Investigation of factors affecting the distribution of Chironomus larvae in Nant Iago
Method: Biotic Data
1.The stream was split into 14 sections and groups of 3 were each assigned to work at different sections of the stream, which began at the beginning, right at the top, and ended further down stream.
2.The method we used for our experiment was Disturbance Sampling.
This was accomplished with the aid of a Surber Sampler which we used in one riffle and one pool in our section. The Surber Sampler consisted of a net which had a 1.0mm2 mesh at the end where all the samples were collected and a quadrat which was placed over a part of the riffle or pool and gave us our area of sampling.
3. By laying the quadrat flat on the bed, I disturbed the stream bed and washed any lying organisms into the mesh. I then washed the water through the mesh to collect any samples in the water.
4. Once this was done the invertebrates were identified, counted and recorded
1. To accomplish my physical data, I measured the pH of the water, the Total Dissolved Solids (TDS), the dissolved oxygen, the depth and the temperature.
2. All were measured using specialised probes which were placed in the water of the riffles or the pool. The probes gave us readings of whichever piece of data we were trying to achieve after 30 seconds. The readings were then recorded.
3. The amount of detritus was estimated using estimation by eye, recorded and measured using a scale of 1-4:
The pool was 4 and the riffles was 2.
4. The substrate was also identified as to what matter was present, i.e. Riffles: gravel’s and pebbles. Pools: twigs, soil, leaves, detritus.
Null Hypothesis: There will be no difference in the numbers of Chironomus between pools and riffles.
Alternative Hypothesis: There will be a significantly higher number of Chironomus in the pools than in the riffles.
Which statistical test?
I am going to use the chi square test because we do not know the normal distribution of the data and because I only have a small data sample of 14. A chi square test is used to see if observe values are different from expected values.
X2 = ? (O-E)2 o = observed
E E = expected
300 = 150
300 = 15
O – E
285 – 150 = 135
15 – 150 = -135
(O – E) 2
(285- 150)2 = 18,225
(15-150)2 = 18,225
18225 = 121.5
18225 = 121.5
121.5 + 121.5 = 243
My chi square value from the experiment was 243. The critical value from the table at 95% confidence showed to be 3.84 at 1 degree of freedom.
Because my experimental chi- square value is of a much greater value than my calculated value, it leaves me with a result indicating that my Null Hypothesis is incorrect. I am 99.9% confident that it is wrong as the critical value at this point is 10.8 and my calculated critical value is 243 which is an extremely larger figure.
Therefore I will in turn accept my alternative hypothesis which states that there will be a significantly larger number of Chironomus in pools than in the riffles. After my statistical analysis, I can see that there was significantly more Chironomus found in the pool. As my density data shows the maximum abundance of Chironomus is 800m2 in the pool, whereas the maximum number of Chironomus found in the riffles is 50m2.
Our aim was to investigate the distribution of freshwater Macroinvertebrates in two microhabitats in an upland stream. A stream is formed due to gravity causing overland flow in water and there are many individual factors which affect the stream and the abundance of its inhabitants. The two different types of factors are Abiotic and Biotic.
The Abiotic factors, which would affect the stream and its inhabitants, are:
The current in the riffles, which is significantly stronger compared to the current in the pools. It would suggest that there are either not as many organisms living in the riffles or that if there are, then they would be specially adapted organisms. Examples of this would be an organism, which is highly streamlined. This would help it be prevented from being washed off the rock. It is also adapted to living in the riffles with its ‘claws’ that help it grip and cling onto the rock to prevent it from being washed away.
In comparison, there is evidence to indicate that there is none or very little current in the pools. This will affect the organisms living in the pools, because there is very little chance for them to be washed away, resulting in there being a higher population of organisms in the pools than there is in the riffles. The low ratings of current mean that there is also a lot of small substrate particles. This means that many other different types of organisms will inhabit the pools as there will be more prey for predators, and in turn, those which are the predators, will attract organisms which hunt for them themselves. These new predators will also inhabit the pools to consume their prey.
The substratum levels and content of them also affects the stream and its organisms. In the pools, there are mostly high levels of detritus as the current is extremely low and so the sediment is allowed to settle on the streambed and be built up. There are, however, lower numbers of detritus, twigs and leaves in the riffles because there is a high current running here and anything which settles here will get washed away. Therefore, there is a layer of gravel and pebbles.
Temperature also affects the distribution of organisms. When there are high temperatures or when the temperatures rise, the respiration of the organism will also rise, affecting where they must live due to the fact that if they are respiring at a higher rate, then there will need to be more oxygen available to them. By living in the pools, this large amount of oxygen is unavailable to them because of the slow flow rate and low dissolved oxygen percentage. If there is a drastic change in the temperature of the water, then the enzymes in most organisms will become denatured and they will die.This is also the case if the temperature becomes too cold. The organisms will not be able to live in those conditions and so will die.
The percentage of dissolved oxygen affects the stream and the distribution of its organisms. When there is a low percentage of dissolved oxygen, there will be a high number of organisms, which are specially adapted to living in these kinds of conditions, such as the Chironomus. This organism has haemoglobin in its body to help it survive in such low levels of dissolved oxygen when it is burrowed in the stream bed. Where there are high levels of oxygen, you will find that there are riffles. This means that there will be very little number of organisms present as they will just be washed away.
Biotic factors affecting the distribution and abundance of the stream and its organisms include:
* Food Availability –> for example,the prey or food particles of the correct size/type
* Predation –>The probability of prey surviving plummets when predator densities increase.
* Competition –>Competition from organisms with similar ecological niches
There are 2 microhabitats in the Nant Iago; the pool, and the riffle. I am now going to describe in more detail the differences the characteristics of the two microhabitats and compare them to assist me in describing the factors affecting the distribution of organisms in Nant Iago Nr. Abergevanny, S. Wales.
Characteristics Of A Pool
To begin, the substrate in the pool mainly consists of small particles such as sand, mud, detritus and twigs. Detritus is dead decaying organic matter. This build up is due to the low flow rate of 0.05m/s in the stream which prevents the substrate from being washed away. The effect is that the substrate settles and is deposited on the streambed as there is not enough force to wash it away and becomes a habitat for many organisms which can be established in the stream. It has made the detritus rating 3.7 which is abundant.
The percentage of dissolved oxygen in the stream is 63.6% which is low. This low amount of dissolved oxygen is due to the fact that bacteria feed on the large amounts of settled detritus. These decomposing bacteria are high in numbers and aerobically respire extensively as they feed on the detritus. In order to respire, oxygen is needed; therefore oxygen levels are exceptionally low and plummet in pools due to the mass of bacteria respiring. This leaves a low ventilation of fresh oxygen in water. The low level of fresh oxygen is also due to the substrate on the streambed accumulating and being so condensed, that the water particles are restricted from moving through it. This, together with the low current, means that the oxygen is not able to be replaced and conditions in pool sediments may become anoxic.
Characteristics Of A Riffle
In a riffle the number of small substrate particles recorded was lower than that of the pool, and this is due to the much faster current flowing through this part of the stream. The flow rate here was 0.431 m/s. This means that it has a considerably higher kinetic energy in comparison to the pool and it results in the diminutive particles and detritus being swept away with ease. This is evidential through the data which I recorded where it demonstrates that the detritus count was 2.4 (little). Organisms which would need to be adapted to these kinds of speeds of water flow. Baetidae is an example. These organisms are extremely streamlined. This assists the organism greatly, as when it hangs onto the rocks, instead of the water taking the Baetidae with its current, the water will instead flow over due to its highly streamlined body.
This is also helpful as when the water flows over the organism, the pressure from the flow causes the organism to be pressed against the rock even more, causing greater resistance to the organism being taken away with the current. The Baetidae also has adapted ‘claws’ which are situated on the ends of its legs. These claws cling onto the rock to help give extra strength against being taken away with the current.
The dissolved percentage of oxygen was read at 64.1%. This however may not entirely be correct as the reading is far too low than expected, which leads me to believe that the reader may have been defected as the value should be closer to 100%. The general trend is superior to that of the pools because there are large gaps between the large substrate particles in the riffles, which in turn results in higher ventilation and faster current/water flow through the sediment.
I will now explain how and why Chironomus are found in pools and are able to live in anoxic conditions.
As a female adult Midge deposits her eggs in water, the gelatinous mass hatches and each larvae which has hatched, burrows into the detritus on the bed of the pool. Once here, it develops a silken burrow to prevent the detritus from collapsing on it. The only time the detritivore Chironomus vaguely leaves this silken burrow is when it will pop its head out briefly to feed on the surrounding detritus. This benthic invertebrate is adapted to existing in anoxic conditions and has become acquired to them through several ways; psyological adaptations, behavioural adaptations and structural adaptations.
* Structural Adaptations –> The Chironomus has diminutive gills at the end of its body which amplifies the large surface area to volume ratio. Therefore oxygen may diffuse in quickly and easily. Its slender cylinder shape means that it can burrow easily in the detritus found on the stream bed.
* Behavioural Adaptations –> The Chironomus exists in silken burrows in the sediment. This means that there is a constrained quantity of oxygen obtainable to them. Therefore to make the oxygen accessible, they undulate their bodies to make water flow through the tube. This enables the haemoglobin in their bodies to become saturated with oxygen from the water in this ventilation current.
* Psyological Adaptations –> Just like a mammalian, in order for oxygen to be transported around the body, it must combine with haemoglobin first. Chironomus has haemoglobin which is very similar to that of a mammal. It consists of 1 or 2 polypeptide chains of 136 to 151 amino acid length. Each polypeptide is folded into a tertiary structure and has a single haem group. This haemoglobin is, however, not found in cells, but is found in the body cavity in a fluid named haemolymph. The only main difference between the Chironomus haemoglobin and mammalian haemoglobin is that Chironomus haemoglobin has a much higher affinity for oxygen. This signifies that the oxygen will bind to the haem groups at extremely low partial pressures and will be released only when needed. This assists them when burrowing in anoxic conditions, in the sediment on the pool bed, where oxygen is very restricted. The haemoglobin act as an oxygen store and will this can be demonstrated on an oxygen dissociation graph.
As shown, the O2 dissociation curve for the Chironomus haemoglobin is to the left of the mammalian O2 dissociation curve. This means that it has a higher affinity and will bind oxygen at especially low partial pressures. The reason why the Chironomus dissociation curve is straight is due to the fact that it has only two polypeptides so it is extremely easy for the molecules to bind to the haem groups.
The mammalian dissociation curve is s-shaped (sigmoid). This is because it has four polypeptides. With the first haem group, it is tough for the O2 molecule to bind to it, but once it has then this makes it easier for the second and third O2 molecule to bind to the haem groups. The reason why the curve lines off is due to the fact that it is harder for oxygen to bind to the fourth haem group.
Benefits of living in anoxic sediments:
Living in anoxic conditions requires special adaptations which can be found in Chironomus. Examples of why it may be beneficial for the Chironomus to live in anoxic conditions are that it helps them avoid predation from predators such as the stone fly nymphs; Perlodidae. It also helps Chironomus avoid competition such as interspecific competition from other organisms, for example, the shrimp (gammaridae) who are also detritivores. The shrimp will feed on the floating detritus but cannot enter the substrates and feed on any buried detritus as it does not have adaptations for anoxic conditions. The Chironomus, however, will feed on the detritus in the pool bed. It means that the shrimp cannot feed on the lower parts of detritus because of the low amounts of dissolved oxygen, they will not survive. This shows the different niches.
Limitations in apparatus:
The limitations in the apparatus equipment may have had an overall affect on my final results. By discussing the limitations with my apparatus, I can then relate it to the affect it had on my results.
The first limitation was with the Surber Sampler we used. In the pools, there is not as much flow as there is originating in the riffles. This can be distinguished on my results table where it illustrates that the mean flow rate for the pools is 0.059m/s in contrast to 0.431m/s found in the riffles. Hence, this signifies that with a low flow rate in the pools, anything disturbed such as Chironomus or other invertebrates which did not get washed into the net could have swum away with ease. In comparison, the riffles had the advantage of having a high flow rate connotating that there was a high chance of invertebrates being washed into the mesh net. This affects the results because it means that there could have been an artificially lower count of Chironomus in the pools. there wasn’t and where there should have been a lower, or perhaps a zero count, of Chironomus in the riffles, there were results that showed up to 5 Chironomus being found.
The second constraint caused by the Surber Sampler we used was due to the net. The net mesh is 1mm2 and this may have been a problem. Chironomus goes through 8 instar stages where at each stage, they shed their skin and grow bigger. At the first instar stage, the Chironomus is especially minuscule, so when we disturbed the pool or riffle, the small Chironomus will have simply washed straight through the net. This brings us to a conclusion that we could only have possibly trapped Chironomus or other invertebrates exceeding the size of 1mm due to the fact that they would have been rinsed directly through the net if any smaller. This affects the results because it demonstrates that there may have been a significantly higher amount of Chironomus in both pools and riffles although they were not recorded as the net was unsuccessful in detaining them, reason being that their size was too diminutive. Nonetheless, this is not a very significant error as it has an equal effect in both the pools and the riffles.
A third limitation with the Surber Sampler was related to the substrate. More rocks can be found in the riffles, whereas in the pools, more sand and silt can be found. The rocks in the riffles prevented the Surber Sampler from lying flat as the rocks are, all, various sizes. As the Surber Sampler cannot lie flat, Chironomus may have been washed away, underneath the Surber Sampler where it failed to touch the stream bed. The way this has affected the results is obvious. Any Chironomus which failed to wash into the net swam away when disturbed, leaving the Chironomus results lower than they should have been in the riffles. This is an important error as it only effects the riffles.
Limitations in method:
The method we all took up comprised of each group being assigned to a certain part of the stream. The limitation of this is that when groups further up stepped into the stream; they would have disturbed the streambed. However, samples which they disturbed were not collected. Instead, the invertebrates which were disturbed could have been washed down stream and rinsed into a net belonging to another group downstream. This is called ‘invertebrate drift’ and it is caused when a large number of people are sampling all at the same moment. When invertebrate drift takes place, the invertebrates are much more likely to settle in pools as they have a very low flow rate and this causes the Chironomus count to be higher than it should be which is called ‘over sampling’. It influences the results because it means that, theoretically, the Chironomus count which some groups collected is more elevated than supposed to. This is a major error source as its effect is limited to the downstream groups and mainly the pool regions.
This is linked to the accuracy of our results being affected by the limitations in the method.
Another aspect contributing to the accuracy of our results being affected by limitations in the method, is that there was no standard method of disturbing. Individuals, who disturbed for their group, will have done so in a different manner to another individual disturbing the streambed further upstream, or downstream. This affects the results because it means that some groups will have collected more data through unsettling the streambed more thoroughly than other groups. Hence, gives the connotation that more invertebrates and Chironomus were found in their sample, which is evident in the pools results where it shows that group 5 collected 80 Chironomus whereas group 11 only collected 3 Chironomus. This is not a major error source as its effect is likely to be equal in both of the pools and riffles.
By analyzing my results, I have seen that there are several anomalous results which can be commented on. I will discuss and compare the anomalous results which have arisen in both the pools and riffles;
In my results, by looking at Surber Number’s 5 and 9, you can evidently distinguish that the recorded amount of Chironomus established in the pools is ’80’ and ’70’. This is a particularly high amount, even for the pools and it may possibly have been caused by invertebrate drift, as explained above, which would affect the results because it means that the Chironomus was over sampled.
This is in contrast to surber sampler’s 7 and 8, where a recorded number of zero Chironomus can be recognized. The reason for this may be due to the detritus reading being ‘2’ with surber sampler number 7. Chironomus feed and bury themselves underneath this detritus sediment which is possibly why there are no Chironomus found in surber sampler 7. This affects the results here, because it leaves us with a lower recording of Chironomus than we are meant to have.
However, this does not explain the findings for surber sampler number 8. The detritus reading here is 4, which may lead us to believe that perhaps the low Chironomus reading is due to predation, being eaten by a fish of some kind preliminary to our sampling.
Perhaps also the Chironomus was in its first instar stage, which would mean that they will not have been trapped by the mesh net. Alternatively, the Chironomus might have just developed into an adult midge, and would no longer be found in the water, but in the air. This would affect the results collected because it would leave us the impression that possibly, if we had sampled the stream on an earlier, or later date, then the Chironomus would be at a trappable larva stage.
Surber sampler’s 7, 8 and 12 have high numbers of Chironomus readings despite of the fact that they are not usually found in this area. Possible reasons for this might be due to invertebrate drift from further upstream, or it could be due to the small particles of detritus being found in these riffles. The detritus could have been collected in sheltered areas such as behind large boulders. After being deposited there and this would have developed a micro habitat, with a mini pool being formed in a riffle, which would explain the high number’s of Chironomus being found.
Another anomalous result which was portrayed in both the riffle and pools results was the dissolved oxygen saturation readings. By observing both of the mean dissolved oxygen percentage saturations, we can see that in the pools it is 63.6% and in the riffles it is 64.1%. This is entirely incorrect due to the fact that in pools, there is supposed to be barely any dissolved oxygen saturated in the water, and in the riffles, there is supposed to be in the vicinity of 100% dissolved oxygen saturated in the water. These erroneous results are down to the oxygen meter being broken. It affected our whole experiment because it meant we were not able to carry out the trial accurately and record correct results which would assist us in our evaluation.
In my opinion, the 2 central error sources in my experiment were caused by the Surber Sampler and invertebrate drift.
Invertebrate drift occurs when large numbers of people are sampling the equivalent lake/stream at the same period in time. It is incurred when someone (upstream) walking through the stream disturbs the streambed, but does not collect the samples with their mesh net, or in other cases it is caused when invertebrates sweep under/ through/ or to the side of the net. Hereafter, any invertebrates disturbed will flow along with the current and settle amid pools (mainly downstream) which affects results because it means that invertebrates and Chironomus have been over sampled.
To prevent invertebrate drift, instead of all groups sampling the stream at the same time, we could allow the group furthest downstream (group 14) to do their experiment first, and then work our way upstream, only allowing groups to do their trial once the group further down has finished their experiment. E.g., group 14 will do their experiment initially and once complete, group 13 will do their experiment. Then once group 13 has completed their experiment, group 12 may carry out their experiment and so forth.
This method of carrying out the experiments will completely avoid the matter of invertebrate drift which improve the accuracy of results achieved.
Another way of shunning invertebrate drift would be to improve apparatus used, which brings me onto the second central error source within the experiment.
The Surber Sampler’s which we used composed only of a quadrat base and a mesh net attached to the end of the quadrat (see drawing). This basic surber sampler meant that when we positioned the quadrat onto the (riffles) streambed, it would not have been laid flat because of rocks being various contours and masses. This affects the results because it means that with the surber sampler lying at an awkward angle, when disturbing, invertebrates are highly likely to be swept underneath the mesh net, or the side, instead of into it.
A way of recuperating this quandary is by utilizing a better surber sampler. One surber sampler which could be used consists of a bottomless box attached to the bottom of the quadrat.
This will improve the experiment and results because not only will it avoid invertebrate drift through the way that nothing will be able to escape (because it will enclose everything within the desired sampling area right down to the streambed), but it will also improve accuracy, giving you a set volume of substrate.
Another alternative to the surber sampler’s which we used is a piece of apparatus called the ‘Eckman Grab’. These ‘grabs’ do not have nets attached to the end of them, but instead act as a set of claws.
After your sample has been picked up by these ‘grabs’, you cleanly drop all of its contents into your tray where after, you can record the results. This will develop the results because one of the core setbacks with surber samplers is that the nets tend to lose samples (through or under), whereas with these ‘grabs’ they do not have nets and so keeping all the samples within its hold will assure the results are more accurate and improved.
As my final conclusion, I will articulate that the trial my group carried out had a quantity of inaccuracies, but not enough for me to completely reject the whole experiment. The results are reasonable as they reflect the way nature works, regardless of the main sources of error. Also, the statistics and chi squared number shows a highly significant difference in the number of Chironomus in the pools and riffles with more found in the pools. The experimental errors are not large enough to cause a 99.9% rejection of the null hypothesis.
To determine the whole pattern of invertebrates and Chironomus along the stream, the experiments should have been carried out every month, instead of just one day. By doing the experiments on just one day we are left with results that are only able to give us a general idea and impression of the patterns of the distribution of invertebrates and Chironomus along the stream.
I had to reject my null hypothesis, as I was 99.9% confident that there was a difference.
In my opinion, if I was to repeat this experiment even with the improvements I have suggested, then the results obtained would still be the same.