BIOL215 Lecture 24 Notes
Do terrestrial and aquatic ecosystems operate in a similar way? It seems that because of differences
in consumptions efficiencies, only a small portion of primary production is used by herbivores in
terrestrial systems compared to ocean and aquatic systems. Thus, large amounts of NPP are either
available or funnel through detrital food web, which are used by decomposers and fungi
Energy Flow in a Grassland Ecosystems:
• 99% of solar energy unavailable for use by second trophic level: primary producers only capture
roughly 1% of sun energy
• Below ground NPP > above ground NPP
• The conspicuous animals contribute little to consumption and secondary production
• As energy losses between trophic levels accumulate eventually there is insufficient energy left to
support a viable population at a higher trophic level
Introduction to Nutrient Cycling:
• The Earth is an open system with respect to energy input (energy is constantly supplied by the
• The Earth is a closed system with respect to nutrient input (ignoring the periodic bombardment by
material from outer space)
• Nutrients are cycled, between biotic and abiotic reservoirs (and among organic and inorganic
abiotic reservoirs) - can get locked in a chemical form that is not useable by all organisms
• Movement, or cycling, of nutrients ultimately requires energy input into ecosystems, e.g., to
initiate chemical reactions - critically dependent on energy supply
What are nutrients?
• Nutrients are the chemical forms of elements used for growth of all organisms
• Nitrogen is one essential element - it exists in many chemical forms in the environment
• Not all of these forms are directly available to all organisms - elemental or nutrient cycling
The conversion of one form of a nutrient to another involves nutrient cycling
Why are ecosystem ecologists interested in nutrient cycling?
• Some nutrients limit primary producers (an consumers)
• Human activities are dominating the cycles of many nutrients causing environmental problems
• Carbon is a nutrient that is accumulating in the atmosphere and warming the planet
Because N is often a nutrient limiting production on land and in the ocean much attention is focused
on the Nitrogen Cycle
Coupling between productivity and nutrient use:
• Organisms need essential elements to grow - these elements are required in constant proportions
• In aquatic ecology this ration is known as the Redfield ratio which applies to primary producers:
106C:16N:1P [6.6C:1N] (as mols) - phytoplankton and roughly 50-150C:1N (as mols) - higher plants
How can we use the Redfield ratio?
To predict which nutrient will be limiting to growth
• i.e. Phytoplankton require N:P = 16:1 and if the environment contains N:P = 30:1, P will ultimately
• If the environment contains N:P = 5:1, then N will ultimately limit growth
• Liebig's Law of the Minimum - The resource is lowest abundance relative to an organism's
requirements will limit plant growth
To predict consumption of one nutrient from another -3 -2 -1
• i.e. Phytoplankton have a C:N = 6.6:1, if N is consumed at a rate of 5 x10 mol m d , then how
much primary production does this represent?
To predict the potential increase in the rate of primary production (NPP) when the limiting nutrient
is added to the ecosystem
• N is limiting phytoplankton production in the coastal ocean, if agricultural runoff supplies 7 x 10 g -2
m yr and all of it is consumed, then how much primary production does this represent?
• Note that calculation 3 underestimates the actual amount of NPP, why?
As we saw last time this view could be overly simplistic - many factors are involved in regulating
Where do autotrophic organisms obtain nutrients from?
• In ecosystems, nutrients for autotrophs come from two main sources: