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
Climate change effect on the seasonality of nutrients in Lake Erken
Table of contents
Abstract...................................................................................................................................... 3
Introduction................................................................................................................................ 4
Materials and methods............................................................................................................... 6
Results ....................................................................................................................................... 8
Discussion................................................................................................................................ 22
Conclusion............................................................................................................................... 24
Acknowledgments.................................................................................................................... 24
Reference list............................................................................................................................ 25
2
Climate change effect on the seasonality of nutrients in Lake Erken
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 project we 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.
3
Climate change effect on the seasonality of nutrients in Lake Erken
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
4
Climate change effect on the seasonality of nutrients in Lake Erken
Seasonality in 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. (Pettersson et 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 nitrogen in 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 concentrations in Lake Erken.
5
Climate change effect on the seasonality of nutrients in 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 point and 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)
6
Climate change effect on the seasonality of nutrients in Lake Erken
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:
𝑁𝑂3 − + 𝐶𝑑 + 𝐻2 𝑂 → 𝑁𝑂2 − + 𝐶𝑑2+ + 2𝑂𝐻 −
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
7
Climate change effect on the seasonality of nutrients in Lake Erken
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 it is 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
Winter
Spring
Summer
Autumn
Year
1972-1976
Standard dev.
1977-1981
Standard dev.
1982-1986
Standard dev.
1987-1991
Standard dev.
1992-1996
Standard dev.
1997-2001
Standard dev.
2002-2006
Standard dev.
2007-2011
Standard dev.
109
21
137
15
104
19
114
30
105
30
94
19
108
5
82
28
45
10
39
9
48
20
84
32
48
10
72
16
61
8
82
37
78
1
90
13
94
9
70
17
105
18
84
21
100
16
87
20
103
21
92
13
82
37
89
8
108
20
113
16
96
12
133
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 to
1976. 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).
8
Climate change effect on the seasonality of nutrients in Lake Erken
Figure 3: Average concentrations of PO4 and total phosphorus (µg P/l) in spring during 40
years (data was calculated from 5 year periods)
PO₄ and total phosphorus concentrations in spring
Concentration (µg P/l)
30
25
20
15
10
MRP (PO₄-P)
Total Phosphorus
5
0
Time periods
During spring time the concentration of total phosphorus and PO4 suddenly 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
45
Concentration (µg P/l)
40
35
30
25
20
MRP (PO₄-P)
15
Total Phosphorus
10
5
0
Date
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).
9
Climate change effect on the seasonality of nutrients in Lake Erken
Figure 5: Average concentration of PO4 (µg P/l) in summer during 40 years (data was
calculated from 5 year periods)
Concentration (µg P/l)
PO₄ concentration in summer
40
35
30
25
20
15
10
5
0
MRP (PO₄-P) Epilimnion
MRP (PO₄-P) Hypolimnion
Time 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 PO4 in hypolimnion and epilimnion in 2011 summer
80
Concentration (µg P/l)
70
60
50
40
30
MRP (PO₄-P) epilimnion
20
MRP (PO₄-P) hypolimnion
10
0
Date
The average concentration of total phosphorus in epilimnion was more than 11 µg P/l (19821986) 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 2 µg P/l (1982-1986) and less than 34 µg P/l (20022006). 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).
10
Climate change effect on the seasonality of nutrients in Lake Erken
Figure 7: Average concentration of total phosphorus (µg P/l) in summer during 40 years
(data was collected from 5 year periods)
Total phosphorus concentration in summer
Concentration (µg P/l)
60
50
40
30
Total Phosphorus Epilimnion
20
Total Phosphorus Hypolimnion
10
0
Time 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
100
Concentration (µg P/l)
90
80
70
60
50
40
Total Phosphorus epilimnion
30
Total Phosphorus hypolimnion
20
10
0
Date
11
Climate change effect on the seasonality of nutrients in Lake Erken
The average concentrations of PO4 (see Fig.9) in autumn were more than 8 µg P/l (19821986) 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 50 µg 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)
Concentration (µg P/l)
PO₄ and total phosphorus concentrations in autumn
60
50
40
30
20
MRP (PO₄-P)
10
Total Phosphorus
0
Time 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 PO4 and total phosphorus in 2011 autumn
80
Concentration (µg P/l)
70
60
50
40
30
MRP (PO₄-P)
20
Total Phosphorus
10
0
Date
12
Climate change effect on the seasonality of nutrients in Lake Erken
The average concentrations of PO4 in winter (see Fig.11) were more than 4 µg P/l (20072011) 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)
Concentration (µg P/l)
PO₄ and total phosphorus concentrations in winter
45
40
35
30
25
20
15
10
5
0
MRP (PO₄-P)
Total Phosphorus
Time 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
45
Concentration (µg P/l)
40
35
30
25
20
MRP (PO₄-P)
15
Total Phosphorus
10
5
0
Date
13
Climate change effect on the seasonality of nutrients in Lake Erken
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 concentration in 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)
Concentration (µg N/l)
NO₃ concentration in spring
160
140
120
100
80
60
40
20
0
Nitrate (NO₃)
Time 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
Concentration (µg N/l)
160
140
120
100
80
60
NO₃
40
20
0
Date
14
Climate change effect on the seasonality of nutrients in Lake Erken
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)
Concentration (µg N/l)
Total nitrogen concentration in spring
720
700
680
660
640
620
600
580
560
Total Nitrogen
Time 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
900
Concentration (µg N/l)
800
700
600
500
400
300
Total Nitrogen
200
100
0
Date
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
15
Climate change effect on the seasonality of nutrients in Lake Erken
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)
Concentration (µg N/l)
NO₃ concentration in summer
100
90
80
70
60
50
40
30
20
10
0
Nitrate (NO₃) Epilimnion
Nitrate (NO₃) Hypolimnion
Time 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
Concentration (µg N/l)
120
100
80
60
Nitrate (NO₃) epilimnion
40
Nitrate (NO₃) hypolimnion
20
0
Date
16
Climate change effect on the seasonality of nutrients in Lake Erken
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 (see Fig.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)
Concentration (µg N/l)
Total nitrogen concentration in summer
800
700
600
500
400
300
200
100
0
Total Nitrogen Epilimnion
Total Nitrogen Hypolimnion
Time periods
Total nitrogen concentration during summer was also quite stable and reached its peak in the
middle of July in the epilimnion and in the end of July in the hypolimnion (see Fig.20). The
amount of total nitrogen in each stratified layer differed and changed with each other.
Concentration (µg N/l)
Figure 20: The concentration of total nitrogen in epilimnion and hypolimnion in 2011
summer
1000
900
800
700
600
500
400
300
200
100
0
Total Nitrogen epilimnion
Total Nitrogen hypolimnion
Date
17
Climate change effect on the seasonality of nutrients in Lake Erken
The mean value of concentration of NO3 varied from 22 µg N/l to 95 µg N/l in autumn (see
Fig.21). The lowest value was reached in the period from 1972 to 1976. The maximum
average value was during the period from 1992 until 1996.
Figure 21: Average concentration of NO3 (µg N/l) in autumn during 40 years (data was
collected from 5 year periods)
Concentration (µg N/l)
NO₃ concentration in autumn
100
90
80
70
60
50
40
30
20
10
0
Nitrate (NO₃)
Time periods
The concentration of NO3 in autumn of 2011 was rapidly increasing throughout the whole
period from the middle of September until the middle of November (see Fig.22). Growth was
the most rapid at the first half of October.
Figure 22: The concentration of NO3 in 2011 autumn
160
Concentration (µg N/l)
140
120
100
80
60
NO₃
40
20
0
Date
18
Climate change effect on the seasonality of nutrients in Lake Erken
The average concentration of total nitrogen differed from 426 µg N/l to 734 µg N/l and the
lowest mean value was reached in the period of 1987-1991 (see Fig.23). The maximum total
nitrogen concentration in autumn was during the period from 1992 to 1996.
Figure 23: Average concentration of total nitrogen (µg N/l) in autumn during 40 years (data
was collected from 5 year periods)
Concentration (µg N/l)
Total nitrogen concentration in autumn
800
700
600
500
400
300
200
100
0
Total Nitrogen
Time periods
The concentration of total nitrogen during autumn of 2011 remained quite stable and no
significant changes occurred (see Fig.24). It dropped a few times in the end of September and
in the middle of November and was the highest in the middle of September when the season
started.
Figure 24: The concentration of total nitrogen in 2011 autumn
800
Concentration (µg N/l)
700
600
500
400
300
Total Nitrogen
200
100
0
Date
19
Climate change effect on the seasonality of nutrients in Lake Erken
During winter the concentration of NO3 was higher than 54 µg N/l and lower than 246 µg N/l.
The lowest concentration of NO3 was noted during the period from 1987 up to 1991 (see
Fig.25). The highest concentration of NO3 was reached in the period of 1977-1981.
Figure 25: Average concentration of NO3 and total nitrogen (µg N/l) in winter during 40
years (data was collected from 5 year periods)
Concentration (µg N/l)
NO₃ concentration in winter
300
250
200
150
100
Nitrate (NO₃)
50
0
Time periods
The concentration of NO3 in winter of 2011 increased until the middle of February and then
remained table until the end of the season in the middle of May (see Fig.26).
Figure 26: The concentration of NO3 in 2011 winter
165
Concentration (µg N/l)
160
155
150
NO₃
145
140
135
130
Date
The mean concentration of total nitrogen varies between 609 µg N/l and 1220 µg N/l in
winter (see Fig.27). The lowest average value was in the period of 1992-1996. The maximum
mean concentration was reached in the period from 1987 to 1991.
20
Climate change effect on the seasonality of nutrients in Lake Erken
Figure 27: Average concentration of total nitrogen (µg N/l) in winter during 40 years (data
was collected from 5 year periods)
Concentration (µg N/l)
Total nitrogen concentration in winter
1400
1200
1000
800
600
400
200
0
Total Nitrogen
Time periods
The concentration of total nitrogen during the winter of 2011 increased more rapidly in the
period from the middle of January until the middle of February and still continued to increase
but at a much slower rate until the end of the season in the middle of March (see Fig.28).
Figure 28: The concentration of total nitrogen in 2011 winter
760
Concentration (µg N/l)
750
740
730
720
710
Total Nitrogen
700
690
680
Date
21
Climate change effect on the seasonality of nutrients in Lake Erken
Discussion
The effect of climate change on the seasonality of Lake Erken
The rise of temperature and other seasonality influencing factors such as human interference
and animal waste have loaded Lake Erken with nutrients, more precisely, nitrogen and
phosphorus. In Lake Erken phosphorus is found as orthophosphate (PO4) which is the form
that plants use as a source of nutrients. One of the forms nitrogen is found is nitrate (NO3),
which dissolves easily, then travels to lake through groundwater and streams and is the source
of nutrients for aquatic plants primary production.
Different sources of phosphorus and nitrogen such as wastewater, fertilizers, animal waste etc.
from lake’s drainage area have access to Lake Erken which resulted in orthophosphate and
nitrate concentrations in the water natural loading. If we look at the results of PO4 and total
phosphorus average concentrations in winter in the time period from 2002 to 2006 (see
Fig.11), we see a sharp increase in MRP (PO4) and in the next period its concentration returns
to normal. Since we have no information available about the human interference in this time
period, we cannot explain this deviation from the long-term average. However, if we look at
the results from NO3 and total nitrogen in winter during the time period from 2002 to 2006
(see Fig.25 and Fig.27), we can see that the concentrations of NO3 and total nitrogen have not
changed significantly, but there is a sudden growth of total nitrogen and a decrease of NO 3
during the time period from 1987 to 1991. From this we can conclude that in Lake Erken the
nitrogen and phosphorus influencing factors differ from each other. (Sources of nutrients and
pesticides) Actually the decrease in phosphate relates to the increase in nitrate. With fewer
phosphates present, there is less nutrients for algae. Less algae means that there will be more
nitrate left in the water. (Kristi Strandberg and Dr. Michael Ross, 2002)
If we look at the results from Figure 5, 7 and 9 in the time period from 1982 to 1986, there is
a slight decrease of total phosphorus. If we had information about total nitrogen in this period
we could determine if the factor causing the decrease of total phosphorus also reduces the
total nitrogen concentration.
According to Ruttner (1953) the plankton algae is able to store more than ten times as much
Phosphorus as they normally contain which means that the plankton algae might be the cause
of reduction of total phosphorus concentration in Lake Erken during its bloom. The
concentrations typically decrease in the epilimnion during summer stratification as nutrients
are taken up by algae and eventually transported to the hypolimnion when the algae die and
settle out. During this period, any "new" input of nutrients into the upper water may trigger a
bloom of algae. (Nutrients, 2004)
If we look at the NO3 and total nitrogen concentrations in winter during the time period from
1987 to 1991 (see Fig.24 and Fig.26), the concentration of Total Nitrogen is abnormally high
while the concentration of NO3 is unusually low. Typically if the concentration of total
nitrogen increases, the concentration of NO3 also becomes higher, but as you can see in
22
Climate change effect on the seasonality of nutrients in Lake Erken
Figure 24 and Figure 26 it is the exact opposite. Total nitrogen concentration was extremely
high compared to other periods.
The period of ice cover has shortened and the mixing periods in autumn and spring have
increased (see Table 1) due to enrichment of nutrients. Since we have no data of the weather
conditions in Lake Erken’s drainage area during this period we can only assume that they
influenced the length of different seasons throughout the year. One of the reasons of
seasonality change can be wind that could have caused too strong surface layer mixing in
summer that resulted in increased water temperature in the bottom layer and thus the length of
summer may have decreased. Also we assume that due to the warmer climate, because global
warming is occurring all over the world, length of winter had decreased during the period of
40 years and thus the days of spring and autumn had increased. Also, the decrease of days of
ice cover can be explained by the fact that there were some errors in observations and
therefore some data of the lake condition (whether it was stratified or there was an ice cover)
were missing and this resulted in not accurate results. (Blenckner, Omstedt & Rummukainen,
2002) Although the processes governing the formation and thawing of lake ice depend on
multiple interacting meteorological and limnological factors, air temperature is considered to
be the dominant of them all. (Adrian R et all, 2009)
Winter mixing brings nutrients up from below, concentrating them near the surface. Spring
warming creates a surface layer that floats on top, halting the supply of nutrients brought to
the surface. As spring turns to summer, nutrients in the surface layer are consumed
by phytoplankton, reducing nutrient availability at the surface. As summer sets in,
phytoplankton die and drift to the bottom, taking the nutrients they ingested with them.
Surface waters are now left with few nutrients available. Through the summer, this situation is
reinforced as the surface waters are warmed and the stable situation of stratification sets in.
Once autumn sets in with cooler days, a limited amount of vertical mixing brings nutrients up
from below. In winter, heavy winds and plummeting temperatures cause strong mixing again.
In general, when nutrients are found near the surface, they are not plentiful at deeper levels
and vice versa. (Part 1-What Causes a Phytoplankton Bloom in the Gulf of Maine?, 2008)
Climate change can be influenced by many natural and anthropogenic factors. Climate itself is
unstable due to the natural factors which humans cannot stop or prevent from happening such
as solar energy output, volcanic eruptions, ocean currents, orbital variations or, so-called,
Milankovitch cycles. However, human activities have added to the already unstable climate
on Earth. Deforestation, greenhouse gas emission and the development of industry have had a
great effect on the increase of the mean yearly temperature.
23
Climate change effect on the seasonality of nutrients in Lake Erken
Conclusion
Lakes are good indicators of climate change. The fluctuation of temperature, changes of the
water level, shifts in the timing of spring and autumn mixing periods, ice duration, and algal
blooms can be some of the signs of climate change.
Our first hypothesis, that the climate change had no effect on total phosphorus, total nitrogen,
nitrate and phosphate concentration, was confirmed to some extent because there were almost
no significant changes in phosphate and total phosphorus concentrations. However, nitrate
and total nitrogen concentration were very varied during the period of 40 years. Our second
hypothesis, that the climate change affected the seasonality of nutrients in Lake Erken, was
partly confirmed because we can clearly see the changes of nutrient concentrations in water
during different seasons throughout 40 years, but the duration of each season has altered due
to global warming and other factors, but it is evident that the climate change is the most
influential and significant factor for such development.
There are no long term changes that would indicate the nutrient concentrations in the future,
but that is the reason to study all the data available to make it easier to predict any shifts.
Acknowledgements
We would like to express our gratitude to our supervisor Kurt Pettersson for guiding us
through this report and for pointing out our mistakes and ways to correct them, to the staff of
Erken Laboratory for letting us assist them during analyses, to Karin Beronius for making this
project happen, to our assistants and the rest of the staff for making our stay here enjoyable.
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Climate change effect on the seasonality of nutrients in Lake Erken
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