Ecosystem energetics

Outline:

• Limits on primary production• Relationship between primary and secondary

productivity• Trophic efficiency

Readings: Chapters 20

Laws of thermodynamics govern energy flow

Laws of thermodynamics govern energy flow

Energy flow in ecosystems

Ecosystem energetics - terminology

• Standing crop biomass – amount of accumulated organic matter found in an area at a given time [g/m2]

• Productivity – rate at which organic matter is created by photosynthesis [g/m2/yr]

• Primary productivity – autotrophs

• Secondary - heterotrophs

• Gross versus net primary productivity

Estimating primary productivity in aquatic ecosystems

Factors limiting primary productivity in terrestrial ecosystems

• Temperature

• Precipitation

• Light

• Nutrients

Controls on primary production in terrestrial ecosystems

Controls on primary production in terrestrial ecosystems

Controls on primary production in terrestrial ecosystems

Controls on primary production in terrestrial ecosystems

Controls on primary production in terrestrial ecosystems

Despite much variation, there is a general trend of increasing net primary productivity with decreasing latitude. a), Grassland and tundra ecosystems. b) Cultivated crops. c) Lakes

Primary production as a function of latitude

Global map of primary productivity

Factors limiting primary productivity in aquatic ecosystems

• Light

• Nutrients

Controls on primary production in aquatic ecosystems

Controls on primary production in aquatic ecosystems

Controls on primary production in aquatic ecosystems

Global map of primary productivity

Energy allocation

Primary production varies with time

Primary production varies with time

Primary production varies with time

Primary productivity limits secondary

productivity

Primary productivity limits secondary productivity

Consumption efficiency = 200/1000

Assimilation efficiency 70/200

Production efficiency = 14/70

Amt produced by trophic level n-1

Amt ingested by trophic level n

Amt egested as feces (waste) by trophic level n

Amt assimilated (i.e. absorbed into body) by trophic level n

Amt respired by trophic level n

Secondary production by trophic level n

Efficiency of energy transfer

I = ingested A = assimilated through gut wallW = expelled as waste

productOf A,

R = respiredP =

production

Efficiency of production

Food chains

Consumption efficiency determines pathways of energy

flow through ecosystem

Note: • Detrital food chain accounts for

most biomass produced in a community

• LCS plays greatest role in phytoplankton-based food chains

FOREST

GRASSLAND

PLANKTON - OCEAN

STREAM COMMUNITY

Energy loss between trophic levels

Amt produced by trophic level n-1

Amt ingested by trophic level n

Amt egested as feces (waste) by trophic level n

Amt assimilated (i.e. absorbed into body) by trophic level n

Amt respired by trophic level n

Secondary production by trophic level n

Example: a herbivore (level n) feeding on a plant (level n-1); values = kilocalories.

Trophic Efficiency = 0.2*0.35*0.2= 14/1000= 0.014

Efficiency of energy transfer

Decomposition and Nutrient cycling

Outline:• Process of decomposition

– Types of decomposers– Controls on decomposition– Decomposition in lakes and

rivers

• Nutrient cycling: generalities• Nutrient cycles

– Carbon– Nitrogen– Phosphorus

Readings: Chapters 21, 22

Decomposition

• Most material = plant• Involves:

• Release of chemical energy• Mineralization (= organic --> inorganic)

• Note immobilization = reverse of mineralization• Net mineralization rate = mineralization -

immobilization

Decomposition involves a variety of organisms

• Microfauna & microflora [<100 μm]– bacteria and fungi; nematodes, protozoa

• Mesafauna [100 μm – 2mm] – mites, potworms

• Macrofauna [2-20 mm] - millipedes

• Megafauna [> 20 mm]- earthworms, snails

Fungi: microfauna

Mites: mesofauna

Megafauna

Vertebrate scavengersConsumers of animal carrion

(highest lignin content)

(lowest lignin content)

Factors influencing decomposition rates

Decomposition of straw

Factors influencing decomposition rates

Factors influencing decomposition rates

Factors influencing decomposition rates

Factors influencing decomposition rates

Immobilization vs. mineralization

Decomposition in aquatic environments

Rate of nutrient cycling

Rate of nutrient cycling

Zones of production and decomposition

Nutrient spiraling in rivers

Nutrient spiraling in rivers

Terrestrial communities:Nutrient sources

• Weathering of rock (K, P, Ca and many others)• Fixation of CO2 (photosynthesis) and N2 • Dryfall (particles in the atmosphere)• Wetfall (snow & rain); contains

– Oxides of S, N– Aerosols

• particles high in Na, Mg, Cl, S• produced by evaporation of droplets

– Dust particles from fires, volcanoes• Ca, K, S

Terrestrial communities:Nutrient losses

• Release to atmosphere– CO2 from respiration– Volatile hydrocarbons from leaves– Aerosols– NH3 (decomposition), N2 (denitrification)

• Loss in streamflow– Dissolved nutrients– Particles

Oceans

• No outflow• Detritus sinks --> mineralization --> nutrients

end up1. Being carried back to surface in upwelling

currents, or2. Trapped in sediment

• E.g. phosphorus: 1% lost to sediment with each cycling

The Carbon Cycle

Daily variation in CO2

Annual variation in CO2

The nitrogen cycle

The phosphorus cycle

Nitrogen saturation

Nitrogen saturation

For next lecture:

• Please read Chapter 6

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