Lecture 20

Earth is an open system with respect to energy but closed with respect to materials. Composition of elements stay the same but they are not static Þ Cycle through the system.

This cycling makes life possible by making nutrients continuously available to organisms.

The paths of this cycling are referred to as biogeochemical cycles.

Major pools ® atmosphere, oceans, soil and earth’s crust.

Major pathways ® water and air (upon which nutrients are borne)

Water cycle (values expressed in units= 1000km³/yr)

Ocean (456)

410 from condensation and precipitation

46 from runoff from land

via evaporation from ocean ® 46 to land + 108 from precip on land(derived from saturated H₂O in air over land), 62 re-enters air mass via evapotranspiration, 46 to ocean via runoff.

Evaporation and convection resulting from high sea surface temperatures (~ 27-28 C), and strong winds (upward convection) leads to formation of rising clouds. At altitude cool temperatures lead to precipitation, i.e. rain. On land, evaporation and evapotranspiration results in convection.

When the size of pool is constant one can calculate the time an average atom or molecule of nutrient spends in the pool. This is referred to as the residence time.

Size of pool / sum of outflows

E.g. for O₂ : 4.8 x 10²¹g in atmosphere

= 20,000 yrs (residence time)

For N₂ : 7.6 x 10²¹g in atmosphere

4.0 x 10¹⁴g/y biologically fixed

= 20 yrs x 10⁶ years !

Bottom line: atmosphere is a storage pool for nitrogen, active pool for oxygen.

Nitrogen is often the limiting factor to growth in both aquatic and terrestrial systems. Of all the inorganic elements used biologically the nitrogen cycle is the most complex.

N₂ is not directly directly available to organisms – requires fixation to NH₃ (ammonia) and NO₃ (nitrate)

2N₂ + 6 H₂O ® 4 NH₃ + 3O₂ Rhizobium (nitrogen fixer)

NO^-₃ ® nitrification, yields energy via two step process

Mediated by bacterial action:

2NH4⁺ + 3O₂ ® 2NO^-₂+ 4H⁺ +2H²O

2 NO^-₂ + O2 ® 2NO^-₃

problem: NO^-₃ is highly soluable ® leached in soils, depleted in aquatic systems.

Nitrogen cycle:

Producers ® consumers ® decomposers

¯

­ ammonia in soil

uptake ¯

­ ­

¬ [Nitrite and nitrate bacteria]

NO₃ in soil & H²O NH3+

Denitrifying bact. ® N₂ in air/water ® Nitro-fixing

Sediments

Carbon is stored primarily in the earth’s crust and ocean as unoxidized carbon. As a consequence the carbon pool is 10³ x smaller than the O₂ pool (where O₂ pool is 4.8 x 10²¹ g O₂)

Since the sum of outflows for CO₂ ~ O₂, the residence time for CO₂ is much shorter (~ 20,000 yrs for O₂) – 10 yrs

So, with a shorter residence time withdrawal of carbon from the unoxidized pool (fossil fuel) has an effect on the atmospheric concentration.

For example, a rate of burning of fossil fuels that would lower O₂ concentration by 0.001%/yr would increase CO₂ concentration by 0.7%.

Current rate estimated to be 0.4 – 0.5% /yr has little effect on O₂ levels (because it is a much larger pool).

Size of reservior (10⁹ metric tons –units)

World vegetation 560

World soils 1,500

Atmosphere 735 ­

Oceans 36,000

Fossil fuel reserves 5,000 –10,000 ¯

In out

Physio-chemical diffusion physio-chemical

water interface diffusion

1. 104

soil respiration photosynthesis

1. 100

plant respiration total = 204

50

Deforestation

2

Fossil fuels

5

This is an alternative approach in ecology. A reductionist (or an accountant’s) approach to predicting species abundance and complexity via an understanding of how much energy is available in the system and how it flows through the community.

[probably resonable approach for vertical (trophic complexity) but not for horizontal (within trophic level) patterns]

Study of food webs – collection of species that feed on another collection of species.

Chain – energy path or sequence of links starting with spp. that eat no other species and end with species not eaten by others.

Length of chain described as the mean chain length of a web, arithemetic mean of the lengths of all chains in the web.

e.g. Figs 1,2, 3

Questions asked:

1. How is the complexity of the food web determined?
2. How many trophic levels can a community support?

This energetic approach unfortunately tells us little about the processes that determine the diversity of potentially competing species with a trophic level, but may tell us something about what effects/determines population size (abundance).

We’ll start out simple to give you a feeling about how energy is distributed in a community. All organisms require energy. Energy enters in the form of sunlight.

Light ® chemical energy ® fixed carbon

(Photosynthesis + H₂O +CO₂)

9.3 K calories/gram of assimilated carbon

This process results in what is referred as:

Primarily production – it is the base of the trophic ladder.

Primary productivity = rate of biomass of plant material produced per unit area per unit time.

For photosynthesis to occur other nutrients are required in addition to light : Nitrate, phosphate, etc.

CO₂ + H₂O + minerals + nutrients ® plant biomass + O₂ + H₂O Ý transpired

Gross production (GP) = total energy assimilated by photosynthesis.

Net primary production (NPP) = energy converted to plant biomass (energy goes off in synthesis and respiration of carbon biomass).

GP – NPP = respiration

Terrestrial systems – NPP usually estimated by increase in plant biomass.

Aquatic systems – Biomass does not accumulate, so more instantaneous measurements are used (e.g. rate of O₂ production).

1. Light - usually maximum level uses only 44% of all available light – photosynthetically active radiation (PAR), 400 – 700 nm wavelength.
2. Temperature - warmer, increase photosynthetic activity
3. Water - necessary for transpiration.
4. Nurients – important in terrestrial but even more so in aquatic systems where nutrients tend to sink out of the photic zone

Under best of all conditions, photosynthetic efficiency is very low.

NPP/energy received = 1 to 2%

Worst case – in deserts only 0.01 to 0.2%

Range of primary production (g/m²/yr) in :

Terrestrial system: 35 - 1,800. Highest in tropical

Rain forest, lowest in tundra.

Aquatic systems : nutrients frequently lacking. In general, NPP is < 10% of terrestrial,

Sargasso sea - 82

coastal Peru (upwelling) 700 -1000

Freshwater:

Iceland lake - 9

Uganda lake - 450

Given primary production, where does the energy go?

Answer: secondary production through the food web.

Task of evaluating trophic links is the fact that they are seldom linear ® often consumption occurs across trophic levels, e.g. omnivores or within trophic levels e.g. cannabalism. Also, parasites and detritivores complicate evaluation of energy flow.

Percent of transfer of energy through the system is called ecological efficiency (within or between trophic levels)

For simple 3 trophic system: production efficiency

Plant ® herbivore ® 1^st level consumer® 2^nd level consumer

Starting with 1000g plant

20% 15% 10%

200 g 30 g 3 g

overall ecological efficiency = 0.3% (3/1000)

In terrestrial systems: biomass accumulates at lower trophic

In aquatic systems: reverse is true because of rapid turnover

To evaluate or predict trophic complexity for a community you need to calculate ecological efficiency between trophic levels.

Three major components:

1. exploitation
2. assimilation
3. net production

Collectively these components describe ecological efficiency with a trophic level.

1. exploitation efficiency