# An investigation into the abundance of freshwater invertebrates in pools and riffles at six different sites of Embercombe and Nutscale Water

Salima Latif

Introduction

An investigation into the abundance of freshwater invertebrates in pools and riffles at six different sites of Embercombe and Nutscale Water, and more specifically into the abundance of swimming mayfly in relation to the velocity of the site where they are found.

The investigation involved sampling freshwater invertebrates by stone washing and counting the abundance of the different species at the different sites. The abiotic factors velocity, temperature, width and depth of the sites were also measured.

Prediction

I think there will be a difference in the abundance of freshwater invertebrates between pools and riffles. I think there will be a greater abundance of freshwater invertebrates in riffles and very few in pools. This is because riffles are ideal if the organism requires a large amount of oxygen as the fast flow of riffles allows more oxygen to dissolve in the water. Also their low depth means that predators such as fish can rarely survive there. This is the reason I think there will be very few freshwater invertebrates in pools, because fish can prey on them, also a lot of carnivorous freshwater invertebrates such as water beetles and water mites are also found in pools which may feed on and reduce the abundance of smaller freshwater invertebrates.

I also think the higher the velocity of the stream, the higher the abundance of swimming mayfly found there. This is because swimming mayflies require quite a lot of oxygen due to their rigid, immobile gills, and the higher the velocity of the water the more oxygen will be present in the waters for them to use.

Statistical Analysis

Mann-Whitney U test

Hypothesis: There is a greater abundance of freshwater invertebrates in riffles compared to pools of Embercombe and Nutscale Water in June 2004.

Null Hypothesis: There is no difference in the abundance of freshwater invertebrates in pools and riffles of Embercombe and Nutscale Waters in June 2004.

The table shows the abundance of freshwater invertebrates found in pools and riffles at the six sampled sites.

The data above reorganised and ranked

∑R1 = 25

∑R2 = 53

U1 = (n × n) + (0.5 × n) (n + 1) - ∑R2

= (6 × 6) + (0.5 × 6) (6 + 1) – 53

= 4

U2 = (n × n) + (0.5 × n) (n + 1) - ∑R1

= (6 × 6) + (0.5 × 6) (6 + 1) – 25

= 32

The lowest value of U is that of U1 which is 4, therefore I have to use this value when looking up the critical value. The critical value for a sample of 6 for both variables is 5. The value of U must be lower than or equal to this critical value to show that there is a relationship between the two sets of data. As 4 is less than this critical value I can reject my null hypothesis at the 5% significance level.

Spearman’s Rank

Hypothesis: There is a correlation between the abundance of swimming mayfly nymph and the velocity.

Null Hypothesis: There is no correlation between the abundance of swimming mayfly nymph and the velocity.

This is a table to show the abundance of swimming mayfly at different velocities.

The above data reorganised and ranked

Rs: 1 – (6∑D2) 1 – (6 × 63.5) _ 1 – 381 = 0.778

n3 – n 123 – 12 1716

Using the table of critical values my rs value should be equal to or greater than 0.777 to show that there is a relationship between velocity and the abundance of swimming mayfly. My rs value is just greater than 0.777 with the value of 0.778 therefore I can reject my null hypothesis at the 1% significance level. Also because my rs value is a positive number, it shows that there is a positive relationship between the two sets of data, so that as the velocity of the water increases so should the abundance of swimming mayfly.