Question

Considering a lake as your ecosystem boundary:

a. Which type of lake has higher dissolved oxygen concentration in the epilimnion than the hypolimnion? Justify your answer in detail.

b. What is generally the main limiting nutrient for plant growth in a freshwater lake environment? Explain in detail why this is.

Answer 1

Answer A

A thermally stratified lake has a higher concentration of oxygen in the epilimnion than in the hypolimnion.

This is because in a thermally stratified lake the depths of the lake are divided into 3 distinct regions

1)epilimnion

2) metalimnion

3) hypolimnion

The epilimnion is the topmost layer which is exposed to the air and the hypolimnion is the deepest layer.

Now this difference occurs due to the different rates at which these regions receive heat. The epilimnion is warm as it receives sunlight and this keeps it on the top and maintains the different regions. the density difference doesn't allow these layers to mix.

Now because the epilimnion is exposed to air it can be affected by wind. Wind can cause turbulence in the epilimnion which facilitates mixing of air components in the water. Thus oxygen can be readily mixed in the water layer.

Now due to the stagnant layer of the water layers, this oxygen cannot be transported to the lower layers

Thus the epilimnion has a higher oxygen concentration than the hypolimnion.

_______________________________________________________________________________________
Answer B

In a freshwater lake environment, phosphurus is the limiting nutrient for plant growth.

This is mainly due to 2 factors

1) slow nature of the phosphorus cycle

2) no natural inflow of phosphorus compounds from the surrounding.

The first point is the main cause of the limiting nature of phosphorus.

Phosphorus is quickly consumed by any plant life in the water, which in turn are consumed by herbivores. Thus the phosphorus very quickly passes through the food chain. Once through it is deposited on the lake-bed in the form of dead remains of the plants and animals. Phosphorus then requires a lot of time to be converted into a biologically accessible form. Thus any phosphorus that is consumed quickly becomes inaccessible to the biome.

Furthermore, this same reason means that any inflow of water that the lake might have would similarly have low phosphorus content, which is not enough to supplant the phosphorus consumed.

Thus phosphorus becomes a limiting nutrient in a freshwater lake environment.