Plankton is the collective term used to describe
some of the smallest plants and animals that live
in the marine environment. Most are incapable
of free swimming and so drift on ocean currents.
There are over 10,000 species of plankton and
most exist in the top 30 metres of the sea in
an area called the ‘photic zone’ (area of water
where sunlight can penetrate).
Plankton are divided into 2 major groups – the
phytoplankton (plants) and the zooplankton (animals).
We shall look at both groups in detail:
Phytoplankton
| The phytoplankton group is made
up of a wide range of photosynthetic organisms
divided into diatoms and dinoflagellates.
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Diatoms: are made up of two halves
of silica case that fits together to form a pill
box shape cell containing chlorophyll. They are
found singulary and/ or in chains and can form
very beautiful structures. The chained cells are
a very good floatation mechanism, helping the
phytoplankton maintain its position in the water
column and therefore prevent it sinking out of
the photic zone.
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| Thallassiosira
spp (chain forming diatom) |
Coscinodiscus
wailessi (single cell diatom) |
Dinoflagellates:
are characterised by having two whip like flagella,
which they use to move through the water column.
They don’t have an exoskeleton like diatoms and
instead have cellulose walls. Dinoflagellates
are usually solitary organisms and rarely form
chains.
Some are capable of producing toxins that are
released into the water. When dinoflagellates
bloom and become very numerous, the cumulative
effect of the toxin can affect other marine organisms
causing mass mortality in fish and invertebrates.
Diruetic Shellfish poisioning (DSP) is caused
by the dinoflagellate Dinophysis and
Paralytic Shellfish Poisioning (PSP) is caused
by the dinoflagellate Gonyaulax. Both
can produce the legendry ‘Red Tides’ seen in some
oceans around the world.
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Dinoflagellate Dinophysis
acuta |
A red tide courtesy
of PJS Franks |
Copepods (Zooplankton)
Copepods are free living planktonic organisms
(resembling a small woodlouse) and range between
1 and 10 millimetres in length. They are capable
of limited free movement by flicking their jointed
limbs and antennae. This is thought to be a predatory
avoidance mechanism. Their antennae also help
to prevent them sinking through the water column.
There are thousands of copepod species – some
live in freshwater, but the majority are marine
organisms and most have the following basic morphology:
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“Copepods
are probablly the most abundant multicellular
organisms on earth. They out number the
insects although insects are more diverse
having more species than copepods. Humes
(1994) estimated that there are some 11,500
species of copepods divided into approximately
200 families and 1650 genera.”
Mauchline J. ‘The Biology of
Calanoid Copepods’ Academic
Press 1998
Copepods can be sexed (male and female
are quite distinct) and it is possible to
determine their developmental stage. Copepods
have a sequential development from nauplii
to adult form – Want
to learn more? |
The copepods are a diverse group
and although some are carnivorous or omnivorous,
the majority are grazers (herbivores) feeding
on phytoplankton. They do this by using a filtering
system or their appendages to manipulate the cell.
The emergence of copepod populations throughout
the year affects the phytoplankton cycles in the
sea.
Other zooplankton
The planktonic community is home
to various types of animal plankton including
the following:
Foraminiferians
Radiolarians
Tintinnids
Molluscs
Marine annelids and chaetagnaths
Cnidaria and Coelentrates (jellyfish)
All of the above groups alongside the copepods
are termed Holoplankton (Have a look at our glossary
of terms)
Meroplankton
Meroplankton is the term used
to describe the larval stages of animal phyla
from other marine habitats. These larval stages
can spend a lot of time in the planktonic community
and can be very numerous at certain times of the
year. The meroplankton includes:
Crustacea – decapods, crabs
Echinoderms – starfish
Barnacle larvae
Fish larvae
Mollusc larvae
Cyphonautes larvae – Bryozoans (sea mats)
Want to try our Plankton
ecology questions?
Think you could identify
marine plankton?
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