The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- Salinity is the amount of dissolved substances in water, usually
expressed in parts per thousand ‰. Seawater salinity in the open ocean
averages 35‰. The principal elements that contribute to the ocean's
salinity are chlorine (55 percent) and sodium (31 percent),
which combine to produce table salt. The primary sources of the elements in sea
salt in the ocean are chemical weathering of rocks on the continents
and volcanic outgassing.
- Variations in seawater salinity are primarily caused by changing the
water content. Natural processes that add large amounts of fresh water to
seawater and decrease salinity include precipitation, runoff from land,
icebergs melting, and sea ice melting. Processes that remove large
amounts of fresh water from seawater and increase salinity include the
formation of sea ice and evaporation. Seawater salinity in the
open ocean ranges from 33‰ to 38‰, with some marginal
seas experiencing considerably more variation.
- The ocean's surface temperature is related to the amount of solar
energy received and varies as a function of latitude. Low-latitude regions
have distinctly colder water at depth, creating a thermocline, which is a
layer of rapidly changing temperature. No thermocline exists in high-latitude regions,
because the water column is isothermal.
- Water's unique thermal properties have caused the ocean's temperature to
remain stable for long periods of time, facilitating the development of life on
Earth. Experiments have been conducted that send sound through the ocean to
determine if the ocean's temperature is increasing as a result of global warming.
- Seawater density is mostly affected by water temperature but also by
salinity. Low-latitude regions have distinctly denser (colder) water at depth, creating a
pycnocline, which is a layer of rapidly changing density. No pycnocline exists in
high-latitude regions because the water column is isopycnal.
- Most open-ocean regions exhibit a three-layered structure based on water
density. The shallow surface mixed zone has warm and nearly uniform
temperatures. The transition zone includes a prominent thermocline and
associated pycnocline. The deep zone is continually dark and cold and accounts
for 80 percent of the water in the ocean. In high latitudes, the three-layered structure
does not exist.
- Marine life is superbly adapted to the oceans. Marine organisms can be
classified into one of three groups based on habitat and
mobility. Plankton are free-floating forms with little power of
locomotion, nekton are swimmers, and benthos are bottom
dwellers. Most of the ocean's biomass is planktonic.
- Three criteria are frequently used to establish marine life zones.
Based on availability of sunlight, the ocean can be divided into the
photic zone (which includes the euphotic zone) and the
aphotic zone. Based on distance from shore, the ocean can be
divided into the intertidal zone, the neritic zone, and the
oceanic zone. Based on water depth, the ocean can be divided
into the pelagic zone and the benthic zone (which includes the
abyssal zone).
- Primary productivity is the amount of carbon fixed by organisms
through the synthesis of organic matter using energy derived from solar radiation
(photosynthesis) or chemical reactions (chemosynthesis).
Chemosynthesis is much less significant than photosynthesis in worldwide oceanic
productivity. Photosynthetic productivity in the ocean varies due to the availability
of nutrients and amount of solar radiation.
- Oceanic photosynthetic productivity varies at different latitudes
because of seasonal changes and the development of a
thermocline. In polar oceans, the availability of solar
radiation limits productivity even though nutrient levels are high. In
tropical oceans, a strong thermocline exists year-round, so the
lack of nutrients generally limits productivity. In temperate
oceans, productivity peaks in the spring and fall and is limited by the lack
of solar radiation in winter and by the lack of nutrients in summer.
- The Sun's energy is utilized by phytoplankton and converted to
chemical energy, which is passed through different trophic
levels. On average, only about 10 percent of the mass taken in at one
trophic level is passed on to the next. As a result, the size of individuals
increases but the number of individuals decreases with each trophic
level of the food chain or food web. Overall, the total biomass
of populations decreases at successive trophic levels.
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