Biology and the environment notes.docx

8/15/2019 Biology and the environment notes.docx

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Biology and the environment notes

Regardless of an ecosystem's size, all energy flowing through it was initially

captured byautotrophs, organisms that synthesize organic molecules from

inorganic building blocks through the process ofprimary production.

Production is the creation of new biomass. New biomass that is synthesized from

inorganic molecules like CO2 and H2O is calledprimary production. Primary

production contrasts withsecondary production, in which new biomass is created

via consumption.

Ecologists refer to the creation of new biomass as production. New biomass

created from existing organic matter, as when animals build new tissue out of the

food that they eat, is calledsecondary production, which is expressed as arate.

Secondary production contrasts withprimary production, in which new biomass

is synthesized from inorganic molecules.

Plants are responsible for the bulk of primary production. They use

photosynthesis to power production. Other primary producers, called

chemotrophs, usechemoautotrophy to build new biomass.

Photosynthesis is the process by whichautotrophic organisms use solar energy

to convert water and carbon dioxide into carbohydrates and oxygen.

The photosynthetic reaction can be summarized as:

6 CO2 + 6 H2O→ C6H12O6 + 6 O2

where C6H12O6 represents the simple sugars that store the energy captured in

photosynthesis.

Chemoautotrophy is a form of autotrophism where an inorganic compound is

oxidized to produce energy.

While the vast majority of autotrophs on Earth capture energy from sunlight, there

are a few that have found other means of gaining energy from inorganic sources.

These chemoautotrophs use free energy contained in chemicals such as

hydrogen sulfide to fix carbon. They are often found in harsh environments such

as hydrothermal vents on the ocean floor.

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Primary production is a rate (i.e., amount per unit time). Ecologists often

distinguish betweengross primary production, the rate at which photosynthesis

occurs, andnet primary production, the rate at which biomass accumulates.

Gross primary production is a measure of the rate at which new organic matter is

being synthesized. Because plants are the most importantprimary producers,

gross primary production is typically a measure of the rate at which

photosynthesis occurs. It does not account for organic matter that is lost through

respiration. Although gross primary production is frequently expressed as

biomass synthesized per unit time, it is more properly expressed as the mass of

carbon incorporated in organic matter per unit time.

Net primary production (NPP) indicates how quickly something is growing. Moreformally, it is the difference betweengross primary production (GPP) and

respiration (R):

NPP = GPP - R

All three entities are expressed as rates of mass per unit time or energy per unit

time. Ecologists may calculate the rate of net primary production for an individual

plant, a group of plants (e.g., a forest stand) or an entire ecosystem (e.g., a lake).

Cellular respiration is the process by which all living organisms convert chemical

energy into energy needed for life's functions. Most frequently, this chemical

energy is stored in organic molecules like glucose, amino acids and fatty acids.

When these organic molecules are broken down, CO2 and H2O are released.

Although there are other important pathways, aerobic respiration may be the

most important. During aerobic respiration, energy is released when sugar

(C6H12O6) is combined with O2 to produce CO2 and H2O. In essence, aerobic

respiration is the reverse ofphotosynthesis.

Primary producers, also known asautotrophs, are organisms that synthesize

organic compounds from inorganic compounds. For example, green plants

capture solar energy and synthesize carbohydrates duringphotosynthesis. In

contrast,heterotrophsgain energy from breaking down organic material (that is,

via consumption). Autotrophs are vital components ofecosystemsbecause they

capture the energy (typically solar energy) that ultimately reaches all trophic

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levels, fueling all life on Earth.

In order to do the work of life, plants may burn up to half the energy initially

captured during photosynthesis, releasing it as heat duringrespiration.1 The

remaining energy is stored as biomass for later use.

Gross primary production describes the rate at which solar energy is captured by

a plant during photosynthesis. It does not account for energy lost via respiration.

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Net primary production describes the difference between photosynthesis and

respiration. It is the rate at which the plant stores energy as biomass.

He emphasized that as energy flows through different compartments, it mustobey thelaws of thermodynamics. Specifically, energy inputs must equal

outputs and, because all spontaneous energy transfers are inefficient, each loses

some energy as heat.

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Earlier in this section we posed the question:Where does the energy in wood

come from? As you have seen, this energy is supplied by the Sun and captured

by autotrophs duringgross primary production (GPP). Much of this energy is

immediately lost duringautotrophic respiration (Ra). The remainder, called the

net primary production (NPP), is available to fuel the tree's growth andreproduction. The relationship between gross primary production, plant

respiration, and net primary production can be summarized as follows:

GPP - Ra = NPP

Although some of the forest's net primary production is used to power human

activities, most is consumed byherbivores or, after the plants die,

bydecomposers, and used to fuel their own growth and reproduction. In turn,

some of the energy and matter in the herbivores and decomposers is passed on

up thefood chain when they are eaten by predators. In this way, primary

producers provide all of the energy and matter needed to support the growth and

reproduction of the ecosystem's highertrophic levels.

The carbon cycle is one of Earth's many nutrient cycles. Over time, carbon (C)

moves between differentpools, including Earth's crust, atmosphere, and biota

(plants, animals, etc.). This movement is described as a "cycle" because carbon

is neither created nor destroyed as it moves. Because carbon is sometimes

incorporated into organic molecules, like those used to build plant tissue, and

sometimes into inorganic molecules, like CO2, this nutrient cycle is an example of

abiogeochemical cycle.

NPP = (m2 - m1) / (t2 - t1)

wherem1 andm2 represent plant biomass density at timest1 andt2, respectively.

Because plants contain a lot of water, samples are typically dried and then

weighed, and NPP is reported as dry organic matter per unit area per unit time.

Acarbon sink accumulates and stores carbon, often in the form of organic

matter, for an indefinite period of time. Carbon sinks such as growing forests and

kelp beds move CO2 from the atmosphere or hydrosphere to the biosphere.

Carbon sinks are consideredcarbon negative because their net emission of

carbon to the atmosphere is negative.

In contract, processes or entities that emit more CO2 than they absorb are

carbon sources. Examples include burning fossil fuels, cement production,

wildfires, and decomposition.

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An endothermic chemical reaction is one that requires energy (heat) to proceed.

Endothermic reactions do not occur spontaneously, and their products contain

more energy (in the form of chemical bonds) than do their reactants.

An exothermic chemical reaction is one that releases energy (heat). Exothermic

reactions can occur spontaneously, and their products contain less energy (in the

form of chemical bonds) than do their reactants.

NPP is always much less thanGPP.

What can you conclude if CO2 in the air around a plant is accumulating

over time (that is, net CO2 is increasing)?NPP is negative (i.e. GPP < R).

If you were to track how the concentration of CO2 in an actively growing

forest changes over the course of 24 hours, when would you expect

concentrations to rise? When would you expect them to decrease?

CO2 will decrease during the day and increase at night.

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At night most CO2 concentration is at the floor while during the day its low

throughout all location.

A positive annual NEP indicates that the forest is actively growing and, on an

annual basis, stores more carbon than it releases.

As you can see, in July (day 190 or so), GPP was much larger than Re, just as in

the simulated July measurements you've been working with. In contrast, during

the winter, without photosynthesis, Re exceeded GPP. This is a typical pattern for

temperate forests. In most years, Re increases early in the spring when soil


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temperatures begin to increase. A little later, when trees sprout leaves, GPP

starts to climb, and by mid-summer it is much larger than Re. In the fall, GPP

drops quickly as trees shed leaves and photosynthesis stops. Respiration rates

also decline during the fall as soil temperatures decline

Forests tend to be least productive when they are very young or very old and

most productive when they are middle-aged.2

Nearly one half of global NPP occurs in the oceans, suggesting that these

ecosystems may be removing significant quantities of CO2 from the water column

and storing it as biomass. In other words, ocean ecosystems may be acting as

important carbon sinks. Most of this production occurs in open-water regions

where tiny unicellular algae and photosynthetic bacteria, collectively called

phytoplankton, are the only primary producers.

Lakes display similar patterns. Phytoplankton dominate primary production in

deep, open waters whilemacrophytes and other bottom-dwelling plants dominate

production along shorelines and in shallow lakes.

How can you calculate daily ecosystem respiration? (Assume that light and

dark bottle DO is measured AFTER an incubation of exactly 24 hours.)

Re = (initial DO) - (dark bottle DO)

Primary production and respiration rates are then estimated as:

NEP = (light bottle DO) - (initial DO)

Re = (initial DO) - (dark bottle DO)

GPP = NEP + Re = (light bottle DO) - (dark bottle DO)

where:

The initial DO is dissolved oxygen in the bottles before the experiment.

Light bottle DO and dark bottle DO are measured after a 24-hour incubation at

the same water depth.

Photoinhibition occurs when light is so intense as to reduce thephotosynthetic

capacity of a plant or alga.

Photoinhibition often occurs becausephotosystem II (part of the photosynthetic

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machinery) can be damaged by intense light at certain wavelengths (e.g.

ultraviolet light).

ecosystem respiration in aquatic systems is typically unaffected by light intensity.

Instead, it declines as temperatures drop or as food for heterotrophs becomesscarce.

The compensation depth (also called the critical depth) is depth beneath the

surface of a body of water that receives just enough light so thatnet ecosystem

production is zero. In other words, at this depth,gross primary production exactly

balancesecosystem respiration in magnitude.

Becausenutrients limit ocean phytoplankton, the most productive ocean

regions are near river mouths and upwelling zones where key nutrients areplentiful.

As in aquatic ecosystems, the availability of both light and nutrients—particularly

nitrogen and phosphorus—can influence NPP in terrestrial systems.

While light and nutrient availability are critically important at both local and

regional scales, the two most important drivers of terrestrial NPP at larger scales

are temperature and precipitation.

NPP tends to increase as temperature increases

NPP also increases as mean annual precipitation increases, but only to a point.

Once annual precipitation exceeds about 2500 mm/year, NPP begins to decline.

Model validation (or model verification) is the process of comparing the results of

a mathematical model with results from field experiments for the purpose of

evaluating the model's validity.

Across the globe, GPP is highest during the summer, but summer occurs

at the opposite time of the year in the Northern and Southern Hemispheres.

In the North, summer is from June to August while in the South summer is

from December to February. This difference is what drives the seasonal

swings seen in this animation.

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What else can you say about the global patterns of GPP?

Seasonal swings in GPP are higher in theNorthern Hemisphere than in the

Southern Hemisphere.

you would expect CO2 to increase during each hemisphere's winter and decrease

during their summers. This means the cycles in the two hemispheres should be

outof synch.

Seasonal swings in CO2concentrations are higher in the Northern Hemisphere.

Additionally, fluctuations in the Southern Hemisphere are out of phase with those

in the Northern Hemisphere and with the global average. Further, seasonal

variation in the Southern Hemisphere is more complex, suggesting that factors

other than terrestrial primary production contribute to observed patterns.

A grazer food chain is afood chain that includes aprimary producer at the lowest

trophic level and a herbivorousprimary consumer at the second level.

Assimilation is the process of converting consumed food into energy for work or

growth. Assimilation requires digestion and nutrient absorption.

Foodingested byheterotrophs is either assimilated oregested. Energetically

speaking:

Assimilation = Ingestion - Egestion

Nitrogen fixation is a process in the nitrogen cycle in which inert atmospheric

nitrogen (N2) is converted into plant-useable forms of nitrogen, such as

ammonium (NH4+) by fungi, bacteria, and othernitrogen fixers.

Below are the formulas for calculating ecological efficiencies for an herbivore

(subscripth) eating plants (subscriptp). Here,I is ingestion, A is assimilation,

andP is production.

Consumption Efficiency (CE):

CE = Ih / Pp

Assimilation Efficiency (AE):

AE = Ah / Ih

Production Efficiency (PE):

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PE = Ph / Ah

Trophic Efficiency (TE):

TE = Ph / Pp =(CE) ( AE) (PE)

endothermic organisms must devote more of the energy they assimilate to

maintaining their energy budget than ectotherms, and consequently will have less

energy available for growth.

Reptiles have the lowest metabolic rates

A detrital food chain is afood chain whose primary consumers aredecomposers,

organisms who consume dead organic matter.

A detrital food chain contrasts with agrazer food chain.

CEs are highest in ecosystems dominated by phytoplankton and benthic

microalgae, both of which tend to be relatively nutrient-rich. In contrast, the

lowest CEs are seen in forests and, to a lesser extent, grasslands, communities

whose plants have lots of unpalatable structural tissues.

biomass pyramids for open oceans are often inverted, with less biomass on the

bottom than on the top.

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terrestrial ecosystems have lower trophic efficiencies than aquatic ecosystems

• Residence time describes how quickly energy flows through an ecosystem.

Residence times are higher where organisms are long-lived (e.g., forests) and

short where organisms are short-lived (e.g., open ocean ecosystems).

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