrations with concentrations in the lake on one occasion.Total nitrogen, total phosphorus, orthophosphate and nitrate concentrations in water samples which we took were measured. During our projectwe found out that in Lake Erken the nitrogen and phosphorus influencing factors differ from each other.

The period of ice cover in Lake Erken has decreased and the length of spring and autumn circulation has grown. The period of summer stratification has not changed significantly. The levels of phosphate and total phosphorus are slowly getting higher, but the concentrations of total nitrogen are stable, but the level of nitrate is decreasing. This could be caused by climate change but there are more factors to consider. The levels of phosphate, total phosphorus, nitrate and total nitrogen in Lake Erken are dependent on the seasons, but since the length of each season has altered because of the climate change, a conclusion stating that the seasonality of the nutrients we examined was affected by the climate change was made.

Introduction

Nutrients

A nutrient is a compound that is essential to all living organisms’ nutrition. Nitrogen and phosphorus are important and often limiting nutrients to the aquatic life. However, in high concentrations they can be harmful. These nutrients occur in a variety of forms. (Mueller and Helsel, 1996) Inorganic nitrogen compounds are present in small amounts even in rain-water and come from the atmosphere in which they occur as the products of electrical discharges, impurities from industrial smoke, terrestrial decomposition, and volcanic eruptions. Elemental nitrogen is fixed in the soil by the nitrogen-assimilating bacteria and becomes available for the metabolism of plants. The phosphorus comes either directly or indirectly from the weathering of phosphatic rocks and from the soil, and is present as dissolved phosphate. It is not easily leached by rain-water. (Ruttner, 1953)

The Lake

Lake Erken is a meso-euthrophic lake which is located in south-eastern Sweden near Norrtälje (5925' N, 1815' E). It has a surface area of approximately 24 km2, the mean depth of 9 m and the maximum depth of 21 m. Water enters the lake through the rainfall, ground waters and 12 small inlets and leaves through one outlet into the river Broströmmen, thus, the water residence time is approximately 7 years. (Hernández et all, 1999)

Figure 1: The map of Lake Erken

Seasonalityin temperate lakes

Spring is the period from the end of the ice cover to the beginning of temperature stratification. The stratified period is summer. The mixing period lasting from stratification until the beginning of ice coverage is autumn. Winter is the period when the lake is covered by ice. (Petterssonet all, 2003)

Figure 2: Graph of four seasons showing the temperature of water and dissolved oxygen level in each season in the Lake

Aims:

1. Compare the concentrations of phosphate, nitrate, total phosphorus and total nitrogen in four different seasons (spring, summer, autumn and winter) throughout a period of 40 years in Lake Erken.
2. Compare the length of each season in five year intervals during the period of 40 years in Lake Erken.
3. Determine if the climate change has had any effect on the concentration of phosphate, nitrate, total phosphorus and total nitrogenin Lake Erken.

Hypotheses:

1. Climate change had no effect on phosphate, nitrate, total phosphorus and total nitrogen levels in Lake Erken.
2. Climate change affected the seasonality of phosphate, nitrate, total phosphorus and total nitrogen concentrationsin Lake Erken.

Materials and methods

We chose to examine the data from 40 years in five year periods (1972-1976, 1977-1981, 1982-1986, 1987-1991, 1992-1996, 1997-2001, 2002-2006, 2007-2011) during four different seasons – spring, summer, autumn and winter and to focus on lake water.

Field work methods

We took samples from the deepest pointand from five major inlets and outlet of the Lake Erken (see Fig 1).The latter was done only in order to compare inflow concentrations with concentrations in the lake on one occasion.We took water samples from the inlets and outlet. First, water temperature and dissolved oxygen concentration were measured with dissolved oxygen meter and noted down. Then, the bottle in the sampler and the sample bottle were rinsed. After that depending on the place either bottle in the holder or sample bottle was used to get the water sample by putting the bottle head down into the water and then turning it 90 into the direction the water was flowing from. Afterwards,water samples at the deepest point of the lake were taken. Temperature and dissolved oxygen concentration were measured in one meter intervals. Then we used a water sampler that was 2 meters in length and took water samples from each two meters. We just needed to open the sampler, put it in water as deep as you want to take samples from and then release the stopper which closes the sampler. This was done due to the fact that in summer lake water is stratified and we need to take integrated samples from epilimnion and hypolimnion.In order to figure out at what depth the epilimnion ends and hypolimnion begins water temperature was measured. In the epilimnion the temperature remains stable with no significant changes however in the hypolimnion it suddenly decreases.

Analyses

Total nitrogen, total phosphorus, orthophosphate and nitrate nitrogen concentrations in water samples which we took from six inlets, one outlet, the epilimnion and hypolimnion of Lake Erken were measured. Nitrate concentration is measured with Flow Injection Analyzer (Wood & Armstrong, 1991) so the laboratory staff just put our samples in it and we measured phosphate concentration with a spectrophotometer (Murphy & Riley, 1962) ourselves.

Total nitrogen

At high temperature and in the presence of a strongly oxidizing agent, all nitrogen forms in water are transformed to nitrate (Broberg, 2003). The formed nitrate is analyzed according to one of the methods for analysing nitrate nitrogen.

Total phosphorus

Organically bound phosphorus is transferred to orthophosphate through oxidative hydrolysis with potassium persulfate. The hydrolysis occurs in a slightly acid solution at high temperature and high pressure in an autoclave. The dissolved phosphate is then analyzed according to the MRP-method. (Menzel & Corwin, 1965)

Orthophosphate (MRP or SRP)

In order to determine molybdate reactive phosphate according to Murphy & Riley (1962), a reagent containing sulfuric acid, ammonium molybdate, ascorbic acid, and antimony potassium tartrate is prepared. In an acid solution ammonium molybdate forms a yellow complex of phosphorus molybdate, which is reduced to a blue complex with ascorbic acid. Antimone is added in order to accelerate the reduction. The colour absorption obeys the Lambert-Beer law up to 1250 µg/l (in this case 50 µg PO4-P/sample), and the colour remains stable for 24 hours. (Murphy & Riley, 1962)

Nitrate-Nitrogen

Nitrate may be reduced to nitrite in a column containing cadmium which has been treated with copper. The Cd2+ ions released in this way are simultaneously bonded to a complex with the help of a buffer in order to prevent the formation of Cd(OH)2 (possibly also CdCO3), which interferes with the efficiency of the column. The reduction can be supposed to proceed as follows:

The efficiency of the reductive column can reach 99±1%. For accurate measurements of nitrate the efficiency must be >80%. Determination of formed and original nitrite is done photometrically in the same way as presented by the NO2-N analysis. The measurements can be done manually or by using an Autoanalyzer (Broberg, 2003).

Field work materials

• Boat
• Car
• Plastic bottles with lids
• Bottle holder
• Dissolved oxygen meter
• Water sampler

Laboratory work materials

• Spectrophotometer
• Finnpipette
• Curvettes
• Flow Injection Analyzer (FIA)
• Tubes

Results

Seasons

According to data from 1972 to 2011(see Table 1) the average length of spring mixing period has increased. Spring usually starts in April and ends in June. The length of circulation in spring has grown almost by half as seen in Table 1. In the time period from 1972 to 1976 the mean was 46 days, but from 2007 to 2011 the mean value of days has increased to 82 days. The summer stratification usually starts in June and lasts until August. The length of this period has not changed significantly. On average the summer stratification lasted for 78 in the period from 1972 to 1976 and for 87 days from 2007 to 2011(Table 1). The autumn usually lasts from September to December. The mean length of the circulation in autumn from 1972 to 1976 is 103 days and from 2007 to 2011 is 133 days. The ice cover usually lasts from January to April. The average length of winter in the time period from 1972 to 1976 is 109 days and from 2007 to 2011 itis 82 days (Table 1). The duration of ice cover has decreased.

Table 1: Average length and standard deviation of seasons in Lake Erken during 40 years in 5 year periods

Season
Year / Winter / Spring / Summer / Autumn
1972-1976 / 109 / 45 / 78 / 103
Standard dev. / 21 / 10 / 1 / 21
1977-1981 / 137 / 39 / 90 / 92
Standard dev. / 15 / 9 / 13 / 13
1982-1986 / 104 / 48 / 94 / 82
Standard dev. / 19 / 20 / 9 / 37
1987-1991 / 114 / 84 / 70 / 89
Standard dev. / 30 / 32 / 17 / 8
1992-1996 / 105 / 48 / 105 / 108
Standard dev. / 30 / 10 / 18 / 20
1997-2001 / 94 / 72 / 84 / 113
Standard dev. / 19 / 16 / 21 / 16
2002-2006 / 108 / 61 / 100 / 96
Standard dev. / 5 / 8 / 16 / 12
2007-2011 / 82 / 82 / 87 / 133
Standard dev. / 28 / 37 / 20 / 37

The concentrations of PO4 and total phosphorus during different seasons

During the spring circulation the average concentration of PO4 is less than 7 µg P/l and more than 1 µg P/l (see Fig.3). The lowest concentration of PO4 in spring was in the time period from 1997 to 2001. The highest concentration of PO4 was during the time period from 1972 to1976. During this season the maximum concentration of total phosphorus was 28 µg P/l(1972-1976) and the minimum concentration was 12 µg P/l(1987-1991).

Figure 3: Average concentrations of PO4 and total phosphorus (µg P/l) in spring during 40 years (data was calculated from 5 year periods)

During spring time the concentration of total phosphorus and PO4suddenly decreases (see Fig.4). PO4 concentration gets to the lowest point almost at the end of the season (middle of May) while total phosphorus concentration reaches its minimum value right after the season starts (middle of April). However, concentrations of both PO4 and total phosphorus slightly increase at the last weeks of spring (end of May).

Figure 4: The concentrations of PO4 and total phosphorus in 2011 spring

During the summer stratification the mean concentration of PO4 in epilimnion was more than 2 µg P/l and less than 6 µg P/l (see Fig.5). The minimum concentration of PO4 was in the time period from 1987 to 1991 and the max concentration was in the time period from 2002 to 2006(Fig.5).

Figure 5: Average concentration of PO4(µg P/l) in summer during 40 years (data was calculated from 5 year periods)

After spring the concentration of PO4 is very low the beginning of the summer (start of June) and does not significantly increase until the middle of July (see Fig.6). Then the concentration of PO4 in epilimnion has a sudden growth until the start of August when it drops rapidly. PO4 concentration in hypolimnion remains low the whole summer. Then from the middle of August both concentration of PO4 in epilimnion and hypolimnion starts slightly increase.

Figure 6: The concentrations of PO4in hypolimnion and epilimnion in 2011 summer

The average concentration of total phosphorus in epilimnion was more than 11 µg P/l (1982-1986) and less than 26 µg P/l (2002-2006) (see Fig.7). The mean concentration of PO4 during this season in hypolimnion was more than 2µg P/l (1982-1986) and less than 34 µg P/l (2002-2006). The average concentration of total phosphorus in hypolimnion was more than 13 µg P/l (1987-1991) and less than 52 µg P/l (2002-2006).

Figure 7: Average concentration of total phosphorus (µg P/l) in summer during 40 years (data was collected from 5 year periods)

Like concentration of PO4, the concentration of total phosphorus in summer of 2011 remains very low after it dropped in spring (see Fig.8). Then in both – epilimnion and hypolimnion – the concentration of total phosphorus starts to increase approximately in the middle of July but the concentration in hypolimnion undergoes much more rapid changes until it drops down in the middle of August and becomes almost parallel to the concentration in the epilimnion. At the end the concentrations in both stratified layers starts to increase until they mix in autumn circulation.

Figure 8: The concentration of total phosphorus in epilimnion and hypolimnion in 2011 summer

The average concentrations of PO4(see Fig.9) in autumn were more than 8µg P/l (1982-1986) and less than 34 µg P/l (2002-2006). The average concentrations of total phosphorus during autumn circulation were more than 19 µg P/l (1982-1986) and less than 50µg P/l (2002-2006).

Figure 9: Average concentrations of PO4 and total phosphorus (µg P/l) in autumn during 40 years (data was calculated from 5 year periods)

During autumn time both concentration of PO4 and concentration of total phosphorus do not undergo any significant changes and remains quite high (see Fig.10). At the middle of September both concentrations start increasing slightly however at the start of October they start going down until they approximately reach the same value as in the start of this season.

Figure 10: The concentrations of PO4and total phosphorus in 2011 autumn

The average concentrations of PO4 in winter(see Fig.11) were more than 4 µg P/l (2007-2011) and less than 15 µg P/l (1982-1986). The average concentrations of total phosphorus were more than 21 µg P/l (1987-1991) and less than 41 µg P/l (2002-2006).

Figure 11: Average concentrations of PO4 and total phosphorus (µg P/l) in winter during 40 years (data was calculated from 5 year periods)

During winter of 2011 the concentrations of PO4 and total phosphorus remained relatively stable with a slight increase of total phosphorus concentration in the middle of February (see Fig.12).

Figure 12: The concentrations of PO4 and total phosphorus in 2011 in winter

The concentrations of NO3 and total nitrogen during different seasons

During the spring circulation the average NO3 concentration was between 10 µg N/l and 143 µg N/l (see Fig.13). The lowest NO3 concentrationin spring was in the time period from 2002 to 2006 and the maximum value was reached in 1982-1986 period.

Figure 13: Average concentration of NO3(µg N/l) in spring during 40 years (data was collected from 5 year periods)

During spring time the concentration of NO3 dropped significantly considering that is was relatively high at the start of this season in the beginning of April (see Fig.14). The first half of May NO3 almost disappeared and the concentration was stable until the middle of may when it started to increase slightly.

Figure 14: The concentration of NO3 in 2011 spring

The concentration of total nitrogen in spring of 2011 has no significant changes during the period of 40 years differing from the average of 609 µg N/l to 702 µg/l (see Fig.15). However, in the period from 1982 until 1986 there was no data of total nitrogen available.

Figure 15: Average concentration of total nitrogen (µg N/l) in spring during 40 years (data was collected from 5 year periods)

Total nitrogen concentration in spring was quite stable without any significant changes with a slight drop at the start of May and in the middle of the same month (see Fig.16). However, during the whole season it stayed relatively high.

Figure 16: The concentration of total nitrogen in 2011 spring

During the summer stratification the concentration of NO3 varied between 4 µg N/l and 73 µg N/l in the epilimnion and between 7 µg N/l and 92 µg N/l in the hypolimnion (see Fig.17). The lowest concentration of NO3 in the epilimnion was in period from 2007 until 2011 and the lowest mean value in the hypolimnion was in the 1982-1986 period. The maximum mean value in the epilimnion was during the period of 1992-1996 and the highest value in the hypolimnion was reached in the period from 1987 to 1991.

Figure 17: Average concentration of NO3(µg N/l) in summer during 40 years (data was collected from 5 year periods)

During summer of 2011 the concentration of NO3 was quite low at the beginning of June as the consequence of low concentration in spring. However, in the epilimnion it started growing in the middle of June and dropped at the start of August while in the hypolimnion is stayed stable throughout the whole season. At the end of August the concentration of NO3 and total nitrogen became approximately the same.

Figure 18:The concentration of NO3 in epilimnion and hypolimnion in 2011 summer

The average concentration of total nitrogen in summer differed from 250 µg N/l to 691 µg N/l in the epilimnion, and from 231 µg N/l to 699 µg N/l in the hypolimnion (seeFig.19). The minimum mean value of total nitrogen in both epilimnion and hypolimnion was during the period from 1987 until 1991. The maximum mean value of total nitrogen in the epilimnion was reached in the period from 2002 to 2006 and the highest value in the hypolimnion was in the period from 1997 until 2001.

Figure 19: Average concentration of total nitrogen (µg N/l) in summer during 40 years (data was calculated from 5 years periods)