Conclusions
My bar graph shows that at every site sampled there were more freshwater invertebrates in riffles than there were in pools, and the Mann-Whitney test shows that there is a significant relationship between the two variables, therefore my prediction was correct.
The high abundance of freshwater invertebrates may be due to the width of the riffles being considerably smaller than that of the pools. This would mean that the water is travelling through a reduced area making it flow faster and therefore increasing its velocity. This causes the water to become well oxygenated as it splashes over stones and rocks. As most organisms need plenty of oxygen to survive, riffles seem more of a suitable habitat for them to live in. These invertebrates have both structural and physiological adaptations to prevent themselves from being swept away by the current, for example they can clamber close to rocks and stones, hang on to rocks using their claws, hooks or suckers. Most of them are also very good swimmers so that if they are dislodged they can recover their position easily. These invertebrates have also developed behavioural adaptations such as adjusting their body parallel to the flow of water to minimise the surface area of their body colliding with the current. Also, because most of the invertebrates are not herbivores they do not need to feed on living plants. Therefore it does not affect them that the strong current of the riffles carries away any plants that may grow there. This is in contrast to pools where the velocity of the water is slow enough for plants to keep a footing and grow. Also in riffles the temperature is low which means the water is able to carry more oxygen.
There may also be far less freshwater invertebrates in pools because they are quite deep, which allow larger predators such as fishes to survive in them and therefore feed on the freshwater invertebrates. If the fish are removed there may be far more invertebrates found in the pools as the current there is slow enough for detritus and plants to collect which becomes a rich source of food for the invertebrates. However, if the herbivores and detrivores go to pools in search of food they probably get eaten by the fish. For example, mayfly feed on producers such as algae and plant material that have collected in pools which they then turn into biomass. This is an important part of the food chain, which is continued by its predators such as other fish, crustaceans and insects. This reduces the number of mayfly in pools, but means they survive in high abundance in riffles where it is too shallow for many fish to survive.
Detritus/algae Mayfly Bull headed fish
On site 3 looking at my bar graph there was a high proportion of freshwater invertebrates in the riffles compared to the other sites. This was probably due to the producers on the banks of the riffles where leaves from these plants were falling into the stream and providing a very good food source. Because the freshwater invertebrates are in the kingdom of animals, they have to feed heterotrophically, meaning they need organic molecules as their carbon source. Their cells do not contain chloroplasts and so they need to count on either primary production on the banks, plants that can survive in the waters or by preying on other freshwater invertebrates. Preying on other freshwater invertebrates will not provide much energy; this means that the abundance of freshwater invertebrates especially those living in riffles rely heavily on the primary production occurring on the banks of the stream because plants cannot usually grow in their fast flowing waters. The niche of these plants is particularly important in providing the energy input for the whole ecosystem of organisms living in the stream, therefore the more detritus falling into the riffles the higher the abundance of freshwater invertebrates. Also riffles get plenty of sunshine because they are not very deep, which encourages diatoms to grow on rocks as well as organic debris, therefore a rich food source for the freshwater invertebrates and therefore increasing their abundance in riffles.
Looking at my bar graph on site 2 and 4 the difference in the abundance of freshwater invertebrates between pools and riffles was not as much as for the other sites. This may be because in site 2 there were a lot of algae on the rocks sampled. Although this would have provided a good food source for the invertebrates, interspecific competition for oxygen between the invertebrates and the algae would have been taking place. This would mean that organisms which require a lot of oxygen will be reduced as they can no longer get enough oxygen to survive, therefore the abundance of freshwater invertebrates needing a lot of oxygen being reduced. In site 4 the difference in abundance of freshwater invertebrates between pools and riffles may have been low because 4 fishes were sampled in the riffles. Therefore this suggests that there is a high population of fish in the riffles of site 4. This would mean that they were feeding heavily on the invertebrates and again reducing their number, whereas no fish were found in the pools of that particular site.
From looking at my scatter graph generally as velocity increases the abundance of swimming mayfly increases. This may be because swimming mayfly nymphs cannot beat their gills because their gills are small and rigid and so need to be in fast flowing water to get enough oxygen to survive. They have morphological adaptations to survive in waters of strong current by having hooks on their feet and fastening themselves onto rocks while feeding on detritus and other invertebrates. They can bring their abdomen up which has gills attached to it to get more water over them and therefore oxygen. Therefore the faster the velocity of the water the more oxygen there is and the more swimming mayfly able to survive. Their high abundance in fast velocity water may be due to them being good swimmers allowing them to escape predators with their agility. Their torpedo-shaped bodies and filaments with hairs attached to each of the three tails allowing them to swim very fast.
However my scatter graph shows that after a certain point, as velocity increases the abundance of swimming mayfly decreases. This may be because high velocity water means that detritus and algae are swept away and the swimming mayfly nymphs are not able to get enough food. Detritus, dead leaves from surrounding plants is a poor food source, its source of nitrogen is low and carbon as cellulose means it cannot be digested and used by macrorganisms, therefore swimming mayfly are also herbivores, this means that they need to be in medium velocity which allows plants such as algae to grow and so as my scatter graph shows are not in a high abundance at very high velocities.
Therefore my prediction was only partly correct as the swimming mayfly probably need a balanced velocity which allows them to get enough oxygen while allowing enough food to remain in the water. Looking at my graph I do not wholly accept my prediction because although generally as the velocity increases so does the abundance of swimming mayfly but after a point, as the velocity increases then abundance of swimming mayfly starts to decrease.
Evaluating Procedures
I think the procedure was suitable to an extent because it allowed us to compare the difference in abundance of freshwater invertebrates in pools and riffles. Other influential factors such as velocity, width, temperature and depth were also measured allowing us to draw conclusions on the abundance of the invertebrates on the basis of these abiotic factors. The sampling method again, to an extent was random as the rocks were picked randomly according to their size. Also, using this particular type of sampling was appropriate, as kick sampling could not be carried out due to the excessive amount of stones in the stream. This meant there was not a smooth surface to rub the foot against and collect the sediment in a net. Also, because most of the organisms live on the rocks washing them meant the sampling would be more representative, as kick sampling on the top surface of the rocks would do nothing for the organisms living under the rocks.
However, there were major limitations to the investigation and the method of sampling. One of the major limitations was the failure to standardise the rocks we sampled the organisms from. Firstly, we tried to standardise the rocks by selecting only those that were palm-sized. However it was hard to find the same-sized rocks. This meant we had to settle for slightly larger or smaller rocks. This would have meant if the rock was larger than others, that more invertebrates could have been present on the rock because there is more space on it. Therefore we would have calculated the abundance in the stream as more, in relation to the other sites just because we had a larger rock. Also some of the rocks had algae on which did make quite a lot of difference to the abundance of the invertebrates because on site 2 there was a lot of algae on the rocks we sampled, which resulted in us finding 89 organisms whereas on the other sites we did not find more than 38 organisms. This resulted in a huge increase and could have given us anomalous results. To improve these, a different part of the stream could have been sampled where there were rocks of the same size as well as rocks with no algae on. This would have made the conditions of sampling the same so that more organisms are not calculated on the basis of a rock being bigger and having algae on.
The second major limitation occurred due to the data being taken from different groups. This may have meant that procedures were carried out differently. For example some groups may have washed the rocks more than others which may mean they obtained nearly all the invertebrates from the rocks, whereas groups who washed them less may have had some invertebrates remaining on the rock which would have reduced their abundance of invertebrates on the basis they washed their rocks less. Some groups may have been slower to identify the organisms so that they did not have enough time to count their entire sample so that the abundance recorded was less than there actually was. This could have been improved by standardising the procedure using a set plan for all the groups, such as washing the rocks against the net nine times. Also enough time should have been given so that all the groups finished identifying and counting their organisms. This would ensure everyone carried out the procedure identically and there was enough time to get the actual amount of organisms that were in the sample, which would give fairer results.
I think the third major limitation was that at some points of the day it began to rain so that when we carried out the study some sites had had more rain than other sites had. For example for the first site the rain stopped while for some of the other sites the rain began again. This would have affected some of the abiotic and biotic factors. For example the depth could have been increased allowing fish to swim into riffles. The velocity may have been increased and displaced many of the invertebrates from their natural habitats. Therefore we would have got unreliable results such as the abundance of invertebrates being reduced in some sites while remaining normal in other sites because there had been no rain there when we sampled there. To improve this, the weather could have been forecasted and the sampling carried out during a constant weather pattern where there is no rain. This would mean that we would get more reliable results of the abundance of freshwater invertebrates in the stream in fairer conditions.
Another major limitation was that errors could have been made when measuring the abiotic factors. For example when measuring the velocity it was hard to make sure the hydropop was kept at a constant depth. We could have moved it without being aware so that it was no longer at the surface of the water. This may have prolonged the time it took for the hydropop to stop therefore giving us a slower velocity than it really was. Also it was difficult to record the exact time the hydropop stopped so that we may have calculated more or less time for the actual time it took for the hydropop to reach the end. To improve our velocity readings we could have used an electronic flow meter which automatically calculates the velocity electronically when placed on the surface of the water. This would give us more accurate velocities than having to time the hydropop and then calculating the velocity using a formula.
Another major limitation was that we only sampled each site for a small amount of time. We spent a longer time at different sites, and we were improving our identification skills as we went along so that later on we were able to identify more of the organisms in the time provided. Therefore the study was not fair across the sites, so that fewer invertebrates may have been recorded for the first sites because of our identification skills being slow. To improve this we could have spent more time at the sites, maybe sampling the organisms regularly for 3 days. This would give us more reliable results.
Evaluating Evidence
I have got anomalous results at site 2 because the abundance of swimming mayflies seems to be very low, an average of 37 was found at a velocity of 23.5cm/s while at site 3 there were 101 at a velocity of 23cm/s. This may be due to sheep faeces being deposited at site 2. Mayflies develop in unpolluted water and therefore their abundance may have been reduced by this.
We have an unusually high number of freshwater invertebrates in site number 3 made up of a high proportion of swimming mayfly. When I looked at the individual data it seems to be only one group that received this high proportion of swimming mayfly. I think at site 3 there was an anomalous result because there is an excessive amount of swimming mayfly compared to the other sites. This may have been because a specific rock was sampled under which swimming mayfly eggs had hatched so that there was a large number of young swimming mayfly nymph. This would not have been representative of the site because these offspring will gradually move down the stream. On the other hand rocks may have been missed with invertebrates living in colonies such as blackfly larvae. I think to make this fairer the study should have been conducted over a few days.
I am going to determine whether the sample for pools and riffles was large enough
Sample size for both pools and riffles was insufficient because a constant cumulative mean was not obtained. This means the data collected was not representative and more sites should have been sampled until a constant cumulative mean was reached.
The limitations to the procedure and errors associated with the investigation makes my conclusion that as velocity increases so does the abundance of swimming mayfly uncertain. Although from looking at my scatter graph there is some correlation, but this is not strong. Also my spearman’s rank test shows that there were a lot of anomalous results present. The result obtained from my statistical analysis was just enough to allow me to accept my hypothesis so that one more slight difference would have caused me to accept my null hypothesis. Therefore it may not be the velocity that attracts the huge number of swimming mayfly but that they prefer to live in riffles which generally seem to have a higher velocity.
I think the conclusion drawn that there are more freshwater invertebrates in riffles than pools may be debatable from looking at my statistical analysis and the limitations discussed above. This is because although I can reject my null hypothesis at the 5% significance level I cannot reject it at the 1% significance level where U has to be less than or equal to 2. Therefore my conclusions are not as reliable as they could be as there is a 5% chance that my conclusion is incorrect. Also the sample size of sites was not enough to draw these conclusions from, as I did not obtain a constant cumulative mean. Therefore my conclusions could be wrong or further improved if more sites of pools and riffles were studied. I think if it was not for the limitations a stronger correlation of the abundance difference in pools and riffles could have been proven. If each individual site did not have many other variables involved such as its size, sheep faeces, weather conditions, etc the results would allow my conclusion to be more accurate. It is very hard to control the conditions for a study such as this and a long period of study with the improvements suggested is needed to increase the validity of the results.
