Testing Your Comprehension
Approximately 71% of Earth’s surface is covered by ocean waters containing, on average, about
3.5% salt. Water temperature declines with depth, and density increases slightly at lower temperatures
and higher salinities. Therefore, deep water tends to be colder, saltier, and denser than the surface water.
Ocean currents are driven by the prevailing wind currents at the surface, by gradients in water
temperature, by gravity, and by the Coriolis effect. Surface currents move horizontally in large
circulation patterns. Vertical currents (upwellings and downwellings) slowly mix the deep waters with
the surface waters, affecting the distribution of nutrients and primary productivity.
Biologically productive areas are concentrated in areas of upwelling, in the shallower waters
along continental margins, and at hydrothermal vents of the deep mid-ocean ridges.
Along the coasts there are kelp forests that shelter invertebrates, smaller fishes, seals, and top
carnivores such as great white sharks. Coral reef communities, which include
sponges, hydroids, tubeworms, molluscs, flatworms, starfish, urchins, and thousands of fish species, are
among the most diverse and productive ecosystems on Earth. Intertidal ecosystems include rocky and
sandy beaches, salt marshes, estuaries, and mangrove forests, which serve to buffer the land from the
effects of storm surges and act as nursery areas for many marine organisms of economic importance,
such as shrimp.