The Great Barrier Reef
142
In temperate waters copepods often release eggs
into the water column that sink to the substratum. Egg
reserves on the substratum can be great and development is usually arrested (they ‘aestivate’) until conditions in the plankton are suitable for hatching; this is
probably not true in the tropics. Some copepods spend
time in the plankton and on the substratum. The harpacticoid Euterpina, for example, shows great variation
in abundance in the plankton and this is partly related
to seasonal use of the substratum.
Demersal plankton
There is great variation in tropical plankton between
day and night. This is largely the result of emergence of
plankters from the substratum at night. Invertebrates
that include copepods, amphipods, mysids (23 species
on GBR), isopods and polychaete worms migrate from
the substratum after dusk. Nocturnal movements are
probably for the purposes of feeding or dispersal.
Demersal plankton provides a rich source of food for
invertebrates and fishes and this can be of greater
importance as an energy source than the import of
plankton from inter-reefal waters. The feeding polyps
of many corals only emerge at night and fishes that
include cardinal fishes and squirrel fishes feed on large
nocturnal zooplankton. Historically, the importance of
demersal plankton was not understood, but is now
viewed as a critical component of the food chain of
reefs.
Vertical migration
Plankton of inter-reefal waters, on the continental shelf
and beyond often undertake nocturnal vertical migrations. A Russian scientist, Vinogradov, referred to a
‘ladder’ of migrations where phytoplankton and zooplankton migrate from different daytime residence
depths toward the surface. Phytoplankton may alter
depth of residence by secreting and absorbing gas, or
by varying their oil or salt content. Phytoplankton often migrate to deeper, more nutrient rich waters during
the night and back to the sunlit surface waters at the
onset of the day. This migration pattern is known as
reversed diel migration. Zooplankton and larger
flagellated phytoplankton migrate by swimming. In
other parts of the world, copepods often swim from
depths of 100 m or greater toward food rich waters at
the surface at night, but there are few data on this for
the GBR. Visual predators are active in the photic zone
during the day and it is thought that gloomy waters at
depth provide refugia for zooplankton. Some studies
have demonstrated that copepods will return to deep
water at night if they detect the predators through
chemical or tactile senses. The timing of some nocturnal migrations varies by reproductive state.
N DISTRIBUTION PATTERNS OF PLANKTON
The influence of oceanography
The movement of water will have a great influence on
organisms that largely drift. However, some directional
control can be obtained by varying their vertical position in water columns that have vertical stratification of
current speed and direction. Currents can influence the
trajectories of plankton and can influence concentration. Some eddies are called ‘phase eddies’ that only
occur at one phase of the tide (e.g. on ebb tides behind
a reef) while other eddies (and larger gyres) persist
over long time periods in mainstream currents. In addition to eddies, currents can interact with abrupt topography such as reefs and seamounts to alter the supply
of plankton through upwelling.
Plankton often concentrate in convergences. In
tropical waters it is common to see blue green algae
(Box 14.2), coral spawn, jellyfishes, spawning appendicularians and other plankters in convergence zones
generated by physical phenomena that include: tidal
and thermal fronts, tidal jets, the edge of eddies, windrows and internal waves. As a result, therefore, convergences are often sites of intense biological activity
and nekton (e.g. planktivorous fishes, piscivores) and
birds are often attracted to the abundant food in these
areas.
Oceanography can also influence the distribution
of plankton with depth. Although the water column
on the shelf of the GBR is often well mixed (see
Chapter 4), thermoclines and haloclines can stratify
the water column into different water masses. For
142
In temperate waters copepods often release eggs
into the water column that sink to the substratum. Egg
reserves on the substratum can be great and development is usually arrested (they ‘aestivate’) until conditions in the plankton are suitable for hatching; this is
probably not true in the tropics. Some copepods spend
time in the plankton and on the substratum. The harpacticoid Euterpina, for example, shows great variation
in abundance in the plankton and this is partly related
to seasonal use of the substratum.
Demersal plankton
There is great variation in tropical plankton between
day and night. This is largely the result of emergence of
plankters from the substratum at night. Invertebrates
that include copepods, amphipods, mysids (23 species
on GBR), isopods and polychaete worms migrate from
the substratum after dusk. Nocturnal movements are
probably for the purposes of feeding or dispersal.
Demersal plankton provides a rich source of food for
invertebrates and fishes and this can be of greater
importance as an energy source than the import of
plankton from inter-reefal waters. The feeding polyps
of many corals only emerge at night and fishes that
include cardinal fishes and squirrel fishes feed on large
nocturnal zooplankton. Historically, the importance of
demersal plankton was not understood, but is now
viewed as a critical component of the food chain of
reefs.
Vertical migration
Plankton of inter-reefal waters, on the continental shelf
and beyond often undertake nocturnal vertical migrations. A Russian scientist, Vinogradov, referred to a
‘ladder’ of migrations where phytoplankton and zooplankton migrate from different daytime residence
depths toward the surface. Phytoplankton may alter
depth of residence by secreting and absorbing gas, or
by varying their oil or salt content. Phytoplankton often migrate to deeper, more nutrient rich waters during
the night and back to the sunlit surface waters at the
onset of the day. This migration pattern is known as
reversed diel migration. Zooplankton and larger
flagellated phytoplankton migrate by swimming. In
other parts of the world, copepods often swim from
depths of 100 m or greater toward food rich waters at
the surface at night, but there are few data on this for
the GBR. Visual predators are active in the photic zone
during the day and it is thought that gloomy waters at
depth provide refugia for zooplankton. Some studies
have demonstrated that copepods will return to deep
water at night if they detect the predators through
chemical or tactile senses. The timing of some nocturnal migrations varies by reproductive state.
N DISTRIBUTION PATTERNS OF PLANKTON
The influence of oceanography
The movement of water will have a great influence on
organisms that largely drift. However, some directional
control can be obtained by varying their vertical position in water columns that have vertical stratification of
current speed and direction. Currents can influence the
trajectories of plankton and can influence concentration. Some eddies are called ‘phase eddies’ that only
occur at one phase of the tide (e.g. on ebb tides behind
a reef) while other eddies (and larger gyres) persist
over long time periods in mainstream currents. In addition to eddies, currents can interact with abrupt topography such as reefs and seamounts to alter the supply
of plankton through upwelling.
Plankton often concentrate in convergences. In
tropical waters it is common to see blue green algae
(Box 14.2), coral spawn, jellyfishes, spawning appendicularians and other plankters in convergence zones
generated by physical phenomena that include: tidal
and thermal fronts, tidal jets, the edge of eddies, windrows and internal waves. As a result, therefore, convergences are often sites of intense biological activity
and nekton (e.g. planktivorous fishes, piscivores) and
birds are often attracted to the abundant food in these
areas.
Oceanography can also influence the distribution
of plankton with depth. Although the water column
on the shelf of the GBR is often well mixed (see
Chapter 4), thermoclines and haloclines can stratify
the water column into different water masses. For