Lecture 9 – Nitrogen & Phosphorus

Nutrient paradox

too little = limited productivity, too much =
degradation

Solutes

materials found in water

nutrients

chemical elements and compounds found in the
environment that plants and animals need to grow and survive

conservative solutes

materials that are essentially unchanged
in concentration during transport; not influenced by biology

reactive solutes

materials whose concentrations change during
transport due to biotic and abiotic processes

dissolved solutes

solutes that are able to pass through a 0.45
μm filter

inorganic nutrients

lacking carbon

organic nutrients

contain carbon

Primary Producers and nutrients

nutrient USERS
• Includes: algae, cyanobacteria, macrophytes

Decomposers and nutrients

nutrient RECYCLERS
• Includes: bacteria, fungi

Consumers and nutrients 

nutrient TRANSFORMERS
• Includes: zooplankton, fish, etc

N Fixation

converts N2 to NH4+ or NO3-, which are usable forms for plants and other organisms

Major N Cycling Processes

N Fixation, Assimilatory uptake, Mineralization, Nitrification, denitrification, anammox, DNRA

Mineralization of N

release of N via OM decomposition

Nitrification

converts NH4+ to NO3-
• Carried out by chemoautotrophic bacteria
• Aerobic process!

Denitrification

converts NO3- back to N2
• Can produce N2O, a potent greenhouse gas

anammox

NH4+ to N2
• Discovered in the 90s in WWTPs
• NO2- is electron acceptor

DNRA

Converts NO3- to NH4+
• Anaerobic process
Major N Cycling Processes

Assimilatory Uptake of N

high primary production is required for streams to exhibit high NO3- uptake rates

Highest DN rates in

Lowest DN rates in

Ag streams

forests

Main driver of DN

High NO3-

Major P Cycling Processes

Weathering & mining,Assimilatory uptake, Mineralization

Weathering & mining

major sources of PO43- to the biosphere

Mineralization of P

release of P via OM decomposition

Sorption/desorption is governed by

pH and O2 concentrations

P in Aerobic conditions

PO43- will adsorb to sediments

P in Anaerobic conditions

release PO43- into solution, especially
below pH 8

Sediments can act as a source
of P when

dry and re-wet

two phases of sediment P release:

• Mobilization from solid→dissolved forms
in the sediment pore water
• Movement of dissolved P from pore
water→surface water

sediments with high legacy P loads =

most likely to release P and experience internal eutrophication when re-flooded

N and P similarities

Both N & P have:
• Dissolved/particulate forms
• Organic/inorganic forms

Key Differences Between N & P

N cycling is largely controlled by biology → microbial transformations!

P: strong sediment interactions (sorption/desorption), accumulates in sediments

Eutrophication

the shift over time from a nutrient- poor to a nutrient-rich status in a water body

Consequences of Eutrophication

Decrease in species diversity

Shading = decreased submerged macrophytes

Harmful algal blooms (HABs) - dead zones

Why can’t we reduce the dead zone?

Largely driven by amount of spring precipitation
• Increasing amount and intensity with climate change

More rain = challenges for nutrient management
• Increases loads overall

Legacy effects

Hydraulic residence time (HRT)

Longer HRT = more time for nutrients to be taken up and recycled, builds up biomass

Why are some systems more vulnerable?

(HRT), temp, light availability, Stratification (prevents O2 from reaching bottom, concentrates decomposition impacts)