PCB 2030: Environmental Science
Chapter 7: Environmental Systems and Ecosystem Ecology
Central Case: The Gulf of Mexico’s “Dead Zone”
Background on the Dead Zone
• The dead zone is a region in the Gulf of Mexico so depleted of oxygen
that it cannot support marine organisms, a condition called hypoxia.
• The spread of the hypoxic zone threatens the Gulf’s fishing industry, one
of the most productive fisheries in the United States.
• Environmental advocates and fishermen across the country joined local
shrimpers in expressing anxiety about the dead zone.
Background on the Dead Zone
• Scientists studying the dead zone have determined that fertilizer runoff
from Midwestern farms is the likely cause.
• In 2002, the dead zone grew to its largest size ever—8,500 square
miles—the size of the state of New Jersey.
• The dead zone forms each April and lasts through the summer, generally
reaching a peak in late July.
Earth’s Environmental Systems
Systems show several defining properties
• A system is a network of relationships among a group of parts,
elements, or components that interact with and influence one another
through the exchange of energy, matter, and/or information.
• Systems receive input, process it, and produce output.
What are Feeback Loops?
• Sometimes a system’s output can serve as input to that same system in
a circular process called a feedback loop.
– In negative feedback loops, output driving the system in one direction
acts as input that moves the system in the other direction.
– In positive feedback loops, the output drives the system further toward
one extreme or another.
Feedback Loops aid in Homeostasis
• The inputs and outputs of a complex natural system often occur
simultaneously, keeping the system constantly active.
• Systems that use negative and positive feedback loops to maintain a
balance are said to exhibit Homeostasis.
Boundaries of systems are often hard to define
• Often, it is difficult to fully understand systems by focusing on their
individual components because systems can show emergent
properties, characteristics that are not evident in the system’s
• Systems rarely have well-defined boundaries, so deciding where one
system ends and another begins can be difficult.
The Mississippi River System
• The Mississippi River is an environmental system.
• Understanding the hypoxia problem requires considering the river and
the gulf together as a system.
• The cause of hypoxia in the Gulf of Mexico stems from excess nitrogen
from the Mississippi River watershed.
• This process of nutrient enrichment is called eutrophication.
Nutrients run off the land and drain into the Mississippi River
Environmental systems may be perceived in different ways
• Earth can be divided into several structural spheres:
– The atmosphere is comprised of the air surrounding our planet.
– The hydrosphere encompasses all water in surface bodies,
underground, and in the atmosphere.
– The lithosphere is everything that is solid earth beneath our feet.
– The biosphere consists of the sum total of all the planet’s living
organisms and the abiotic portions of the environment with which they
Ecosystems are another level of key environmental systems.
• An ecosystem is defined as all interacting organisms and abiotic factors
that occur in a particular place at the same time.
• Ecosystems differ from communities in that they include abiotic elements
and involve the flow of energy and nutrients.
Many ecosystems link together to produce the biosphere
• Ecosystems are open systems, so they interact with other ecosystems.
• Landscape ecology is the study of geographical areas that include
• Many animals move between ecosystems; thus they must be studied on
the landscape scale.
• Ecotones = transitional zones where ecosystems meet
• Considering these linkages has proven to be a useful tool in
Energy in ecosystems
• Energy for most ecosystems comes from the sun and is converted to
biomass by producers through photosynthesis.
• Producers convert solar energy into biomass (matter in organisms)
• Rapid conversion = high primary productivity (coral reefs)
• Rapid plant biomass availability for consumers = high net primary
productivity (wetlands, tropical rainforests)
Net primary productivity
Different ecosystem types show varying net primary productivities.
Satellites can help us look at Global Net Primary Productivity
Nutrients can be a limiting factor for ecosystem productivity
• Nutrients are elements and compounds that organisms consume and
require for survival.
• Nutrients stimulate production by plants, and the lack of nutrients can
• Nutrients move through ecosystems in nutrient cycles or biochemical
• All organisms require 24 of the naturally occurring chemical elements to
Artificial Nutrification Experiment
Satellite Image of Algal Bloom off the Mouth of the Mississippi
The nitrogen cycle
• Nitrogen makes up 78% of the atmosphere and is the sixth most
• Inert nitrogen gas becomes ―fixed‖ so that plants can use it by: nitrogen
fixation or nitrification.
• Consumers get the nitrogen they need by consuming plants or other
The Nitrogen Cycle
• Decomposers get nitrogen via dead and decaying organic matter in the
form of nitrates
• Nitrates in the soil or water are converted back to nitrogen gas by
The Nitrogen Cycle has changed dramatically in recent decades
• Many human activities, such as burning fossil fuels and using fertilizers,
alter the nitrogen cycle.
– By fixing nitrogen, we speed up its movement into other areas of the
– Human Activities increase the amounts of nitrogen available to aquatic
plants, producing a boom in aquatic plant growth.
• Primary consumers acquire phosphorus from water and plants, and pass
it on to secondary and tertiary consumers.
• Decomposers return phosphorus to the soil.
• Concentrations of available phosphorus in the environment are very low.
• Humans influence the phosphorous cycle by releasing sewage and using
The phosphorus cycle plays a key role in energy conversion
• The element phosphorus is a key component of DNA and RNA.
• Organisms also use elemental phosphorus to build ADP and ATP.
• Phosphorus is most abundant in rocks, and weathering releases
phosphates into water.
The carbon cycle moves organic nutrients through
• Through photosynthesis, producers pull carbon dioxide out of the
atmosphere to produce oxygen and carbohydrates.
• During respiration, consumers and decomposers break down
carbohydrates to produce carbon dioxide and water.
• Remains of organisms may settle as sediments in water bodies to
eventually be converted to fossil fuels.
• The world’s oceans are the second largest reservoir in the carbon cycle.
• The burning of fossil fuels, forests, and fields has altered the relative
rates at which parts of the carbon cycle occur.
The hydrologic cycle influences all other cycles
• Water moves into the atmosphere via evaporation and transpiration.
• Water returns to the surface as precipitation, most of which flows into
water bodies as runoff.
• Some precipitation and surface water soaks down through the soil and
rock to recharge underground water reservoirs known as aquifers.
• Human activity affects the water cycle.
The rock cycle is a slow, but important, environmental system
• The Rock Cycle is the process where rocks are formed, reshaped and
eventually recycled in the lithosphere.
• Three types of rocks are found in the rock cycle:
• Rocks that form when magma cools are called igneous rocks.
– Intrusive igneous rocks form when magma cools slowly well below
– Extrusive igneous rocks form when magma is spewed from a volcano
and it cools relatively quickly
• Sedimentary rock is formed when dissolved minerals seep through
sediment layers and crystallize and bind sediment particles together.
– Limestone and rock salt are chemical sedimentary rocks.
– Sandstone and shale are classic sedimentary rocks.
• When great heat or pressure is exerted on rock, it is transformed into
• Limestone becoming marble and shale becoming slate are examples of
this metamorphic process.
Earth’s Lithosphere has layers
• Earth’s lithosphere consists of the
Plate tectonics shapes the Earth
• The crust is divided into several plates that float on the mantle. As the
mantle moves, it drags the plates of crust along.
• New crust is formed where two plates are pushed apart.
• Old crust is destroyed when one plate slides under another, a process
• Plates may also collide and cause uplift of the crust.
• Approaching questions by taking a systems approach is helpful in
• The case of the Gulf of Mexico’s hypoxic zone provides evidence that
systems thinking can lead the way to solutions.
• The ecosystems and other environmental systems that we see on Earth
today are those that have survived the test of time
• Many times we alter natural cycles before we even fully understand