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14. Plankton
M. J. Kingsford, K. Heimann, C. G. Alexander & A. D. McKinnon
N PLANKTON
Coral reefs of the world are surrounded by pelagic blue
waters. Inter-reefal waters of the Great Barrier Reef
(GBR) are mostly 10 m to 60 m deep on the continental
shelf. Beyond the shelf, waters plunge to 4000 m and
more. Despite the clarity of the water column a broad
diversity of tiny organisms are found in the pelagic environment. Plankton are the ocean’s ‘wanderers’ (from the
Greek ‘planktos’) and are tiny organisms that drift or
have limited powers of locomotion. The plants of the sea
(phytoplankton, Fig. 14.1) are autotrophic and generate
energy, in the form of sugars, by photosynthesis. All
other planktonic organisms (bacteria, zooplankton etc.,
Fig. 14.2) are heterotrophs. Some plankton are ‘mesotrophic’, that is, both autotrophic and heterotrophic
(e.g. protists, derived from the Greek ‘first of all’). Phytoplankton are the basis of pelagic and benthic food chains
that are now known to be complex, and involve many
trophic transformations within microbial food webs.
Plankton provide important links with reefal and interreefal environments as a source of food to suspension
feeders (e.g. sponges, clams and corals), fishes and ultimately detrital feeders associated with the substratum
(e.g. sea cucumbers).
Plankton that spend all of their lives in the pelagic
environment are ‘holoplankton’ (Fig. 14.1) while others
are temporary visitors. Meroplankton are larval forms
of reef associated organisms such as corals, crabs, sea
urchins and fishes (Fig. 14.4). At times of the year when
animals are spawning, larvae can constitute a major
component of total plankton; this is especially the case
for corals that have ‘mass spawning’ (also worms, molluscs and echinoderms) and benthic algae that have
mass releases of spores into the water column. Larvae
range from the small and ciliated planula larvae of jellyfish and corals (about 0.5 mm across) and the transparent brachiolaria larvae of starfish, to large and essentially
nektonic (organisms that can swim and move independently of currents) presettlement reef fishes. The pelagic
phases of surgeonfish can be over 50 mm Standard
Length (SL) at settlement and tank tests have demonstrated they may swim up to 120 km without food. The
larvae of many invertebrates are often bizarre in shape
and are very large compared to many other plankters.
Spindly phyllosoma larvae of lobsters and sinister looking stomotopod larvae are up to 30 times the size of
adult copepods and are voracious predators.
Marine snow and other structures
Historically, the pelagic environment was viewed as
being occupied by a hierarchy of organisms of different
sizes, from protists to jellyfishes. This perception occurred because the majority of samples were collected
with nets or water bottles that disrupted or destroyed
delicate structures such as are found within gelatinous
zooplankton. In addition, flotsam and drifting algae
14. Plankton
M. J. Kingsford, K. Heimann, C. G. Alexander & A. D. McKinnon
N PLANKTON
Coral reefs of the world are surrounded by pelagic blue
waters. Inter-reefal waters of the Great Barrier Reef
(GBR) are mostly 10 m to 60 m deep on the continental
shelf. Beyond the shelf, waters plunge to 4000 m and
more. Despite the clarity of the water column a broad
diversity of tiny organisms are found in the pelagic environment. Plankton are the ocean’s ‘wanderers’ (from the
Greek ‘planktos’) and are tiny organisms that drift or
have limited powers of locomotion. The plants of the sea
(phytoplankton, Fig. 14.1) are autotrophic and generate
energy, in the form of sugars, by photosynthesis. All
other planktonic organisms (bacteria, zooplankton etc.,
Fig. 14.2) are heterotrophs. Some plankton are ‘mesotrophic’, that is, both autotrophic and heterotrophic
(e.g. protists, derived from the Greek ‘first of all’). Phytoplankton are the basis of pelagic and benthic food chains
that are now known to be complex, and involve many
trophic transformations within microbial food webs.
Plankton provide important links with reefal and interreefal environments as a source of food to suspension
feeders (e.g. sponges, clams and corals), fishes and ultimately detrital feeders associated with the substratum
(e.g. sea cucumbers).
Plankton that spend all of their lives in the pelagic
environment are ‘holoplankton’ (Fig. 14.1) while others
are temporary visitors. Meroplankton are larval forms
of reef associated organisms such as corals, crabs, sea
urchins and fishes (Fig. 14.4). At times of the year when
animals are spawning, larvae can constitute a major
component of total plankton; this is especially the case
for corals that have ‘mass spawning’ (also worms, molluscs and echinoderms) and benthic algae that have
mass releases of spores into the water column. Larvae
range from the small and ciliated planula larvae of jellyfish and corals (about 0.5 mm across) and the transparent brachiolaria larvae of starfish, to large and essentially
nektonic (organisms that can swim and move independently of currents) presettlement reef fishes. The pelagic
phases of surgeonfish can be over 50 mm Standard
Length (SL) at settlement and tank tests have demonstrated they may swim up to 120 km without food. The
larvae of many invertebrates are often bizarre in shape
and are very large compared to many other plankters.
Spindly phyllosoma larvae of lobsters and sinister looking stomotopod larvae are up to 30 times the size of
adult copepods and are voracious predators.
Marine snow and other structures
Historically, the pelagic environment was viewed as
being occupied by a hierarchy of organisms of different
sizes, from protists to jellyfishes. This perception occurred because the majority of samples were collected
with nets or water bottles that disrupted or destroyed
delicate structures such as are found within gelatinous
zooplankton. In addition, flotsam and drifting algae