Unit 4 - Nutrient
Cycles in Marine
Ecosystems
AICE Marine
Detritus:
1. What is it?
2. How does it fit in to
a food web/chain?
3. Who/what
consumes it?
Detritus:
1. What is it? Dead, decaying organic matter
2. How does it fit into a food web/chain? Returns
nutrients (so they aren’t “lost” from the system)
3. Who/what consumes it? Detritivores AKA
Decomposers (fungi & bacteria)
•
Detritus is broken
down and released as
chemical elements
into the soil, water, &
air where P.P. can
recycle them into
organic compounds
•
Without detritivores,
life would cease!
Essential nutrients
would be locked up &
unavailable to the
ecosystem
Chemical Cycling:
• Nutrients move between organic and
inorganic parts of the ecosystem in
biogeochemical cycles.
• Cycles may be global or local. Nutrient
cycles with a gaseous component (carbon,
sulfur, nitrogen) are global whereas
phosphorus, potassium and calcium cycle
more locally (at least on short time scales).
2 Major Types of Biogeochemical Cycles:
**Based on the Primary Source of nutrient input**
1. Gaseous cycles—atmosphere & ocean (global
circulation patterns)
2. Sedimentary cycles—soil & rocks of the Earth’s crust;
these vary from one element to another, but essentially
each has two abiotic phases:
a) Salt Solution phase
b) Rock phase
Draw me!
Fig. 1
Fill me in!
Fig. 1
General Structure of Nutrient Cycles:
• Nutrients can move from one reservoir to another by
a variety of processes
• Some examples:
– Photosynthesis (makes organic nutrients available
from inorganic)
– Respiration (makes inorganic nutrients available
from organic)
– Decomposition (makes inorganic nutrients
available from organic)
– Excretion (makes inorganic nutrients available
from organic)
Nutrient Cycling: Global Scale
Precipitation
Terrestrial
Food Web
Uptake
Bioelements
in solution
H2O (+ volatile
biochemicals)
Volatile
bioelements only
Decomposition
Volatile
bioelements only
Death
Detritus
Weathering
Losses by water runoff
Terrestrial Biosphere
Evaporation
Marine Food
Web
OCEAN
Detritus
Sinking
*Volatile=evaporates rapidly
Decomposition and nutrient cycling rates
• Rate of nutrient cycling is affected by rate of
decomposition
• In the tropics, warmer temperatures cause
organic material to decompose 2-3 X faster
than it does in temperate regions!
Decomposition and nutrient cycling rates
• In aquatic ecosystems decomposition in anaerobic sediments can be
very slow (50 + years)
• As a result, sediments are often a nutrient sink and only when there
is upwelling* are marine ecosystems highly productive.
• (*Upwelling is when ocean currents move water vertically from the
depths to the surface—more to come)
Human effects on nutrient cycles (1):
• Recent studies suggest that human activities
(fertilization & increased planting of legumes) have
approximately doubled the supply of nitrogen available
to plants.
• Fertilizers that are applied in amounts greater than
plants can use or that are applied when plants are not
in the fields leach into groundwater or run off into
streams and rivers therefore getting to the ocean.
Human effects on nutrient cycles (2):
• A major problem with intensive farming is fertilizer runoff.
• The heavy supply of nutrients causes blooms of algae and
cyanobacteria as well as explosive growth of water weeds.
• Because respiration by plants depletes the oxygen levels at
night this process of eutrophication can cause fish kills.
• Eutrophication of Lake Erie, for example, wiped out
commercially important populations of fish including lake
trout, blue pike and whitefish in the 1960’s.
Limits on primary productivity
• Nutrient limitation is a major factor affecting NPP
in aquatic biomes.
• In marine environments the nutrients limiting
primary productivity are usually nitrogen and
phosphorus, which are scarce in the photic
zone.
5 Marine Nutrient Cycles:
1. Carbon
2. Nitrogen
3. Phosphorus
4. Magnesium
5. Calcium
Carbon Cycle
• Carbon forms the basis of all organic molecules (organic
chemistry is the study of carbon chemistry). Fats, sugars, DNA,
proteins etc. all contain carbon.
• Photosynthetic organisms take in CO2 from the air and using
energy from sunlight join carbon atoms to make sugars (energy
is stored in the chemical bonds).
Carbon Cycle
• Major reservoirs of carbon include: atmospheric CO2, ocean
(dissolved carbon compounds), biomass of organisms, fossil
fuels and sedimentary rocks.
Carbon Cycle
• Photosynthesis removes large amounts of atmospheric CO2
• An approximately equal amount of CO2 is returned to
atmosphere by cellular respiration.
• Burning of fossil fuels adds large amounts CO2 to the
atmosphere.
Nitrogen Cycling
• N is an essential element in
– Protein: each amino acid has at least 1 N
– Nucleic acids: nitrogenous bases
• N2 makes up about 79% of the atmosphere
• N2 is only usable by prokaryotes with the
enzyme nitrogenase
– Cyanobacteria like Anabaena carry their
nitrogenase in a low oxygen package, the
heterocyst.
The bacterial processes involved in nitrogen cycling.
The width of each arrow is an approximation of the
process rate.
Fixation
Converts N2 to
– ammonia (bacterial)- NH4 : 100 - 200 kg N/ha=90%
N2 --> 2N
2N + 3H2 --> 2NH3
• Rhizosphere bacteria: mutualistic, some nodule formers
• Free-living bacteria (+cyanobacteria) ~ 12,000 species
– Nitrate (high energy discharge, like lightning) - NO3
– NO3 in atmosphere + water vapor = H2NO3
– Atmospheric fixation low: 8.9 kg N/ha/yr
Mineralization (ammonification)
• Major step in N cycle
• Biomolecules (proteins,nucleic acids) from
dead organisms broken down by bacteria and
fungi to amino acids then to CO2 + H2O + NH4 +
energy
• Fate: dissolved in H2O, trapped in soil, fixed in clay
Nitrification
• Nitrosomonas bacteria use ammonia in soil
as sole E source
NH3 + 11/2 O2 -> HNO2 + H2 + 165 kcal
HNO2 -> H+ + NO2
• Nitrobacter bacteria use the remaining E in
nitrite ion and oxidize it to nitrate
NO2 + 1/2O2 -> NO3
• Pollution can result if nitrates build up in
aquatic ecosystems
Denitrification
• Nitrates reduced biologically to N2
• Denitrifiers are facultative anaerobes
– Bacteria: Pseudomonas
– Fungi
Nitrogen Cycle
Phosphorus Cycle:
Terrestrial & Aquatic