Análisis de la oferta

Since preindustrial time until present, the influence of external inputs and the forcings (an environmental influence that causes change in a system) on the global coastal ocean has increased enormously, and consequently, the role of the coastal ocean in the cycling of carbon and other elements has changed signifi- cantly . Increased industrialisation, transportation, and agricultural activities globally, particularly since the end of World War II, and the dramatic rise in the growth of the world’s economies are responsible for the enhanced fluxes that add materials from human activities to the Earth’s landscape, its atmosphere, and ocean (Fig . 9 .1) .

Co ur te sy o f A . C on ra d N eu m an n.

Figure 9.1 Cartoon of human interference in the natural biogeochemical cycles of the ecosphere from the cover of the book Man’s Contributions to Natural Chemical

Human interference in the natural biogeochemical cycles, including the activities of land-use changes such as deforestation, agriculture, mining, and construction of factories, homes, roads, etc ., have increased rates of erosion and the load of nutrients and organic material transported via rivers and groundwater runoff toward the coastal ocean (Fig . 8 .9) . Erosion may have increased by a factor of three to four at the global scale compared to natural levels and probably has

increased by an order of magnitude locally (Vörösmarty et al ., 2004) . However,

the construction of dams and irrigation practices have increased the residence time of continental runoff on land, led to the retention of increasing amounts of sediments behind dams, and generally altered the hydrological cycle . Although human-controlled regulation of the flow of rivers has stabilised the flow for use by humans, it has changed habitats and migration paths for aquatic organisms

(Vörösmarty et al ., 2004) . The total river load of dissolved N and P has doubled at

the global scale, but in some regions such as Europe and North America, it has increased by as much as a factor of 10 to 50 (Meybeck, 1982) . In addition, more than 80% of the world’s rivers exhibit a human imprint . In concert with changes

in the material inputs to the coastal ocean, atmospheric CO2, other greenhouse

gas concentrations, global temperature (Fig . 8 .9), and the average heat content and sea surface temperature of the ocean (Fig . 9 .2) have also increased during the Anthropocene .

Figure 9.2 Trends in ocean heat content (a) and sea surface temperature (SST) (b). The time series for global annual heat content in joules is for the 0 to ~700 m depth range in ocean waters. The various curves represent estimates by three groups: black line with blue shading for the 90% confidence limits, Levitus

et al., 2005; heavy dashed curve, Ishii et al., 2006; and light dashed curve nearer

end of record, Willis et al., 2004 (in IPCC, 2007). The time series for sea surface ocean temperatures (two estimates: one in red, one in blue) is generalised and exhibits an average trend of ~ 0.65 o_{C per century (after Smith et al., 2005).}

There are many examples of the changing water chemistry and sediment loads due to human activities of riverine systems that deliver materials to the

coastal ocean . One that is especially well documented is shown in Figure 9 .3 .It

can be shown that a correlation exists between the amount of nutrients entering coastal environments and the population of people living in watersheds upstream

of these environments . As the population density of a watershed increases, the nitrate and soluble, biologically reactive phosphorus fluxes of a major river draining that watershed and entering a coastal environment increase . This relationship portends for the future increasing nutrient inputs into coastal environments and their enhanced cultural eutrophication (eutrophication due to human activities) because of increasing population density and the increase in agricultural, trans- portation, and urban activities associated with that population . The potential for enhanced eutrophication, coupled with increased inputs of pathogenic bacteria, viruses, heavy metals, and synthetic organic compounds, suggests increased degradation of coastal margin ecosystems and loss of habitat and species .

Figure 9.3 Relationship between the population density in a watershed and the export of dissolved NO3- (a) and phosphorus (b) as soluble, biologically available P by

a river to the coastal ocean. The dark line represents the linear trend though the data and the dashed lines are measures of the degree of confidence in the relationship [(a) after Cole et al., 1993; (b) after Caraco, 1995)].

Figure 9.4 The Gulf of Mexico hypoxic zone. The extent of bottom water hypoxia (yellow area, less than 2 mg l-1 _{dissolved O2) during July 21 to July 25, 1998 (a) The}

area of the hypoxic zone in the northern Gulf of Mexico from 1986 to 2007 in sq km. (b) (after Mackenzie, 2011).

An excellent example of what can happen to a coastal environment because of excess nutrient delivery via rivers is that of the development of the Gulf of Mexico hypoxic zone (Fig . 9 .4) . The Mississippi River and its tributaries in the United States drain two thirds of the conterminous area of the country and deliver nutrients via runoff to the Gulf derived from natural sources and steadily increasing anthropogenic sources of fertiliser and farm wastes, sewage, industrial and urban wastes, and combustion products of nitrogen . The loading of the land- scape by nitrogen from nitrogenous fertilisers and by deposition of combustion nitrogen is particularly heavy in the Midwest of the country . The excess nutrients derived from anthropogenic sources reaching the sea enhance algal growth in coastal waters of this region of the Gulf (Fig . 9 .4a) . The bacterial decomposition

of the dead algae sinking through the water column leads to depletion of oxygen

(O2) dissolved in the water column, and when bottom water levels reach 2 mg l-1

O2 or lower, the water is considered hypoxic . Such low levels lead to the death

of most benthic organisms and impact significantly the abundance of fishes and shrimp living in the area . The hypoxic zone is especially well developed in the summer because of the development of a shallow seasonal thermocline . Figure 9 .4b shows the comparative size of the Gulf of Mexico hypoxic zone from 1986 to 2007 . Owing to the average westward flow of nearshore, shallow-water

currents in this coastal region of the Gulf, the pattern of O2 depletion mimics to

some extent the flow pattern of the water .

Hypoxic zones in coastal environments throughout the world are devel- oping more frequently, mainly because of nutrient pollution and subsequent phytoplankton blooms and later decay of organic material . In addition, as average sea surface temperatures increase, the solubility of oxygen in seawater decreases, further complicating the problem . The largest concentrations of these hypoxic and anoxic zones are found in off shore areas of the United States and Europe . These degraded and oxygen-deficient coastal waters can strongly affect the

CO2-carbonic acid system in coastal water bodies and air-sea exchange of CO2 .