Practical 5

Practical 5 Mesocosms

Like a rainforest, the bottle contains primary producers (microscopic algae), primary consumers (the brine shrimps), and decomposers (micro-organisms). Provided the bottle ecosystem has an external source of light energy it is self- sustaining.

Brine shrimps and the algae in their ecosystem affect the chemical balance of the water they are in.

Using a bottle ecosystem with two recording probes set in it, together with a data logger and a computer, the changes in the pH and the dissolved oxygen levels may easily be measured. The graph below is from one bottle ecosystem data-logged for these two variables over 24 hours. Time 0 is at midnight. The first sunlight reached the bottle at 6 hours and the sun finally went down at 18 hours. There were a large number of shrimps in the bottle and a rich algal community.

The brine shrimps never need feeding and never run out of oxygen because the algae on which they feed carry out photosynthesis, grow and multiply by asexual reproduction.

The algae never run out of carbon dioxide, water or mineral salts because they are recycled. There are micro- organisms in the water, which cause decay of dead algae, dead shrimps, shrimp droppings and even dead micro-organisms.

When the population density of brine shrimps is low there is abundant food - everyone can see the green colour of the water. The brine shrimps can be seen mating and eggs can be seen, which soon hatch. The population increases exponentially until mounting competition for food (the green colour of the water disappears) halts further expansion. The populations fluctuate in a way reminiscent of the text book predator-prey graphs (although this is actually a grazing example) and over time may tend towards an equilibrium.

Other possible questions:

If light intensity was increased, how would you expect the graph to differ and why?

How would a change in temperature lead to a change in the graph?

Why do the algae never run out of carbon dioxide?

How could the wavelength of light effect the graph?

Why is the water initially quite green and then become clearer?

Below is an experiment set up by Le Rosey IB students to investigate pH and dissolved oxygen concentration over a 24 hour period. The fall in pH corresponds can be seen during the period of darkness.