Page 215 - (DK) Ocean - The Definitive Visual Guide

CYCLES OF LIFE AND ENERGY 213

Productivity Upwelling

Throughout the world’s oceans, the abundance of The open-ocean surface water can become
marine life varies dramatically. The ocean is more impoverished, as nutrients are constantly absorbed
productive in some places and at some times than by phytoplankton and fall with detritus to the sea
others. The amount of sunlight is a major influence floor. Nutrient-rich water can be restored to the
on productivity and changes with latitude and time surface on a large scale by vertical ocean currents
of year. The supply of nutrient-rich water from the in a process called upwelling (see p.60).
sea floor and light for photosynthesis is affected by Near land, coastal upwelling is caused by surface
changing water movements and day length, affecting currents, such as the Humboldt Current off South
plankton levels. Temperature also affects productivity America (see p.58). In the equatorial waters of the
as it influences the rate Pacific and Atlantic, mid-ocean upwelling occurs
of photosynthesis. when water masses are driven north and south by
the trade winds, and cooler, nutrient-rich water
CLEAR, TROPICAL OCEAN rises to take their place. Polar upwelling can
Tropical waters are not mixed happen where winter storms cause intense water
seasonally, so few nutrients are RICH, MURKY TEMPERATE SEA movement. When upwelling occurs and there is
returned to the surface, and little In coastal and temperate areas, water
plankton growth is possible. Here, a turbulence circulates nutrient-rich water sufficient sunlight, phytoplankton multiply rapidly
solitary turtle cruises in crystal-clear that supports a variety of algae, such to support a vast number of organisms, creating
surface waters near Hawaii. as this kelp forest in the Pacific. the most productive ocean waters in the world.

NUTRIENT-RICH WATERS
Where there is upwelling, large numbers of small
fish gather to feed on the plankton. They, in turn,
attract larger predators like these copper
sharks feeding on sardines off the coast
of South Africa. OCEAN LIFE