The Coastal Environment

Jump to: [Types of Coastal Zones][What Makes Coastal Zones Biologically Productive][Threats to Coastal Zones][Vulnerability to Hazards][Threats to Coastal Fisheries][Case Studies][Conclusions]


Format for printing

In this lecture period, we wish to learn:

  • How are coastal environments important to society?

  • What are specific types of coastal ecosystems and how is their biological productivity being threatened?

  • What human hazards are associated with living near coasts?

  • How can we minimize or mitigate risks to human welfare and coastal resources?

Types of Coastal Zones

Coastal zones include many types of environments:

Upwelling areas were described in a previous lecture.  They are highly productive because nutrient rich water from depth reaches the surface where ample sunlight fuels primary production.

Coral reefs are amongst the most impressive and diverse, as well as productive structures on earth.  Although we will not discuss reefs in detail, you should be aware that coral reefs face a variety of serious threats. Reefs of the Tropical Americas are at particularly high risk.

  • About 58 percent of the world’s reefs are at risk of degradation because of coastal development, destructive fishing and pollution from inland runoff from deforestation and farming. 

  • Coastal reefs of Southeast Asia are most threatened, with more than 80 percent at risk, primarily from coastal development and fishing. 

  • In the United States, most reefs face some risks, with all reefs off Florida facing potential degradation because of coastal development and farming-related pollution. 

  • Two-thirds of the Caribbean reefs are in jeopardy, including high risks to reefs off Puerto Rico, the U.S. Virgin Islands, Jamaica and Barbados. The principal culprits are overfishing and pollution including high levels of nutrients that cause algae growth.

  • Slight warming of surface waters due to climate change can cause coral bleaching and death 

The Coastal shelf is characterized by shallow (200-m depth) water, and extends 50 (Cape Hatteras) to 150-km (Cape Cod) offshore.  The Shallow depth allows re-suspension of nutrients due to storms, and so biological productivity usually is high.

Figure 2. Cross-shelf transect of high resolution bathymetry on the continental shelf near Duck, NC. Continental shelf zones, surf zone, inner-shelf, mid-shelf and shelf break, are labeled.

Estuaries form where freshwater from rivers and streams flows into the ocean, mixing with the salty sea water.  Important examples include San Francisco Bay, Puget Sound, Chesapeake Bay, Boston Harbor, and Tampa Bay.  Estuaries act as a natural buffer between the land and ocean, absorbing flood waters and dissipating storm surges.  Often they exhibit vertical stratification, with outward-flowing freshwater resting on a layer of denser salt water along the bottom.

Salt marshes line much of the Atlantic Seaboard.  These highly productive ecosystems serve as nursery grounds for diverse fish and shellfish, and habitat for birds and other wildlife.  They trap sediments and nutrients as well.  The cord grass Spartina is the most important plant in many easter seaboard saltmarshes.

Mangroves are composed of mangrove trees that have specially adapted aerial and salt-filtering roots and salt-excreting leaves that enable them to occupy the saline wetlands where other plant life cannot survive.  Mangrove is not a taxonomic category, but a diverse group of salt-tolerant plants.  They provide habitat and stabilize currents, allowing many organisms to dwell amongst them.

Coastal Shore and Barrier Islands

Scenic and rich in wildlife, coastal regions have high recreational value.  Roughly 300 barrier islands occur on Atlantic and Gulf Coasts.  Extremely dynamic land masses, they are retreating landward in response to rising sea levels

Figure 3. A highly developed barrier island (left) exposes large numbers of people to meterological hazards.  An undeveloped barrier island (right) exposes no humans to risk and protects a natural feature.

What makes Coastal Zones Biologically Productive?

As we will learn in the lecture on fisheries, most of the the harvestable production of the seas comes from coastal areas and upwelling areas (which are coastal as well).  The open ocean is vast, and due to its large extent, contributes the bulk of the oceans primary production, but only a fraction of the harvestable fish and shellfish. 

Why are coastal zones productive?  River export of nutrients, localized nutrient upwelling, water column mixing and resuspension of nutrients during storms and ample light for photosynthesis, all contribute to high productivity in coastal zones.

Threats to Coastal Zones

Globally, coastal zones are stressed by population growth.  Population pressures include increased solid waste production, polluted urban runoff, and loss of green space and wildlife habitat.



Figure 4. 50% of world’s population lives within 6 km of the sea.  Some 14 of 15 largest mega-cities are coastal.

Coastal zone population trends document the human influx to these areas.  About 53% of the US population resides along its coastal fringe (excluding Alaska), although coastal counties account for only about 17% of the land area in contiguous U.S.  Fourteen of 20 largest U.S. cities are located in coastal zone.


Figure 5. Coastal population trends shown spatially (left panel) relate to non-coastal population growth (right panel)

Michigan’s Coastal Zone is amongst its most valuble resources.  With 3,288 miles of coast, Michigan has the world’s largest freshwater coastline.  The Department of Natural Resources of Michigan manages coastal activities such as shipwreck salvaging, building piers and marinas, and development.  Tourism, boating, fishing, commercial shipping, agriculture and manufacturing are the State’s largest coastal industries. 

The prevailing strategy adopted for preservation of these ecosystems is that of Integrated Coastal Management, which includes integration across levels of authority (federal, state, regional, local), economic sectors (recreation, agriculture, industry, energy), traditional disciplines (science, engineering, law, etc.), and management tasks (wetlands, restoration, fisheries, etc.). Funding is provided to coastal states that take on a coastal management effort for wetlands, lagoons, reefs, and other habitats. Improved recreational use, minimization of property damage from coastal hazards, and intergovernmental cooperation are also emphasized. Thirty-two states have approved plans for adopting the Coastal Zone Management Act by developing new standards in coastal management.  The Office of Ocean and Coastal Resource Management works to effectively manage multiple uses of the nation's coastal and ocean resources.

Vulnerability to Hazards

Hurricane Fran struck the North Carolina coast on Sept 5, 19996.  At landfall, estimated winds were 115 mi/hr.  The resulting coastal storm surge was 12 feet high.  Hurricane Fran dramatically redefined the shoreline, eroded the protective dune system, and caused extensive property damage.


Extreme events of nature are natural, and not human hazards until humans find themselves, or place themselves, in the path of storms and hurricanes.

The increased population density along coasts increases risks to human safety and property.  Annual, nationwide disaster losses were $4.5 billion in 1970; today direct loss estimates are $10-20 billion dollars.  Of an estimated $500 billion total losses 1975-1994, 80% were caused by meterological events and only 17% were insured.

Figure 6. This coastal residence, although guarded by sand bags, was totally destroyed by Hurricane Fran.

Reducing the Impacts

To minimizing the impacts of coastal zone hazards requires preparedness and a better informed public,  including improved capability in:

  • predicting hazard events and impacts

  • mobilizing the public to evacuate or shelter

  • reducing overall vulnerability of people and property

Figure 7. Cost of Hurricanes Damage, in billions of dollars (left panel) and Loss of Life (right panel). While the cost of natural disasters are rising sharply (left), loss of life is falling sharply (right).

Threats to Coastal Fisheries

While over-harvesting is a serious threat to many fisheries, additional human threats exist.  They include destruction of estuaries and marshes where much reproduction occurs and juveniles spend their early lives, shifts in biological activity, often due to nutrient enrichment, and the development of oxygen-depleted regions, also due to nutrient enrichment.

Nutrients include nitrogen, phosphorus, and other elements that stimulate plant growth.  Excessive nutrients leads to high levels of plant growth, termed eutrophication.  Lake Erie was famous in the 1970's for this problem.  All of this biological production eventually is consumed by microbes and their metabolic activities, which can deplete the oxygen available to very low levels.

Hypoxia is defined as < 2-3 mg/L O2 and anoxia is 0 mg/L O2.  Sea water is often 6-8 mg/L O2, and organisms are stressed by low oxygen levels.  The de-oxygenated zone is usually confined to the bottom waters, and if severe, is devoid of most life.  While fish probably leave dead zones, less mobile, bottom-dwelling organisms may be killed.

Dead zones (hypoxic or anoxic waters) are known from a number of locations.  Chesapeake Bay and Gulf of Mexico dead zones have attracted much attention due to their size, and the loss of valued biological resources.

Nutrient Enrichment

To understand nutrient enrichment, lets digress for a moment to consider how excess nutrients affect biological production.  Phosphorus usually limits freshwater production, whereas nitrogen tends to limit oceanic production.

In order to limit nutrient loading to aquatic ecosystems, we need to identify where the excess nutrients originate.  We do this using the mass balance approach.

inputs = outputs +/- storage

The inputs of N include atmospheric NOx from internal combustion engines and coal-fired power plants, fertilizer, and sewage.  Outputs of N include export from river mouths (which causes coastal eutropication and dead zones), and biological processing en route (which helps reduce the N-load to the seas).  Storage occurs in channel sediments and floodplains.  The mass balance approach demonstrates that fertilizer is the source of the increased nitrogen delivered to the sea by large rivers.

Case Studies

Now, let's look at two case studies of coastal areas affected by nutrient enrichment.

Chesapeake Bay
The Chesapeake Bay is the largest estuary in the U.S., encompassing 64,000 mi2.  Its near-area population is home to 15 million people.  "The Bay" is of enormous value.  It is the largest source of oysters in the U.S. and largest producer of blue crab in the world.  The Bay is also important for shipping, recreation and fishing.  Unfortunately, it receives waste from point and non-point sources throughout a huge drainage basin.

Figure 8. The drainage basin of the Chesapeake Bay.  Phosphate and nitrate plant nutrients have risen sharply, causing algal blooms and oxygen depletion. Primary sources are sewage plants for phosphates; non-point agricultural run-off for nitrogen.

Figure 9. Due to nutrient loading from its rivers, Chesapeake Bay’s anoxic zone is expanding. Compare the darkest (anoxic) zones in the two images above, between 1950 (left) and 1990 (right). 

For more Chesapeake Bay information check out the Chesapeake Bay Program

Gulf of Mexico Hypoxia

The Gulf of Mexico now experiences an anoxic zone twice the size of the entire Chesapeake Bay.  This dead zone has been devastating to bottom organisms, including its famous shrimp, and drives fish away.  The anoxic zone was first discovered in the 1970's, and has grown greatly in size.  Are excessive nitrogen loads from the Mississippi River responsible?  If so, what accounts for the increase in nitrogen in the river and what can be done about it?

Mass balance calculations attribute 80% of Mississippi’s nitrogen load to fertilizers, originating in the agricultural heartland of the USA.  Nitrate loads of the Mississippi are three times higher than in the 1950’s, and chemical fertilizer use in the Midwest has tripled over this period.  The evidence seems clear.  Fertilizer use in the upper Mississippi Basin is the culprit.  Iowa farmers and Louisiana shrimpers are in conflict.  However, the farming communities of the Midwest are not necessarily in agreement.

What are possible solution to excess nutrient export?

  • Use less fertilizer: low cost entices farmers to over-fertilize, hoping for a good year
  • Use nitrogen-fixing crops
  • Retain wetlands and riparian buffers, sites of denitrification and nutrient uptake by plants
The state of Maryland's "Smart Growth" program is a positive example of coastal zone management.  "Smart Growth" legislation discourages low-density suburban development, funds infrastructure (sewers), and mandates 100-m setbacks.


  • Coastal ecosystems are biologically productive, yet extremely vulnerable to direct and indirect sources of pollution.
  • Legal recognition and definition of these ecosystems is an important step towards preservation.
  • Successful policy solutions must encourage cooperation between regulatory agencies, farmers, and fishermen, while providing increased support for research.