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Life becomes rarer still with increasing depth. Unique forms of life are found in the very deep waters surrounding hydrothermal vents, where geothermally heated water comes from volcanically active places like tectonic plate spreading centers. Chemosynthesis, the biological conversion of carbon molecules and nutrients like hydrogen gas and hydrogen sulfide into organic matter,3.203 support giant tube worms, clams, and shrimp colonies.
The Shallow Seas provide the environment for some of
the most abundant life on earth. Starting at the sea floor, this life includes
reefs. Many reefs result from abiotic processes, such as the deposition of sand and wave erosion planning down rock outcrops. However, the best known reefs are the coral reefs of tropical waters. These reefs are developed through biotic
processes, dominated by the growth of corals, calcareous algae, and shells, like those from oyster beds.3.204 Ancient coral reefs play an important role in the geologic record. Along with ancient pelagic ooze (organic material that fell into the deep oceans), these ancient reefs formed the limestone found in the geologic record. Limestone is the primary source of cement, a fixture of modern society, and the primary storage site
for ancient greenhouse gases. Dust from the giant deserts, algal blooms, pollution, and overfishing are threatening modern reefs. The combined impact of (1) mining ancient coral reefs and (2) losing these significant carbon dioxide sinks
we call coral reefs results in (3) loosing the most important control there is on the amount of CO2 greenhouse gases in the atmosphere.
The shallow seas are also where the largest volumes of plankton grow. Plankton
occur throughout the oceans, in the seas, and in large lakes. The local abundance of plankton varies horizontally, vertically, and seasonally, as a function of light and of the availability of nutrients.3.205 If there are excess nutrients, particularly phosphorus and nitrogen), then there can be a very rapid growth of algae and green plants (see Figures 58a and 58b).3.206 Satellite images can be enhanced to emphasize subtle differences in the quantities of marine life. A single blue whale eats about 4 tons (3,600 kilograms) of seafood each day.3.207 At the opposite end of the spectrum, HNLC ("high-nutrient, low-chlorophyll") areas are ocean life deserts. Although there may be sufficient nutrients, the HNCL areas are probably due to a lack of iron.3.208
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paradoxes
