In this lecture period, we wish to learn:
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.
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.
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.
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.
Reducing the Impacts
To minimizing the impacts of coastal zone hazards requires preparedness and a better informed public, including improved capability in:
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).
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.
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.
Now, let's look at two case studies of coastal areas affected by nutrient enrichment.
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?