For the first time in Earth's history, there is a species (homo sapiens) that can affect the Earth system on a global scale in a very short time -- and tip the balance into chaos. But paired with that awesome potential for destruction is our ability to study these changes on a global scale, and with the knowledge we gain perhaps avoid radical jolts to Earth's systems.
Yet even without human influence, the Earth would be a dynamic, changing system. The Earth has its own cycles and trends. NASA exploration is instrumental in our efforts to determine whether the global changes we are seeing are natural, set in motion eons ago, or caused by human action. Either way, global change is already affecting life on Earth.
"Dead zones" are areas where the bottom water (the water at the sea floor) is anoxic - meaning that it has very low (zero) concentrations of dissolved oxygen. These dead zones are occurring in many areas along the coasts of major continents, and they are spreading over larger areas of the sea floor. Because very few organisms can tolerate the lack of oxygen in these areas, they can destroy the habitat in which numerous organisms make their home. Landsat image of the Mississippi Delta shows shades of muddy water flowing into the Gulf of Mexico. The colors change from muddy brown to aqua as the water moves further into the gulf and sinks.
The cause of anoxic bottom waters is fairly simple: the organic matter produced by phytoplankton at the surface of the ocean sinks to the bottom, where it is subject to breakdown by the action of bacteria. Bacteria use oxygen and give off carbon dioxide. The oxygen used by bacteria is the oxygen dissolved in the water, and that's the same oxygen that all of the other oxygen-respiring animals on the bottom (crabs, clams, shrimp, and a host of mud-loving creatures) and swimming in the water (zooplankton, fish) require for life to continue. Most of the time, oxygen is renewed by wind mixing the water, especially in winter. In summer, when winds are weak and sun warmed water at the surface increases stratification, oxygen becomes depleted.
Dead zones grow if phytoplankton produce more organic matter to feed the bacteria. Phytoplankton productivity can be increased by agricultural fertilizer, runoff from cities and outflow from sewage plants.
Who fertilizes the phytoplankton? No one, not on purpose anyway. Fertilizer runs off the fields into the streams and rivers of a watershed. City dwellers put fertilizer on their lawns, and everyone produces sewage. When the nutrients reach the ocean, they supplement the natural nutrients available to the phytoplankton, so the phytoplankton do what they do best: they grow and multiply. Which leads to more organic matter reaching the bottom, more bacteria, and more dead zones.
Algal Blooms - good & bad algae
Phytoplankton blooms normally occur in response to either an increased supply of nutrients, or increased exposure to sunlight. Most of the phytoplankton blooms occurring in the oceans are induced by natural causes and seasonal cycles. The North Atlantic Bloom is the largest seasonal bloom observed every year, but there are also large blooms in the Arabian Sea due to the monsoon cycle, and increases in productivity associated with the rainy season in South America can be seen emanating from the mouths of the Orinoco and Amazon rivers. Because these blooms take place due to natural processes, it's possible to call them 'good' blooms.
However, the activities of mankind are causing an increasing incidence of what can be termed 'bad' blooms: blooms induced by high nutrient concentrations due to pollution, enhanced runoff due to loss of vegetative cover and erosion, or agriculture. These blooms, rather than enhancing oceanic productivity, tend to interrupt the natural cycle, providing an overabundance of organic matter to the ocean bottom (see Creeping Dead Zones) and frequently enhancing the growth of noxious or toxic phytoplankton.
Coral reefs are hotspots of biodiversity in the ocean. Though they cover only 0.2 percent of the ocean's floor, scientists estimate that the reefs are home to over 4,000 species of fish, 700 species of coral, and thousands of other forms of plant and animal life.
Algal blooms cause the water over coral reefs to become increasingly cloudy. Since corals must have clear water to thrive, this turbid water slowly extinguishes many coral species. Excess nutrient concentrations may also encourage the growth of encrusting macroalgae directly on the coral, as shown below, even in waters that appear relatively clear and pristine. Furthermore, sewage can carry with it bacteria and viruses that can induce coral diseases, which may cause rapid die-off of living coral formations.
Fish kills can have varied causes, including pollution and harmful algal blooms. A red tide is caused by the bloom of a single species of phytoplankton that produce toxins (such as dinoflagellates) and lead to discoloration of surface waters. This production of toxins causes mass mortality of marine organisms. Another common cause of fish kills in this region is oxygen-depleted water upwelling to the surface. Knowing why a fish kill is happening is important to the fishing industry. Oxygen depletion raises no concerns about food security, while concerns about toxic blooms can seriously damage fish exports and local consumption.
In some areas, such as the Gulf of Oman and Arabian Sea, there are oxygen-poor water at depths of about 100 meters (300 feet) below the surface. An oxygen-poor layer is related to highly productive (fertile for growth of plankton and other microorganisms). Although phytoplankton and other marine biota have relatively short lifespans (roughly 1-3 days), under the right conditions they have the capacity to reproduce, or "bloom," into exponentially large numbers in a matter of days. Over time, these biota die at the surface and begin to sink to the bottom as detritus. As this detritus sinks it decays, thereby using up oxygen in the water column. Sometimes, due to shifts in the overlying wind field, these deep oxygen-poor waters upwell to the surface. Images of the Gulf of Oman on August 21, 2000 and August 31, 2000 showing sea surface temperature and wind data. Small flag icons appear over the sea surface area indicating the speed and direction of the wind.
Satellite imagery can give a quick and early indication of an upwelling event along the coast. In the case of a fish kill in the Gulf of Oman, images showed some of the water was being upwelled in the Batinah region where the fish kill was located, and some oxygen-poor water was being upwelled to the south and carried into the region as part of a narrow coastal current. Analysis of remotely-sensed sea surface temperature data showed that cool upwelled water appeared at the surface along the Batinah coast as early as August 21, 2000, reaching coolest temperatures by the time of the peak of the fish kill on September 4, 2000. Thus, the remote sensing data, combined with in situ data helped determine that the fish kill was due to oxygen-depleted water and not a toxic red tide event, allaying the fears of the local populace as well as the fisheries.