Give three explanations for the observed differences in light attenuation in these lakes.

Question

The light intensity versus depth profiles for two lakes are listed in the table below. Calculate the extinction coefficient (η) for both lakes using all of the data provided (i.e., you need to calculate the slopes of these curves). Give three explanations for the observed differences in light attenuation in these lakes.

Here are the extinction coefficients that I calculated using excel.

Black Lake: -0.5

Tan Lake: -0.3

I just having trouble with the 3 explanations for the observed differences. I don't know what the extinction coefficient is telling me. If someone could help me figure out some explanations, that would be helpful! Thanks in advanced!

Answer 1

The vertical extinction coefficient, or k value measures the amount of surface light that is absorbed or scattered in a 1m vertical column of water. The lower the k value, the deeper the light can penetrate.

I can think of many reasons that light may penetrate less deeply in one lake than another:

1. One lake is more oligotrophic then the other. Oligotrophic is nutrient poor, so very little dissolved organic matter or simple algae in the water column. The opposite of oligotrophic is eutrophic.
2. One lake has a run-off problem at the moment. The two lakes could be at the same trophic stage, but one is temporarily suffering from extra particulate suspension, either rain (mud) or agricultural runoff.
3. One lake is shallower than the other. The bottom being closer will change the absorption.
4. One lake has a floating plant infestation: duckweed, water hyacinth, Pistia... The light is blocked by the floating plants.
5. There has been a sudden algal bloom. Kind of like the floating plant example, the algal bloom is blocking light from penetrating the water column.

These are just a few possibilities. Water chemistry will effect how light moves through the water as well. Some of these are temporary situations, others more permanent. You will have to look at them and choose what makes sense in your context.