Ocean Drifters
Dr Richard R Kirby, Marine Institute Research Fellow, Plymouth University
A secret world beneath the waves
Figure 1
Ocean Drifters introduces the amazing
world of a microcosm of life rarely seen,
the plankton in the sea: microscopic
phytoplankton – plant-like cells, and the
animals that eat them, the zooplankton.
These myriad creatures float freely in the
sunlit surface of the sea from where they
underpin the marine food chain, provide
the world with oxygen, and – through their
birth, growth, death and decay – play an
essential role in the global carbon cycle.
Without the plankton the oceans would be
a barren wilderness, there would be no fish
in the sea, we would have no oil reserves,
and the Earth’s climate would be quite
different. At present, as their habitat alters
rapidly due to rising sea temperatures as a
consequence of global climate change, the
distribution, abundance and seasonality of
the plankton is changing in the seas around
our coasts with ensuing ramifications for
the whole marine food web.
So, how much do you know
about plankton?
Figure 2
How big are plankton? The word Plankton
is derived from the Greek word planktos
meaning ‘drifter’ and so it encompasses
everything that drifts at the mercy
of the ocean currents. Plankton can
therefore range in size from the invisible
picoplankton, the marine viruses – the
virioplankton – and the free-living marine
bacteria – the bacterioplankton – to
organisms as big as the Nomura’s jellyfish,
Nemopilema nomurai, which can reach 2 m
across and weigh more than 200 kg.
How does the plankton underpin the
marine food web?
At the very base of the marine food
chain are the phytoplankton, the ocean’s
‘primary producers’. Despite their small
size, they account for approximately
50% of all photosynthesis on Earth,
and so 50% of the oxygen in the air we
breathe. The phytoplankton are grazed
by the herbivorous zooplankton; these
vegetarians range from simple, singlecelled protozoa to complex multicellular
animals such as euphausiids and
copepods. In turn, the herbivores are
eaten by predators such as carnivorous
copepods, jellyfish, and the planktonic
larvae of animals that live on the seabed
such as crabs. Together all these
creatures are the food of fish larvae and
planktivorous fish such as the sandeel and
sardine, which themselves are the food of
tuna, sharks, and fish-eating seabirds like
the puffin and albatross. At the very top of
this food pyramid are animals like dolphins,
killer whales, seals, and polar bears. The
zooplankton are also the food of one of
the largest sharks in the sea, the basking
shark, and for the world’s largest mammals
- the baleen whales - such as the Southern
Ocean right whale that feeds on the
Antarctic krill; they are also the natural food
for many surface-feeding oceanic seabirds,
such as fulmars, petrels and kittiwakes. In
this way, the plankton underpin the marine
food chain.
Figure 1.
Phytoplankton - various diatoms
Figure 2.
A copepod - Subeucalanus crassus
Figure 3.
A spider crab larva - Maja squinado
Figure 3
How are the plankton changing
around our coasts?
Living at the surface of the sea, the plankton are sensitive
to changes in sea surface temperature (SST). The North
Sea has warmed by 1ºC since the mid-1980s due to hydroclimatic change, and as a result the North Sea ecosystem
has changed fundamentally. Phytoplankton appears to
have increased, while a copepod that prefers colder water,
and is an important food source for larval cod, has declined
in abundance by over 60%.
The planktonic larval abundance of many animals that
live on the seabed has also changed. Decapod and
echinoderm larvae have increased in abundance while the
numbers of bivalve larvae have reduced.
Plankton Facts
you may not know
New warm-water species of crabs whose larvae are
planktonic predators have also invaded the North Sea,
and jellyfish have increased in frequency. Predator-prey
interactions in the marine food web link all these trophic
levels and we have shown that these linkages can act to
amplify the effect of a change in sea temperature to bring
about a ‘new ecosystem dynamic regime’. In the North
Sea this new ecosystem appears to favour decapods and
detritivores in the benthos, and jellyfish in the plankton,
over productive fisheries for cod and plaice. We have
therefore called this new mechanism for ecosystem
change the ‘Trophic amplification of climate warming’.
The copepod life cycle can take a whole year to complete,
and it may involve a period of winter dormancy at great
ocean depths, often up to 2km below the surface where
these animals rest in huge numbers. Many of the baleen
whales seek the overwintering copepods as a source of
their wintertime food.
Phytoplankton Facts
Swirls of foam on the sea surface, also known as spindrift
signal that phytoplankton have bloomed in the water
below. This foam is often thought, wrongly, to represent
pollution, especially when it’s blown onto beaches by the
wind.
Many zooplantkon have weird shapes that are believed to
be adaptations to their planktonic life. Most crab larvae
have long spines that are thought to be a defense against
predators. And, just as we can float better in the sea by
extending arms and legs, the young crab’s spines may help
it to float.
Some phytoplankton release chemicals when they die that
enter the atmosphere and create the characteristic smell
of the seaside that the Victorians called the ‘sea air’. One
of these chemicals, called Dimethyl Sulphide, reacts with
sunlight to create particles that attract water droplets to
form the clouds in the sky.
When atmospheric carbon dioxide dissolves in seawater it
forms a weak acid. Scientists are concerned that increasing
ocean acidity due to rising levels of atmospheric carbon
dioxide, may cause the calcium carbonate shells of some
plankton to dissolve affecting the marine food web.
England’s newest National Park, the South Downs, is
made mainly of the remains of coccolithophores - a type
of phytoplankton that covers their surface with plates of
calcium carbonate – that were deposited on the seabed
over 65 million years ago.
Scientists have suggested that fertilising the sea with
iron, a nutrient that is often in short supply at the sea
surface limiting phytoplankton growth, could promote
phytoplankton growth to draw carbon dioxide from the
atmosphere.
Zooplankton Facts
There are two types of zooplankton: those animals that
complete their whole life cycle, from egg to adult, in the
plankton - like copepods and krill, and those animals that
spend just some of their life in the plankton, these are the
juveniles of animals that live on the seabed, such as crabs
and worms.
To help them avoid being eaten, many copepods spend
the daylight hours far below the surface in the safety of
darkness, only rising to the surface at night to feed. Some
copepods make a daily round trip of over 800m.
Living near the surface of the sea, the plankton are
particularly sensitive to changes in sea surface
temperature. As the sea surface warms due to current
climate change the plankton are changing where they
occur.
In the northeast Atlantic, copepods that like warm water
have moved northwards by over 1200 km in the last 50
years as the sea surface has warmed; those liking colder
water have retreated towards the North Pole. This climatedriven shift in the plankton has had a knock-on effect
for the whole marine food chain - including commercial
fisheries.
Ocean Drifters supported by
The Rolex Institute • Carl Zeiss Ltd • Natural Environment
Research Council • The Fishmongers’ Company •
Valeport
Additional Information /Logos
You can find out more about the mechanism of Trophic
Amplification and the changes to the North Sea ecosystem
in the paper by Richard R Kirby and Gregory Beaugrand,
which is titled Trophic Amplification
of Climate Warming.
This paper is available free online:
http://rspb.royalsocietypublishing.org/
content/276/1676/4095.full