Phosphorus and
Nitrogen in Three Lakes
A study of the nutrient status in Lake Erken, Lake
Limmaren and Lake Largen
Uppsala University
Erken laboratory
Research school
Summer 2011
Supervised by Kurt Pettersson
Johan Sundin and Linda Čukure
Abstract
Lakes have a large influence on the climate of the world. They contain many different species,
including many plants, animals, bacteria, etc.
Phosphorus and nitrogen are components of outstanding value in many organisms. Due to
this, substances containing phosphorus and nitrogen are very important nutrients. In this
project, the concentrations of total phosphorus, total nitrogen, phosphate and nitrate and nitrite
have been measured. This has been done in the pore water, the lake water and the inlet water
of three different lakes, to be able to compare their nutrient status. The lakes are Lake Erken,
Lake Limmaren and Lake Largen. The concentration of nutrients in a lake also gives a hint of
if there is too high spillage of nutrients in the area, perhaps caused by humans.
A number of samples were taken from the surface and the bottom water, the inlet water
and the pore water in the sediments from the three lakes. These lakes are known to have very
different levels of nutrients. The nutrient concentration in the pore water and in the inlet water
made it possible to analyze the internal and external loading of nutrients in the lakes.
However, no water was found in the inlet of Lake Limmaren.
The concentration of nutrients was generally highest in Lake Limmaren. This shows that
Lake Limmaren is the most eutrophic of these three lakes. The second highest concentrations
were, for almost all the substances, found in Lake Erken. This gives a hint of that Lake Erken
is a mesotrophic lake. Roughly, the lowest concentrations were found Lake Largen, which
shows that the lake probably is oligotrophic. Lake Limmaren had, most likely, the highest
level of internal loading of nutrients, Lake Erken the second and Lake Largen the lowest. The
inlet of Lake Largen had the lowest concentration of nutrients and also the lowest amount of
water. This shows that Lake Largen had the lowest level of external loading of nutrients and
that the level of external loading of nutrients in Lake Erken was a bit higher.
One explanation of the results is the different amount of agriculture round the lakes.
2
Index
1. Abstract
2
2. Introduction
4
3. Method and material
7
4. Result
10
5. Discussion
12
6. Conclusion
15
7. Acknowledgements
15
8. Reference list
16
3
Introduction
Lakes have a large influence on the world, containing many different types of organisms. The
lakes in Sweden, defined as a relatively still body of fresh water in the inland, cover more
than 40 000 square kilometres, which is nine per cent of the country’s area (Björk et al, 2011).
This is a quite large area, compared with many other countries in Europe.
Different parts of a lake
A lake is divided into different parts. These parts are very dissimilar and contain different
types of organisms.
The littoral zone is the part of the water closest to the shore. This is the part of the lake,
where sunlight penetrates all the way to the bottom. This allows macrophytes (aquatic plants)
to grow there. The next part of a lake is the limnetic zone. It is the open water area. Where the
sunlight does not penetrate the water all the way down to the sediments is called the profundal
zone (Hall et al, 2011). There, it is very low primarily production, due to the lack of sunlight.
The productivity and activity of this zone depends highly on the species that live there in the
specific case and the level of nutrients. In this zone organics matter is also decomposed, by
bacteria. The euphotic zone is the layer from the surface and to the down to the area of the
lake where the concentration of light is too low for photosynthesizes.
In the summer only the top layer of the water is mixed. This leads to a clear line in the
temperature profile of the lake. One warmer layer, epilimnion, is established at the surface
and one colder at the bottom, named hypolimnion.
The particles on the bottom are, by the waves and the wind, naturally transferred to the
deepest spot in the lake. Small particles are faster transferred there than, for example, bigger
stones, however. Therefore, smaller particles are mostly found in the sediments in the deeper
parts of the lake and stones and rocks are more common by the shore.
Nutrient
Nutrients are a requirement for all organisms. Without nutrients, no life can exist. Due to this,
nutrients become a limiting factor for how large a population can become, as well as the size
of an organism or its’ longevity.
The elements nitrogen [N] and phosphorus [P] are two of the most important components
of organisms (Granhall and Lundgren, 1970). Due to this, nitrogen and phosphorus are
nutrients of extraordinary value. These two elements have, for a long time, been difficult to
detect in, inter alia lake water, because of their low concentrations. With the technology
today, however, we are able to measure the concentration more exactly than before. Due to
this, the nutrient status in lakes can be investigated very accurately (Ruttner, 1952).
The nutrient status in a lake is one of the most important factors for the organisms in a
lake. Many factors can disturb the nutrient balance in lakes. Some examples are fertilisation
from farms that increases the concentration of nutrients, ditching that transfers the nutrients
bound in the soil or unnatural algae blooms that consume all nutrients and oxygen. It is then
very important that the nutrient status is regularly controlled.
Phytoplankton
Phytoplankton are the component in the plankton community which are autotrophic. They are
very dependent of the level of nutrients in a system (Reynolds, 1984).
4
The phytoplankton population can, if the nutrient level is high, rapidly increase. This
happens in many eutrophic lakes. The consumption of oxygen is then also increasing, due to
the increase of organisms that consumes oxygen. A greater factor concerning the consumption
of the oxygen is, however, the decomposition of the phytoplankton. The phytoplankton sink
to the bottom and are decomposed, which requires a large amount of oxygen. It creates
oxygen free bottoms, which is a total devastation for many of the other the species in the
water.
Nitrogen
Small amounts of inorganic nutrient compounds are found in the rainwater. They dissolve in
the water in the atmosphere, which rains down and gathers in streams and lakes.
One major decomposition product from animals and plants is ammonia. Nonetheless, in
the present of oxygen, the ammonia is transformed into nitrate, by nitrifying bacteria. Due to
this, ground and spring water generally contains nitrogen in the form of nitrate.
The most common form of inorganic nitrogen in lakes is nitrate. In eutrophic lakes,
however, the concentration can, during spring and summer, be close to zero. The nitrate is
taken up by phytoplankton (Ruttner, 1952), which leads to a decrease of nitrate concentration
in the water. The concentration can, therefore, be higher during summer in oligotrophic lakes
than in eutrophic lakes.
At greater depths, with low concentration of oxygen in the hypolimnion, the nitrate almost
totally disappears and the level of ammonia increases. Without oxygen, bacteria and chemical
reactions can’t create nitrate of the nitrogen (Ruttner, 1952).
Total nitrogen is the total concentration of all forms of nitrogen.
Phosphorus
The phosphorus is transferred, directly or indirectly, to streams and lakes from the soil and
from phosphatic rocks.
Phosphate is, generally, eagerly held by the soil. This is the reason why spring water does
not contain so high concentrations of phosphorus as nitrate (Ruttner, 1952). Lake water
contains, normally, very small concentrations of phosphate.
Phosphorus is not only taken up by algae to mitigate their need of phosphorus. A surplus
is stored, if the conditions allow it. Some species store up to ten times as much as they
normally need. This very important factor can, under optimal circumstances, result in a major
decrease of phosphorus in the water (Ruttner, 1952).
Total phosphorus is the concentration of phosphorus in all forms (Minnesota River basin
Data Center, 2004).
Exchange between sediments and free water
Like, for example, the soil on land, sediment in lakes is very complex systems of organic and
inorganic colloids, crystalloid mineral components in a very wide range of particle sizes and
living organisms. Because of this, recognition of the role of a single component is very hard,
in normal conditions (Ruttner, 1952).
The water that is bound in the sediments is named pore water.
The sediment of a lake has a very important role in the metabolism of nutrient in the lake.
The substances closest to the water reach after a while the water and are then transferred to
the water of the lake. Another exchange proceeds naturally, in the opposite direction, forming
equilibrium – substances are also, by the sediments, withdrawn, from the water of the lake.
5
This means that nutrients can be transferred to the water in a lake by internal loading, from
the pore water, as well as by external loading, from the inlets.
Transport of nutrients between the sediments surface and the water mainly occurs when
the water closest to the sediment moves. The movements are mainly caused by the wind. The
sediment layers are warmer in the winter and colder in the summer than the water above it.
This temperature difference affects the transferring of nutrients between the water and the
sediment. The difference in temperature leads to a streaming of the water just above the
sediment. Due to this, the transport of materials from the sediments to the water increases
under these circumstances. Worth mentioning is also the movements of animals, not only
fishes and larger animals, but also smaller animals in the surface of the sediment (Ruttner,
1952).
Short description of the lakes
In this project, three very different lakes, located in Uppland, in the areas around Norrtälje,
were investigated. The lakes are Lake Erken, Lake Limmaren and Lake Largen.
Lake Erken is a mesotrophic lake. Since it is the main source of drinking water in
Norrtälje municipality, many earlier studies been done in the lake, including its’ nutrient
status.
The nitrogen fixation in the lake has been studied by Granhall and Lundgren (1970). The
nutrient level in the outletstream of Lake Erken has been investigated by Lingsten (1974). A
study about “the impact of physical and chemical factors on denitrification and phosphorus
release from sediments” was done by Boström and Pettersson (1987).
Lake Limmaren is an eutrophic lake. Not so many studies have been done in the lake. A
wide-ranging study was, however, done by Lindquist and Pettersson (1993).
Lake Largen is an oligotrophic lake. A very detailed study of the lake was done by
Bakunaite et al. (2000).
Of the areas round Lake Limmaren around 14 % is covered by agriculture, as show in the
study by Lindquist, 1993 and round Lake Erken it is about 10 % (Weyhenmeyer, 1999). Of
the areas round Lake Largen only a very small percent is covered by agriculture (Bakunaite et
al., 2000).
Aim and hypotheses
The aim of the project is to collect enough information about the nutrient status in the three
lakes to be able to compare their nutrient status, as well as mapping the internal and external
loading of nutrients.
The hypotheses are that the highest concentrations of total phosphorus, total nitrogen,
phosphate and nitrate and nitrite are found in the water of Lake Limmaren, since it is an
eutrophic lake. Lake Limmaren’s pore water also has the highest concentrations of total
phosphorus, total nitrogen, phosphate and nitrate and nitrite, as well as its’ inlet. The second
highest concentrations are found in the Lake Erken, since it is a mesotrophic lake. The lowest
concentrations are found in Lake Largen, since it is an oligotrophic lake.
6
Methods and material
Three lakes have been investigated in this project. The lakes are Lake Erken, Lake Limmaren
and Lake Largen. These lakes are all located in the south-eastern part of Sweden. The area of
Lake Erken covers 24 km2. The maximum depth of the lake is 21 m and the average depth is
9.0 m (Figure 1a). Lake Erken has a theoretical water residence of 7 years. Lake Limmaren
has an area of 5.5 km2. The maximum depth is 7.8 m and the average depth is 4.6 m (Figure
1b). The lake has a theoretical water residence time of 3 years. Lake Largen has a surface area
that covers 1.5 km2. The maximum depth of the Lake Largen is 21 m and the mean depth is
8.3 m (Figure 1c). The water residence time is 9 years. The watershed areas of all three lakes
are mainly covered with forests.
Fieldwork
One sample of the two first centimetres of the accumulation sediment was collected from the
three lakes. This was done at a deep part of each lake (Figure 1a, b and c), with an “Ekman
grabber”. The water in the “Ekman grabber” was removed and two centimetres of the top
layer of the sediment was collected. For information about conditions and the sampling
locations, see Table 1 and Figure 1.
The temperature in the lake water was measured, both at the surface (one meter below the
surface) and at the bottom. Water was collected with a “Ruttner sampler” from a boat, and
then the temperature in the water was measured with a thermometer. The temperature profile
was then analysed. If the temperature difference was too big, two water samples were taken
from the lake, otherwise just one sample. In the case of a high temperature difference, one
water sample was taken at the surface (one meter below the surface) and one at the bottom
(approximately at the depth of 15 meters in Lake Largen and Lake Erken). The water samples
were collected at a deep part of each lake (Figure 1a, b and c).
One sample of water was taken from the greatest inlet. The water sample was collected
with a flask, fixed on the top of a two meter long stick, in the inlet of Lake Erken. The stick
was put out from the beach of the inlet, and then, approximately a half meter below the
surface, water streamed into the flask. No water was found in the inlets of Lake Limmaren
and therefore no water sample from the inlets of could be taken. The inlets of Lake Largen
were just ditches. There the water sample was collected directly with a flask, in one of the
ditches.
Table 1.
Lake
Conditions at fieldwork location
Date of
Time of Surface Bottom Secchi
sample
samptemp.
temp.
depth
taking
ling
[°C]
[°C]
[m]
Weather
Lake
Erken
Lake
Limmaren
Lake
Largen
June 21st,
2011
June 22nd,
2011
June 22nd,
2011
11.00
am
11.30
am
1.00 pm
17.2
13.4
5.3
17.0
15.5
1.1
16.5
4.5
5.8
7
Mostly sunny with
occasional clouds, light wind
Mostly sunny, light wind
Mostly sunny, with
occasional clouds, light wind
Location of sediment, surface and bottom water sampling
Location of inlet water sampling
Figure 1a. Map of Lake Erken, with sampling stations (Granhall and Lundgren, 1970)
Figure 1b. Map of Lake Limmaren, with sampling stations (Lindquist and Pettersson, 1993)
8
Figure 1c. Map of Lake Largen, with sampling stations (Bakunaite et al., 2000)
Laboratory work
The time between the analyses and the sampling was, for phosphate and nitrate, round five
hours and for total phosphorus and total nitrogen between 30 to 60 hours.
The pore water, in the sediments, was separated from the sediments by centrifugation.
Eight 10 ml centrifugation tubes were filled with the sediments. They were centrifuged for ten
minutes. The pore water was collected with a pipette. The water was diluted (see result).
Phosphorus, that originally was bound, was transformed to orthophosphate, through
oxidative hydrolysis, including potassium persulfate, as described by Broberg (2003). The
concentration of the total phosphorus was then calculated. This was done with the water
sample from each lake (both surface and bottom samples), with the water from the inlets and
with the pore water.
The concentration of total nitrogen was analysed, as described by Broberg (2003). This
was done with the water samples from the lakes (both surface and bottom samples), the water
from the inlets and the pore water from the sediments.
The concentration of phosphate was analysed as described by Broberg (2003). This was
done with the water sample from each lake (both surface and bottom samples), with the water
from the inlets and with the pore water.
The concentration of nitrate was analysed as described by Broberg (2003). This was done
with the water sample from each lake (both surface and bottom samples), with the water from
the inlets and with the pore water.
9
Results
Pore water
Table 2 shows the results from the pore water tests. The first table, Table 2a, shows the
concentration of total phosphorus and total nitrogen. The second table, Table 2b, shows the
concentration of phosphate and nitrate and nitrite (NO3 and NO2).
Table 2a.
Lake
Lake Erken
Lake Limmaren
Lake Largen
Total phosphorus and total nitrogen concentrations in pore water
Total phosphorus
Total nitrogen concentration [μg
concentration [μg P/l]
N/l]
568
4870
791
9440
664
12400
The sediment of Lake Limmaren contained the highest concentration of total phosphorus.
Lake Largen contained the second highest and Lake Erken the lowest concentration of total
phosphorus.
The highest concentration of total nitrogen was found in the sediments of Lake Largen.
The second highest was found in Lake Limmaren and the lowest in Lake Erken.
Table 2b.
Lake Erken
Lake Limmaren
Lake Largen
Phosphate and nitrate concentrations in pore water
Phosphate concentration [μg
Nitrate and nitrite
P/l]]
concentration [μg N/l]
171
53
529
25
108
55
The sediments of Lake Limmaren had the highest concentration of phosphate, the sediments
of Lake Erken the second highest and the lowest concentration was found in Lake Largen.
Lake Largen’s pore water and sediment contained the highest concentration of nitrate and
nitrite. The second highest concentration of nitrate and nitrite was found in Lake Erken and
the lowest in Lake Limmaren.
Lake water
Table 3 shows the results from the measurement in the lake water, both the surface water and
the bottom water.
Table 3a.
Lake
Lake Erken
Lake Limmaren
Total phosphorus and total nitrogen concentrations in lake water
Total phosphorus
Total nitrogen concentration [μg
concentration [μg P/l]
N/l]
Surface
Bottom
Surface
Bottom
18.3
22.3
546
584
84
--1260
---
10
Lake Largen
8.6
20.7
402
585
The highest concentration of total phosphorus was found in Lake Limmaren. The lowest
concentration was found in Lake Largen, both in surface water and bottom water. The bottom
water in Lake Limmaren was, however, not determined (see “Method and materials”).
The water of Lake Limmaren (the surface water) contained the highest amount of total
nitrogen. The lowest value of total nitrogen concentration was found in the surface water of
Lake Largen.
Table 3b.
Lake
Lake Erken
Lake Limmaren
Lake Largen
Phosphate and nitrate concentrations in lake water (ND = not
detectable)
Phosphate concentration [μg Nitrate and nitrite concentration
P/l]
[μg N/l]
Surface
Bottom
Surface
Bottom
3
14
4
33
11
--ND
--ND
ND
1
106
Lake Limmaren had the highest concentration of phosphate. The value in Lake Largen was
the smallest, both at the bottom and at the surface. The concentrations in Lake Largen were
below the detection limit of the measurement instrument.
The concentration of nitrate and nitrite was, at the surface, the highest in Lake Erken and
lowest in Lake Limmaren, where it was below the measurable limit. At the bottom the highest
concentration was found in Lake Largen and lowest in Lake Erken. The sample was not
determined in Lake Limmaren.
Inlet water
Table 4a and Table 4b show the results of the samples from the inlet water of each lake,
except Lake Limmaren.
Table 4a.
Lake Erken
Lake Limmaren
Lake Largen
Total phosphorus and total nitrogen concentrations in inlet water
Total phosphorus
Total nitrogen concentration
concentration [μg P/l]
[μg N/l]
30.4
1050
----20
449
The values in the inlet to Lake Largen were about half of the concentration in the inlet of
Lake Erken.
Table 4b.
Lake Erken
Lake Limmaren
Lake Largen
Phosphate and nitrate concentrations in inlet water
Phosphate concentration [μg
Nitrate and nitrite
P/l]
concentration [μg N/l]
20
237
----6
11
11
The inlet of Lake Erken contained the highest amount of phosphate and nitrate and nitrite.
They were significantly higher than the concentrations in the inlet of Lake Largen.
Discussion
Pore water
The explanation of the low result for total phosphorus in Lake Erken is probably a
combination of the water residence time and the area of the lake. The lake is the deepest of the
three lakes. On the other hand, the area of the lake is much larger than the other lakes. This
makes it possible for the wind to make the water circulating. This statement is, in some ways,
confirmed by the small difference in temperature between the surface and the bottom water
(Table 1). The water residence time of Lake Erken is quite long – seven years – but it is still
less than the water residence time of Lake Largen. When the temperature at the bottom is
high, more phosphorus is released from the sediments to the pore water and later to the lake
water. The concentration of phosphate in the lake was the second highest, but also quite low,
compared with Lake Limmaren.
Lake Limmaren is a quite shallow lake. This leads to a high circulation between the
bottom and the surface water, which also is shown by the temperature. When the water at the
bottom is moving, there is a high exchange between particles in the sediments and the water.
Since the water in Lake Limmaren contained high levels of total phosphorus and phosphate
(Table 3a, b), it is not strange that high levels of total phosphorus and phosphate also were
found in the pore water.
The bottom water in Lake Largen, however, contained very little amount of total
phosphorus and phosphate. This indicates that it, at this point, is a very little amount of
internal loading of phosphorus, from the sediments to the lake water. At the surface the
temperature was 16.5 ºC and at the bottom it was only 4.5 ºC. The bottom water was almost 4
ºC, which shows that there was almost no circulation of water in the lake. If there is almost no
circulation, there can only be very little exchange of particles between the bottom sediments
and the water. The low level of circulation in the lake can be explained by the depth of the
lake. The lake is relatively deep, although it is quite small and surrounded by forest. This
means that there is very little wind that can make the water moving. The water residence time
is also nine years, which is very long, compared with the other two lakes. This means that the
exchange of water is very little, over all, in the lake. That is, of course, a factor that adds time
to the exchange of particles between the bottom water and the sediments.
The total nitrogen and the nitrate and nitrite concentrations do not exactly follow the same
pattern as the concentration of total phosphorus.
The second highest concentrations of total nitrogen and the lowest concentrations of
nitrate and nitrite were found in the pore water of Lake Limmaren. The water of the lake
contained very small concentrations of nitrate and nitrite as well – in fact below the
measurable limit. This indicates that phytoplankton in the lake consume large amounts of
nitrate and nitrite. This statement is supported by the very low “Secchi depth” in Lake
Limmaren, compared with the other lakes. The “Secchi depth” in Lake Limmaren was 1.1 m,
in Lake Erken it was 5.3 m and in Lake Largen it was 5.8 m. Since the level of water
circulation in Lake Limmaren is very high, nitrate and nitrite from the pore water and the
sediments are dissolved in the water. It is the nitrate and nitrite that limits the size of the
12
populations of phytoplankton. This is the reason why there are so low concentrations in the
pore water.
The highest concentration of total nitrogen nitrate and nitrite was found in Lake Largen,
which also had the highest “Secchi depth” and also the highest concentration of nitrate and
nitrite in the lake water. The lake contained a very small amount of phytoplankton in the
water, as shown by the “Secchi depth”, this due the lack of, most likely, phosphorus.
However, the concentrations of total nitrogen and nitrate and nitrite in the pore water of the
lake are quite similar to the concentration in Lake Erken. On the other hand, the “Secchi
depth” in the two lakes was also very similar, compared with the “Secchi depth” of Lake
Limmaren, and in some ways, also the concentration of nitrogen in the water.
Perhaps, the level of internal loading in Lake Erken is a bit closer to the level in Lake
Largen than the level in Lake Limmaren, according to the results. In the samples from the
inlet of Lake Erken, the concentrations of nutrients were higher than in the inlet of Lake
Largen. This shows that the level of external loading of nutrients in Lake Erken was higher
than the level in Lake Largen.
Lake Water
Since no water was found in the inlets, the high concentrations of total phosphorus and total
nitrogen need to be explained with a high level of internal loading of nutrients.
The concentration of total phosphorus was almost the same as in the study made by Lindquist
and Pettersson, 1993, from the same time of the year in 1992. It was just a bit lower. This
indicates that the concentration of phosphorus in Lake Limmaren has not changed that much
in the past twenty years. In some ways, this is a positive result. Lake Limmaren contains more
nutrients than it naturally should. Since phosphorus is a nutrient, this might indicate a small
reduction of the nutrient level of the lake. In the study by Lindquist and Pettersson, 1993,
phosphate in Lake Limmaren has also been investigated. The phosphate concentration was
measured both in 1991 and 1992. The value from 1991 was slightly higher than the result
from this study and the value from 1992 at this time is also a bit higher than the result. This
supports the statement that the nutrient level in Lake Limmaren is decreasing. However, the
difference is very small and the value can probably change dramatically in a short period of
time.
The second highest concentrations of total phosphorus and phosphate were found in Lake
Erken. This value also matched the level of agriculture around the lake. Higher concentration
of total phosphorus was also found at the surface of the lake than in the bottom. The
difference was, however, not that large. In a study made by Weyhenmeyer, 2000, the mean
concentration of total phosphorus in the lake is 27±9.6 μg P/l. The mean result is around the
named value. The value can, however, change in a short period of time during the seasons.
Lake Largen had the lowest concentrations of total phosphorus and phosphate. It also
matched the level of agriculture in the surrounding areas. The difference between the surface
and the bottom concentration was, however, higher than in Lake Erken. The reason is
probably the slower circulation of the water in the lake. The water was then not mixed as
much as in Lake Erken and more phosphorus gather at the bottom water. Due to a study made
by Bakunaite, 2000, the concentration of total nitrogen was, in March 4, 2000, quite similar to
the result from the surface water in this study. However, the study was made earlier in the
year, so the results can’t be used for a closer comparison.
The highest concentration of total nitrogen and the lowest concentration of nitrate and
were found in the water of Lake Limmaren. The concentrations of nitrate and nitrite in Lake
Limmaren were almost zero. The explanation is probably the high level of phytoplankton in
the water, as described in “Discussion”, “Pore water”. The phytoplankton, have probably
13
consumed the entire concentration of nitrate and nitrite, since it, in this case, is a limiting
factor, as described above. The entire concentration of nitrate and nitrite was, when the
sample was taken, consumed, but earlier in the year it was probably much higher – almost
certainly higher than in Lake Erken and Lake Largen. This means that the population of
organisms could grow almost limitless. This is supported by the low “Secchi depth”, which
indicates a large population of organisms in the water. When the population is big, it
consumes a higher level of nutrients than a small population. After a while, the concentration
of nutrients in the lake starts to decrease. This was probably the case with the concentration of
nitrate and nitrite in Lake Limmaren.
Lake Erken contained the highest concentration of nitrate and nitrite at the surface and the
second highest at the bottom. The level of organisms in the water was quite normal.
Therefore, not as much nitrate and nitrite as in Lake Limmaren was consumed. The water in
the inlet of Erken also contained quite much nitrate and nitrite, compared with the water in the
inlet of Lake Largen, which explains the result.
The water in Lake Erken contained the second highest concentration of total nitrogen. The
explanation for this is the same as for Lake Limmaren. There was a higher concentration of
total nitrogen in the bottom water, compared with the surface water. The result is reflecting
the values from the total phosphorus. The lake also contained the highest concentration of
nitrate and nitrite at the surface, due to the second highest level of phytoplankton in the water
consuming nitrate.
The lowest concentration of total nitrogen and the second lowest concentration of nitrate
and nitrite at the surface were found in Lake Largen. However, in the pore water of Lake
Erken the concentration of nitrate and nitrite was higher than in Lake Largen. Probably this
was a cause of lower concentrations of nitrate and nitrite in the inlet water and not a cause of
higher concentrations of organisms in the water. This statement is supported by the “Secchi
depth”. The difference between the concentration of nitrate and nitrite in the surface and the
bottom water was very big in Lake Largen, due to the low level of mixing of the water.
Inlet water
No water was found in the inlet of Lake Limmaren.
The highest concentration of total phosphorus and total nitrogen was found in the major
inlet to Lake Erken. The concentrations were almost twice as high as in the inlet of Lake
Largen – probably due to the agriculture in the area. This almost certainly shows that
agriculture has a vast impact of the nutrient status in lakes. However, the inlet of Lake Largen
was only a ditch with very low amount of water in it, while the inlet of Lake Erken was much
larger. Therefore, no exact conclusions can be formed and this must only be read as
speculations.
The inlet of Lake Erken also contained the highest concentration of phosphate and nitrate
and nitrite. The explanation for this is most likely the same as the explanation for the
concentrations of total phosphorus and the concentration of total nitrogen in the inlets.
Hypotheses
The hypothesis concerning the concentration of total phosphorus was proven right.
The hypothesis concerning the concentration of total nitrogen has been falsified. The pore
water in Lake Largen contained the highest concentration of total nitrogen, the pore water of
Limmaren the second highest and the pore water of Lake Limmaren contained the lowest
concentration of total nitrogen. This was explained with the large population of phytoplankton
14
in Lake Limmaren that consumed a large amount of nitrate. Concerning the lake water and the
water in the inlet, however, the hypothesis was proven right.
The hypothesis concerning the concentration of phosphate was proven right.
The hypothesis concerning the concentration of nitrate and nitrite was proven wrong. The
pore water of Lake Largen contained the highest concentration of nitrate and nitrite, Lake
Erken the second and Lake Limmaren the lowest. The surface water of Lake Erken had the
highest concentration of nitrate and nitrite, Lake Largen the second highest and Lake
Limmaren the lowest. The bottom water of Lake Largen contained the highest concentration
of nitrate and nitrite and the concentration in the bottom water of Lake Erken the lowest. The
large amount of phytoplankton in Lake Limmaren should have been taken into account.
Potential continuation of the project
It would be preferable if more parameters, like the oxygen level and the chlorophyll level,
were investigated. If these samples also were taken several times during a year they would
give pretty good and comparable pictures of the nutrient status in the three lakes.
Conclusion
The results clearly show that Lake Erken is a mesotrophic lake, that Lake Limmaren is an
eutrophic lake and that Lake Largen is an oligotrophic lake. All the values point in this
direction – all but the concentration of nitrate and nitrite, and the concentration of total
nitrogen in the pore water. This was, however, explained with the high amount of organisms
in the water of Lake Limmaren, which consumed almost all nitrate and nitrite.
It was also shown that Lake Limmaren had a high level of internal loading of nutrients.
Lake Largen had the lowest level of both internal and external loading of nutrients. Lake
Erken had the second lowest level of internal loading of nutrients and the highest level of
external loading of nutrients.
Enough information was collected to give a hint of the nutrient status in the lakes and
enable a quantitative comparison between them.
Acknowledgements
We would like to express our gratitude to Kurt Pettersson, our supervisor, for support, help in
the planning of the project, for assistance in the field work and for constructive criticism. We
would also like to thank Pia Larsson and Karin Beronius and the other personnel in the Erken
laboratory for their help, especially in the laboratory work, but also in the sampling process.
15
Reference list
Bakunaite, Jurgita; Bilén, Anna-Karin; Chowdhury, Tarun; Darth, Ann-Charlotte; Gustafsson,
Tommy; Hancke, Magnus; Hillman, Magnus; Holmgren, Alexandra; Jakimaviciute, Irma;
Karlberg, Andreas; Langhamer, Olivia; Lilliequist, Peter; Sandra, Lindahl; Lundin, Maria;
Lönn, Åsa; Persson, Jonas; Svedin, Lillemor; Oscar, Säwström; Åkerblom, Lena. 2000.
Largen Limnologisk undersökning Mars 2000. Page 3 – 20. Institute of Limnology, Uppsala
University.
Björk, Sven; Forsman, Arne; Grip, Harald. Nationalencyklopedin. 2011. Sjö. WWW
document June 17th, 2011: http://www.ne.se/lang/sjö/306619. Date visited June 17th, 2011.
Boström, Bengt;Pettersson, Kurt. 1987. Batch experiments on the impact of physical and
chemical factors on denitrification and phosphorus release from sediments. Institute of
Limnology, Uppsala University.
Broberg, Anders. 2003. Water and sediment analyses. Page 24 – 58. Fourth edition.
Department of limnology, Uppsala universitet.
Granhall, Ulf; Lundgren, Alf. 1970. Nitrogen fixation in Lake Erken. Department of
Microbiology, Agriculture College and Institute of Limnology, Uppsala University.
Hall, Ragnar; Kullberg, Anders; Åse, Lars-Erik. Nationalencyklopedin. 2011. Strand. WWW
document June 18th, 2011: http://www.ne.se/lang/strand/316732#. Date visited 18th, 2011.
Lindquist, Ulf; Pettersson, Kurt. 1993. Sjön Limmaren med tillflöden. Page 4 – 17. Institute of
Limnology, Uppsala University.
Lingsten, Lars. 1974. Nutrients in the outletstream of Lake Erken. Uppsala University.
Minnesota River basin Data Center, Minnesota State University, Mankato. 2004. Phosphorus.
WWW document September 15th, 2004: http://mrbdc.mnsu.edu/mnbasin/wq/phosphorus.html.
Date visited June 18th, 2011.
Reynolds, C. S.. 1984. The ecology of freshwater phytoplankton. Cambridge studies in
ecology.
Ruttner, Franz. 1952. Fundamentals of limnology. Page 87 – 157. Third edition. Waleter de
Gruyter Co., Berlin, Canada.
Weyhenmeyer, Gesa. 1999. Lake Erken. Page 4. Department of Limnology, Uppsala
University.
16