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Design an investigation to see if there is a significant relationship between the number of bladders and the length of longest frond in Fucus species of seaweed at two different locations on a rocky shore

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Bethany Weston

Design an investigation to see if there is a significant relationship between the number of bladders and the length of longest frond in Fucus species of seaweed at two different locations on a rocky shore



Robin Hood’s Bay is situated on the Yorkshire coast of England. It is a well-studied, large rocky shore and consists of shelved rocks which form a series of ledges and gullies. This shelved structure of the shore enables it to be clearly identified as three separate areas; lower, middle and upper shores. The bay is home to a range of organisms, including Fucus vesiculosus, which is also known as ‘bladder wrack’, due to its structure. Fucus vesiculosus is brown/green algae, easily recognised by its paired bladders occurring on either side of a prominent midrib. The frond is generally not strongly spiralled, and does not have a serrated edge. In optimum conditions, the fronds on this particular seaweed can grow up to 2m in length. Fucus vesiculosus is shown in the picture above.

I am going to design and carry out an investigation in order to explore the relationship between the number of bladders and the length of the longest frond on Fucus vesiculosus at low and middle shore.

Null Hypotheses:

For this investigation I will make two null hypotheses, stated below;

  1. There is no significant relationship between the length of longest frond and number of bladders on that frond for Fucus vesiculosus. Any relationship found is due to chance.
  1. There is no significant relationship between the location of the Fucus vesiculosus on the shore and the length of frond and number of bladders. Any relationship found is due to chance.


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I have opted to sample the Fucus vesiculosus using an interrupted belt transect 100m in length, taking samples at 2m intervals. This is as, using a shorter transect or larger intervals, would make my sample less representative of the area being sampled, and so my results would lack reliability. However, if I was to use a transect of a longer length, the transect may start to enter different shore zones. For example, a longer transect at lower shore may result in part of the middle shore being sampled. Using a longer transect, or a continuous belt transect of the same length would also be incredible time consuming, and so not ideal.

I have decided to use a 1m x 1m quadrat as Fucus vesiculosus can grow up to 2m long, and at the time when I’m executing my investigation, it’s likely that the seaweed will be at it’s maximum length. In light of this fact I felt that a smaller quadrat would cause me to have too small a sample. Conversely, a larger quadrat would mean I’d have to take a larger sample for it to be representative, which would be too time-consuming to do.

I have chosen to use a tape measure to measure the lengths of the longest fronds. This is to limit the margin of error available whilst carrying out my experiment. The tape measure will be able to measure to an accuracy of 0.001m. If a smaller degree of accuracy was used, it’s possible that very large errors could be made. I am unable to use a greater degree of accuracy due to the limitations of the range of equipment available to me. However, by using this degree of accuracy I am limiting the maximum error in my measurements to 0.

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The standard deviation will then be calculated using the following formula;


The numbers are then placed in to the formula for the t-test, the degree of freedom is calculated, which in this case is the sum of measurements from both locations, minus 2. Again I will choose to use the 5% probability, read off the tabulated value, and compare that calculated with the tabulated one. Once again, if my calculated value is greater than the tabulated value, I will reject my null hypothesis.

The second statistical test I will carry out is Pearson’s correlation. I’ll conduct this side by side with my scatter graph of length of longest frond and number of bladders for both shores. This way my result for Pearson’s correlation will support my graph, and hopefully prove my null hypothesis to be incorrect. Calculating the correlation will provide an index to the degree to which two variables are related- correlation coefficient (r).

If r is           +1 (perfect positive)

                    -1 (perfect negative)

                     0 is no correlation

Pearson’s is the test that that I will use to calculate r. This uses actual values as opposed to Spearman Rank Correlation Coefficient which uses ranked raw data. As a result, Pearson’s is more suitable in this situation.

Finally I will see if there is a different between the distributions of Fucus vesiculosus on the two shores. I will also see if there is a large amount of bare rock and other species of seaweed at the two locations. By analysing these facts I can see which shore is more competitive and I will take this factor into account whilst analysing my results. I will also see if the distribution of Fucus vesiculosus varies as I move up the shores. This will allow me to observe any graduations in the habitat. If a graduation exists I will then go on and try to explain the observation using scientific knowledge.

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