EXAM 1: Nitrogen and Phosphorus

What are nutrients in a lake critical for?

ecosystem function

-Required for autotrophic growth and reproduction.

-Include various ions and compounds

What is nutrient limitation?

nutrient availability sets upper limit to rate at which "algae" can photosynthesize.

-sets upper limit to primary production and therefore, algal biomass

Nutrients often limit what?

the rate at which energy enters an ecosystem

Higher phosphorus concentrations are associated with?

greater algal biomass

Different algal types may be limited by? What are examples?

may be limited by varied factors.

-Diatoms are frequently limited by silica

-Flagellates are limited by certain vitamins.

What are the most important nutrients? What does this mean?

nitrogen and phosphorus this means they are the most limiting.

Since N and P are most limiting, what does this affect?

this affects freshwater productivity.

-nitrogen is only occasionally limiting.

-Phosphorus is nutrient limiting in freshwater productivity

What are the five major elements found in all living organisms?

oxygen

carbon

hydrogen

nitrogen

phosphorus

What form of nitrogen is available in large quantites?

atmospheric N2

-atmosphere has roughly 78% N2 gas

What kinds of animals can use free N2 from the atmosphere?

only nitrogen fixers.

-they convert N2 into NH4+

-In aquatic systems, cyanobacteria is the most abundant at doing this.

What are the most important utilizable forms of nitrogen?

NO3-N and NH4-N

-these are the ONLY forms that are available for photosynthetic algae and plants

In a lake, how is most of the nitrogen found?

as organic matter

-amino acids, nucleotide, proteins, and humic substances

What are the sources of nitrogen in a lake?

fallout & precipitation

runoff & groundwater (N-fixing bacteria in soil supply NH4-N)

N-fixation by Cyanobacteria (blue-green algae) and by N-fixing bacteria in sediments.

What are the losses of nitrogen?

outflow

denitrification

sedimentation of organic and inorganic N

What is denitrification?

done by anaerobic bacteria

converts NO3 to N2

continuously released to atmosphere

What are the four steps in the nitrogen cycle?

fixation, nitrification, assimilation, denitrification

What is assimilation?

NO3 being incorporated into plant living tissues like phytoplankton

What are the human influences on the nitrogen cycle?

agriculture (fertilizer) >fossil fuels (rainfall)> sewage (manure)

Where do the human influences on the nitrogen cycle manifest at?

in "dead zones" in the Gulf of Mexico

During fall and spring turnover, what does the N2 curve look like?

uniform top to bottom

During summer stratification, what does the N2 cure look like?

it decreases in the epilimnion (due to increase in temperature)

-Can be due to N-fixing by cyanobacteria

When is N-fixing lowest?

when NH4-N is high

When is N-fixing the highest?

when NO3-N and NH4-N are low

In a eutrophic lake during summer stratification, what is the N2 curve like?

it increases in the hypolimnion due to denitrification

In an oligotrophic lake during summer stratification, what is the N2 curve like?

little change from top to bottom except for epilimnetic temperature decrease

What is the main source of NH4-N?

ammonification

and production of NH4 waste by aquatic animals

What kind of bacteria does ammonification?

heterotrophic bacteria (decomposers)

Where is NH4-N quickly utilized?

in euphotic zone

-usually low in aerobic epilimnion

In eutrophic lake, is NH4-N low or high in the epilimnion?

low in the epilimnion due to photosynthesis and nitrification

In eutrophic lake, is NH4-N low or high in the hypolimnion?

high in anaerobic hypolimnion.

-no nitrification due to no O2

-Little or no photosynthesis utilization

-High release from sediments due to decomposition (ammonification)

In a oligotrophic lake, what does the NH4 curve look like?

not much change from the top to the bottom.

What is the most "important" form of nitrogen?

NO3

In a eutrophic lake, is NO3 high or low in the epilimnion?

relatively high because of

-input

-Algal/plant preference for NH4-N when available

-Nitrification

In a eutrophic lake, is NO3 high or low in the hypolimnion? What is this due to?

low in anaerobic hypolimnion

-Due to denitrification

When does denitrification occur?

in anaerobic water when no O2 is available for heterotrophic bacteria.

-oxygen is stripped from NO3 by heterotrophic bacteria in a step wise reaction

What is DON?

dissolved organic nitrogen

What is PON?

particulate organic nitrogen

More than 50% of nitrogen in lakes and streams is DON or POn?

DON- dissolved organic nitrogen

What are the usual DON:PON ratios?

usually around 5:1 up to 10:1

As a lake becomes more eutrophic, what decreases?

the DON:PON ratio