An investigation into whether varying light intensity at a stream affects the species diversity

Foulden common has a variety of different habitats including grassland, young woodland, ponds, swamps, fenland, set-a-side and streams which enables a diverse range of organisms to be present in an area.

Life for plants and animals in any medium is a constant struggle. Besides the physical environment, predators, parasites and other competitors that have to be contended with; but compared with a terrestrial existence, fresh water offers a relatively stable environment.i

The small stream that runs a long the side of Foulden common is a tributary of the River Wissey, which eventually flows into the Great Ouse. The stream is spring fed so has a fairly high chalk content and so the waters are slightly alkaline. This is good for fresh water organisms, as the majority of fresh water organisms requite chalk (limestone: CaCO3) because they either have an exoskeleton or shell, both of which contain large amounts of chalk.ii

The stream is quite shallow so available light can penetrate the bottom easily however light intensity levels vary considerably along the stream due to trees and bushes which are quite dense in places, spaced well apart in other places and totally absent in other regions.

Aim: The aim of my investigation is to see whether varying light intensity at a stream affects the species diversity. Light intensities will be recorded in specific zones and samples will be taken in the stream at the zones to see if a difference in light results in a change in species diversity.

Background information: Rays of light falling on the surface of the water do not penetrate very far, and sedimentary matter, even organisms themselves, will absorb light. The most obvious effect of light is to promote the growth of the plants. The plants are mostly emergent plants so add little to the fresh water habitat and if anything maybe net removers of oxygen yet the plants do contribute food and shelter. Probably the most important species is foolcress, which is both submerged and emergent and will provide increased oxygen levels and shelter.iii

Plants will in turn affect animal life with greater number of species being present where submerged plants are predominantly found. This is mainly because they provide food and shelter yet the oxygen aspect is not so important as although plants will produce oxygen it will soon get washed downstream.

Photosynthesis is the fixation of carbon dioxide followed by its reduction to carbohydrate, using hydrogen from water. The necessary energy comes from absorbed light energy.iv There are two sets of reactions involved: the light-dependent reactions in the chloroplast lamellae and the light-independent reactions in the chloroplast stroma.

During photolysis in the light dependent stage of photosynthesis, water is broken down by light:

H2O –> 1/2O2 + 2H+ + 2e-

Oxygen is released which, in this case, is then released into the steam and may be a cause of an increase in species diversity. On a sunny day in a stream containing a number of water plants, the water can become super-saturated with oxygen in the region of weeds. At night, when photosynthesis ceases, the dissolved oxygen is at a low level due to the respiratory processes of the animals and plants themselves. Light directly controls the growth of plants and since plant life forms the basis of the animal food chains, a stretch of water rich on plant life will support a large animal community. Light also enables us to see and to be seen which is important when considering food relationships and predation.

There are two types of competition: Intra-specific which is the competition between individuals of the same species and inter-specific, which is the competition of individuals of different species

In intra-specific competition population numbers are constantly changing due to the following cycle: Abundant food is available to species so there is a high feeding rate, immigration of species and successful reproduction. The numbers of the species increase rapidly. The food supply then becomes scarce causing competition for food.

Inter-specific competition will result in the competing populations increasing in size more slowly than normal. This type of competition may result in the extinction of one of the competing populations. Examples of inter-specific reproduction are predator – prey relationships and competition for resources. In this situation as the prey population crashes, it is followed by a crash in the predator population due to a reduction in food. Consequently the prey population will then increase since the predation pressure is reduced, this is closely followed by a rise in predator numbers. This will have an effect on my results as depending on the stage of the cycle that is occurring, the number of individuals in a sample will be affected. In natural populations one species rarely out competes the other such that the population of the latter declines to zero because there are sufficient parts of the niches for two species to co exist.

Ecological succession is the changes in a community structure over time, for example in the stream the amount of plants, algae and detritus present. This may result in starvation of the species, lack of reproduction and emigration, which in turn decreases the number of individuals so the feeding rate declines and the food once again becomes abundant and the cycle, continues. This has an effect in my results as at the time that I took my sample a species may have been at any strange in the cycle which would have an obvious effect on whether the species was more or less abundant than at another time that the sample was taken.

A Niche is defined as the precise place in a habitat where an organism lives which includes its ‘role’ in the community. As habitats become more complex, such as the large variety of plant species in the stream and the varied substrate as shelter, the population density increases which leads to a natural emigration of prey from the defined area. With no sediment involved in the cycle it results in both population crashing as there is nowhere for the prey to hide so the predators have an advantage, consume the prey and then have a poor numbers of prey to feed on so starve or emigrate.

Many factors affect population density, which results in the population density constantly varying.

Prediction: From the background information above I predict that, as long as other variables (discussed below) remain constant, as the light intensity increases the species diversity will increase, as more plants will be present providing shelter, shade and oxygen for the species.

Variables : Many variables need to be controlled as explained in the theory above. These variables include :

* Oxygen levels – As long as the flow rate is kept as constant as possible the oxygen level will remain fairly uniform. It is not a factor that i can control but is a factor that iam aware of. To most organisms (excluding those capable of living in anaerobic conditions) oxygen is essential to their life processes. Oxygen becomes dissolved at the surface of the stream where air is in contact with the water. A shortage of oxygen can prevail in water which contains a large amount of decaying animal and plant matter, for the process of decomposition uses up the oxygen present. Certain aquatic worms and the larvae of a few species of insects possess special adaptations for living in conditions of extremely low oxygen levels.v Some aquatic insect larvae and other insects have made special adaptations of a thin body covering making a direct exchange of oxygen between their body tissues and the surrounding water. Where oxygen levels are high more species will be present as more species are adapted to coping where oxygen is present and living in aerobic conditions.

* Flow rate – The flow rate wil be monitored by timnig a cork floating down the stream to find it’s velocity in ms-1. I will attempt to find zones in which the flow rate is as constant as possible. In preliminary experiments the flow rate was meaured in all zones (the method of determining zones is explained below) and ranged from 12.5ms-1 to 14.0ms-1. I feel that this is not a significant difference but will be a variable that i am aware of. Current in a river or a stream results in the constant erosion of the stream bed and sand and silt being weashed away to setlle under large stones where the current is slacker. vi

* Depth of stream – The depth of the stream will affect the temperature and the number of species able to live there. It will be monitored by using a meter ruler to find zones with similar depths. The depth of the zones ranged from 0.33m to 0.41m so are fairly uniform.

* PH of stream – The pH of the stream was monitored by taking a sample of water from each zone and testing the sample with universal indicator. The pH ranged between pH7.5 and pH 8.0 which is not a significant differecen to affect the species present. Water dissociates using the equation : H2O ? H+ + OH-. The number of grammes of hydrogen ions per litre is a measure of the pH.Hard waterwith high calcium content has ahigh pH value as it is alkaline. A chalk stream may contain as much as100mgl-1 of calcium. Some creatures such as fresh water snails prefer hard water as their shells are composed largely of calcium carbonate. vii

* Temperature – If the flow rate and depth of the stream remain as constant as possible the temperature will not be signifiacntly different. The amount of oxygen that a given amount of water will hold in equilibrium decreases as the temperature increases. With a rise of temperature the rate of metabolism of many invertebrates may be trebled and an oxygen shortage can be easily created in the water. The temperature can increase when a stringer light intensity is reaching the water so this will need to be taken into consideration when carrying out my investigation.

* Substrate – the nature and amount of mineral salts present in any body of fresh water will depend on the geology of the surrounidng land since they enter in the drainage water from the soil. Mineral nutrients are also regenerated from the mud on the bottom of the stream by bacterial action.The land at Foulden common is very alkaline which will affect the salts present.

In limestone districts the soil will contain considerable quantities of calcium salts, which in the prescence of carbox dioixde become changed into bicarbonates and carbonates. Water with a high concentration iof these salts is alkaline. Carbon dioxide is very soluble in water and can eneter directly from the atmosphere. It combiones readily with the salts of calcium and magnesium as soluble bicarbionates and carbonates whihc sink to the bottom and become mixed with the mud. This will have an affect on my data as the different bicarbionates and carbonates present will affect the species living there.

The substrate at the bottom of the stream at Foulden common varies from sand to gravel to mud and is a factort hat i am aware of but unfortunately cannot control. When analysing the results of the investiagtion, the tyoe of substate will also be noted as the substate present may have an affect on the species diversity and number of individuals present.

* Chemical factors: The origin of all the fresh water of ponds, lakes and streams is the atmosphere in the form of rain. Apart from a small amount of dissolved carbon dioxide, rain-water is, however, absent of most substances available for the maintenance of life. The composition of fresh water varies with each body of water as fortunately very little of the water reaching the bodies of water actually fall directly on them as rain. Most of the water arrives as the result of drainage from the surrounding land and gaseous and solid substances become dissolved in it making the water a suitable medium, which can be colonised by animals and plants. The substances that have dissolved into the stream at Foulden Common may have an effect on the species present.

Nitrate levels – must be monitored to check that samples are not taken where there is an excessivly high nitrate concentration as light intensity and oxygen levels will be reduced due to eutrophication. An increased rainfall causes an increase in nitrogen content of soil and water supplies as nitrate is highly water-soluble and is leached from soil and washed into rivers and streams from surrounding areas. Grassland, which is grazed by animals, accumulates more nitrates in the soil than grassland, which is cut to make hay because animals produce urea, and excretion, which will be broken down into nitrates. The cut grass is taken away which removes nitrates from the area. This is a factor that should be taken into consideration as if an area by the stream has animals grazing it will have an effect in the nitrogen concentration in the stream.

Excess nitrates in water cause eutrophication and a chain of effects. Algae use the nitrate that has leached into the stream for growth and there is an increase in reproduction, which leads to an algal bloom. This results in less light penetrating the water, which means that algae and other plants cannot photosynthesise and therefore die. Aerobic putrefying bacteria decompose the plants and so consequently the dissolved oxygen levels decrease and the invertebrates and fish suffocate. The nitrate levels will have an effect on the species as the oxygen levels will be decreased and also the light intensity decreased.

* Sewage levels – must be monitored as this will have an effect on the species diversity. However, the stream is situated away from houses and humans and there are no signs to sewage being present at the stream. Sewage outflow into a stream results in there being a larger species diversity downstream as there is less pollution

As light intensity is the factor that iam varying, it is important that cloud cover remains constant throughout the investagion to ahcieve fair results at eah zone, despite the varying light intensities. I will therfire only take readings when cloud covers the sun as when there are no clouds present the light intensitywould increase at the specific zone beign sampled.

Methods involved with monitoring pH, depth of stream and flow rate of stream : Once the desired zones were marked, the light intensities recorded and the optimum number of sweeps and repeats per sample were established from the preliminary work above the pH of the zones were recorded by placing some of the stream water, from directly in front of the marker from the specific zone, into a testing point and three drops of universal indicator was added. This pH was then recorded.

At each zone the depth of the stream was tested by placing a meter ruler in the stream directly in front of the marker as above. The flow rate was then recorded by making a point 1.5m either side of the marker and timing how long it took a cork to travel the 3m down the stream. This was repeated three times and the average flow rate in ms-1 was calculated.

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