Page 214 - (DK) Ocean - The Definitive Visual Guide

212 INTRODUCTION TO OCEAN LIFE

Cycles of Life and Energy

ALL LIFE DEPENDS ON ORGANISMS that harness energy Recycling
from either chemicals or the Sun to produce food.
All living things need a supply of chemical nutrients, such as nitrates,
These organisms, whether phytoplankton, seaweeds, or
phosphates, and silicates, to grow and reproduce. They are taken up by
bacteria, are called primary producers and form the first primary producers then passed along the food chain. Although some
link of a food chain. This first link is just one point in nutrients are available from seawater, most are derived ultimately from
the sea floor. When an organism dies, any parts that are not eaten by
a cycle that processes chemical energy and nutrients
other animals gradually sink to the sea floor, where they are broken
through the entire community of life in an ecosystem, down by bacteria and other decomposers. Fecal matter also ends up
into the physical environment, and back again. on the seabed and is processed by detritus feeders or decomposers.
Eventually, the nutrients are released into the environment in their
Energy Flow mineral, nonliving forms. They may then remain at depth, or they
may be returned to surface waters by circulating water currents
As each organism in an ecosystem is eaten in turn by the next organism within an ocean basin (see upwelling, opposite).
in the food chain, food energy flows from prey to consumer. The
primary producers—the organisms such as diatoms and bacteria at the upwelling of nutrients NUTRIENT CYCLE
beginning of the food chain—are eaten by organisms called primary released by bacteria
Small particles of organic matter, or
consumers, which are eaten by secondary consumers, and so on to top phytoplankton absorbs sunlight detritus, are found in the water column.
predators—animals not preyed upon by anything else. In land-based and use nutrients to grow They may be eaten by scavengers or
ecosystems, the total mass of organisms at each succeeding food-chain zooplankton feed broken down still further by bacteria
level decreases, leaving very few top predators. However, in marine on phytoplankton present in the water. However, many of
them rain down on the ocean floor where
ecosystems with phytoplankton as producers, the mass is greatest at the they decompose, releasing nutrients. The
primary consumer level. This is possible because phytoplankton grow nutrient cycle is completed by upwelling
so rapidly that they provide great turnover despite having little mass. water currents that then carry the nutrients
back to the surface where they can be
utilized by the phytoplankton.
FOOD-ENERGY PYRAMID BIOMASS PYRAMID
At each level of a food chain, energy is lost as The biomass pyramid (below) for a system detritus
heat, so less is available to the next consumer. with plankton producers is partly inverted, WARM WATER falls
The diminishing energy at each level can because the producers have low total mass. HUMAN IMPACT
be represented by a pyramid (below) and Despite this, the rapid reproduction of the COLD WATER fish eats OVER-HARVESTING
accounts for the scarcity of top predators. plankton keeps the food chain supplied. detritus
These fishermen are harvesting
top predators top predators detritus Pacific cod. Cod populations have
predators predators falls to
sea floor drastically declined and many
consumers consumers
important stocks have collapsed
primary producers primary producers because too many are being caught
bacteria process detritus on for human consumption before
TOTAL ENERGY TOTAL BIOMASS
detritus sea floor they can reproduce successfully.
The imposition of quotas by
governments has not solved the
PRIMARY PRIMARY SECONDARY TERTIARY QUATERNARY
problem, although numbers are
PRODUCERS CONSUMERS CONSUMERS CONSUMERS CONSUMERS
phytoplankton starting to recover in some areas.
krill
baleen whales COD FISHING
More than 1.5 million tons of cod (Atlantic and
birds
Pacific) were caught in 2010 using various
methods including trawls and longlines.
protozoans

carnivorous penguins seals
zooplankton

pelagic FOOD WEB
fish Many food chains have been
decomposer squid combined to form this complex
bacteria
food web, extending from primary
copepods
producers to quaternary consumers
(top predators) for a Southern Ocean
ecosystem. Each arrow shows the
OCEAN LIFE seaweed decomposer on sea demersal small toothed killer whales predator, grazer, or decomposer. It
flow of food energy from prey to
detritus
shows how organisms depend on
whales
invertebrate
floor
fish
one another for food. Some animals
feed on organisms from several
different levels of the food chain,
adding to its complexity. Food webs
are delicately balanced and easily
upset by human interference.