Phosphorus and Nitrogen in Three Lakes



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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.



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

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).

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. 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.



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.

Conditions at fieldwork location

Lake

Date of sample taking

Time of samp-ling

Surface temp. [°C]

Bottom temp. [°C]

Secchi depth [m]

Weather

Lake Erken

June 21st, 2011

11.00 am

17.2

13.4

5.3

Mostly sunny with occasional clouds, light wind

Lake Limmaren

June 22nd, 2011

11.30 am

17.0

15.5

1.1

Mostly sunny, light wind

Lake Largen

June 22nd, 2011

1.00 pm

16.5

4.5

5.8

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)






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.



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.

Total phosphorus and total nitrogen concentrations in pore water

Lake

Total phosphorus concentration [μg P/l]

Total nitrogen concentration [μg N/l]

Lake Erken

568

4870

Lake Limmaren

791

9440

Lake Largen

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.

Phosphate and nitrate concentrations in pore water




Phosphate concentration [μg P/l]]

Nitrate and nitrite concentration [μg N/l]

Lake Erken

171

53

Lake Limmaren

529

25

Lake Largen

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.

Total phosphorus and total nitrogen concentrations in lake water




Total phosphorus concentration [μg P/l]

Total nitrogen concentration [μg N/l]

Lake

Surface

Bottom

Surface

Bottom

Lake Erken

18.3

22.3

546

584

Lake Limmaren

84

---

1260

---

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.

Phosphate and nitrate concentrations in lake water (ND = not detectable)




Phosphate concentration [μg P/l]

Nitrate and nitrite concentration [μg N/l]

Lake

Surface

Bottom

Surface

Bottom

Lake Erken

3

14

4

33

Lake Limmaren

11

---

ND

---

Lake Largen

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.

Total phosphorus and total nitrogen concentrations in inlet water




Total phosphorus concentration [μg P/l]

Total nitrogen concentration [μg N/l]

Lake Erken

30.4

1050

Lake Limmaren

---

---

Lake Largen

20

449

The values in the inlet to Lake Largen were about half of the concentration in the inlet of Lake Erken.




Table 4b.

Phosphate and nitrate concentrations in inlet water




Phosphate concentration [μg P/l]

Nitrate and nitrite concentration [μg N/l]

Lake Erken

20

237

Lake Limmaren

---

---

Lake Largen

6

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 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 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 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.



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