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What conditions are required to have a dimictic lake?

Mixes 2x per year. Moderately deep and located in a region (temperate: 40 - 75 lat) where the lake can freeze.

What conditions are required to have a warm polymictic lake?

Mixes many times per year. Relatively shallow. Located in a sub-tropical region where lake can weakly stratify and then be wind mixed repeatedly.

What are the intermediate nitrogen species of denitrification ending in N2 gas? What happens to the N2 gas?

NO3 - → N2O → NO2 - → NO → N2 The N2 gas escapes to the water column and ultimately the atmosphere

: Explain why the NO3 and NH4 levels are where depicted in the two lake profiles below.

A. In epilimnion there is high N loading, high conversion of ammonia to nitrate in the presence of oxygen, high rate of assimilation of ammonia. In hypolimnion there is no ammonia to nitrate conversion because there is no oxygen, no nitrate is assimilated because there is no light. B. low external loading, phytoplankton use up ammonia and nitrate as it is available. In hypolimnion nitrification occurs but there is no light so nitrate is not assimilated.

Which of the following help with limiting phosphorous availability and which do not? Why? Al S Fe

Aluminum helps as it forms a permanent bond with phosphate. Sulfur hurts as it forms a permanent bond with iron, which then cannot bind up phosphate. Iron helps by forming a precipitate with phosphate, but only under oxic conditions.

Lakes that act as phosphorous sinks have what two characteristics?

Deep lakes with large oxic hypolimina and long water residence time

Name and define the three layers of a stratified lake.

Epilimnion: water body mixed by winds & currents Metalimnion: transition zone defined by location of the thermocline Hypolimnion: water body isolated

Explain the importance of internal cycling in the Phosphorous cycle of a lake.

P that enters a lake can be stored in the sediments, bound to other molecules. In the spring in fall when the lake turns over that P can be resuspended and made available to phytoplankton. Internally cycled P is hard to get rid of and can become a persistent problem in eutrophic lakes.

List four ways that lakes with too much phosphorous can be remediated.

Point source reductions, sewage treatment plants, diversions, construct wetlands, buffer strips, precipitate P from water column, remove P rich sediments, withdraw P rich hypolimnetic waters, add oxygen to hypolimnion, Add predators to the top of the food web.

What did the results of experiments in ELA Lake 226, shown below, demonstrate regarding the cause of nuisance algal blooms in freshwater bodies?

The addition of Phosphorous from detergent, as a limiting nutrient, was likely the cause of nuisance algal blooms.

You are a phytoplankton cell. How does the wind create physical forces (spirals) that keep you away from the compensation depth? How does an observer know these cells are active?

The wind blows across the surface water creating Eckman spirals. The Eckman spirals swirl clockwise downward to the top of the metalimnion. Downward force causes water at the bottom of the epilimnion to upwell in a Langmuir spiral. The Langmuir spiral pushes plankton to the surface where it

How is solar radiation important to lake stratification and lake processes (4 ways)?

Warms the lake, drives winds that mix the lake, base for biological productivity, regulates chemical and biological reactions