14 – Plankton
137
whether they are harmful or harmless. In any case,
these blooms are spectacular at night, as Noctiluca
scintillans is a bioluminescent dinoflagellate (capable of
producing and emitting its own light), and causes
‘phosphorescence of the sea’. In contrast, many diatoms (i.e. Pseudo-nitzschia multiseries) and dinoflagellates (i.e. Karenia breve in Florida) produce potent toxins
and their blooms result in large fish and marine mammal mortality. Human health can be adversely affected
by consuming contaminated shellfish (shellfish filter
algae and accumulate toxins in their body tissues) or
via inhalation of organisms contained in ocean spray.
These blooms develop when environmental conditions
are perfect, that is, warm surface temperatures, stagnant and stratified water masses, low turbulence, and
high light intensities are critical factors in bloom
establishment and duration. An example of a bloom
considered of fundamental importance in nitrogen
cycling in tropical waters is produced by the cyanobacterium Trichodesmium. It fixes large amounts of atmospheric nitrogen and thus contributes significantly to
the availability of nitrogen in these otherwise oligotrophic waters (see below).
An amazing variety of dinoflagellates, centric and
pennate diatoms exist within the central GBR region
in this size class of phytoplankton (Fig. 14.9). Compared to the nanoplankton, their surface to volume
ratio is reduced due to their larger size and their
rates of cell division are slower. Also, their nutritional strategies differ from those of nanoplankton in
that these organisms can exploit nutrient patches by
storing these nutrients inside their cells (e.g.
phosphates). Thus, the presence and abundance of
microplankton may be ‘decoupled’ from nutrient
availability. Unlike the much smaller nanoplankton,
sinking out of the photic zone is possible for larger
phytoplankton. There are many means that phytoplankton use to avoid sinking. Buoyancy may be increased by the presence of gas vesicles or the storage
of light oils, surface area increased by the production
of elaborate cell surface extensions as observed in
the centric diatoms Bacteriastrum and Chaetoceros,
and mobility increased by powerful flagellation
in the dinoflagellates. Susceptibility to grazing is
size- dependent, with grazing pressure inversely
related to cell size. The evolution of spines (setae)
and the ability to form buoyant mats (Rhizosolenia) is
a valuable anti-sinking strategy and may deter some
herbivores.
Net plankton (organisms 0.1–1.00 mm)
Net plankton are often the focus of studies because they
represent a critical link between microbes and larger organisms such as fish. These organisms include a broad
range of zooplankton including nauplii and juveniles
(e.g. copepodites) as well as adult holoplankton and
many larval forms. Copepods (Subclass Copepoda)
often dominate zooplankton samples (up to 80% of
catches and 193 species on the GBR) and common genera in waters of the GBR include: small sized Parvocalanus and Bestiolina; medium-sized Acartia, Paracalanus,
and Temora (~ 0.7 mm to 1.1 mm adults), and larger
(~ 1.0 mm to 3.0 mm) Candacia, Undinula, Eucalanus, Centropages, Labidocera, and Pontella of the Order Calanoida;
small to medium-sized Oithona and Oncaea (~ 0.7 mm to
1.1 mm adults), of the Orders Cyclopoida; and Euterpina
and Microsetella of the Order Harpacticoida (Fig. 14.10).
Other important members of the Phylum Crustacea that
are found in net plankton include Penilia (Order
Ctenopoda).
Common elongate plankton include Oikopleura,
Fritillaria (larvaceans or appendicularians; eight species
on GBR) and small arrow worms (Phylum Chaetognatha, 12 species on GBR). Larvaceans feed on picoplankton, while chaetognaths are voracious predators
on meso- and megaplankton. The abundance of larval
forms varies with the timing of spawning of different
taxa, but common crustacean larvae include barnacle
nauplii (usually too small for mesoplankton nets) and
cyprids as well as crab zoea, prawns (e.g. Acetes),
stomatopod larvae (Squilla) and Lucifer. Fish larvae can
be very abundant especially after the mass release of
larvae from demersal eggs around new and full moons.
Coral spawn can be abundant in nets at times of mass
spawnings, causing plankton nets to go pink. Other
common larvae include polychaete worms and gastropod larvae. Although the cells of Trichodesmium are tiny
masses of cells they can clog mesoplankton nets.
137
whether they are harmful or harmless. In any case,
these blooms are spectacular at night, as Noctiluca
scintillans is a bioluminescent dinoflagellate (capable of
producing and emitting its own light), and causes
‘phosphorescence of the sea’. In contrast, many diatoms (i.e. Pseudo-nitzschia multiseries) and dinoflagellates (i.e. Karenia breve in Florida) produce potent toxins
and their blooms result in large fish and marine mammal mortality. Human health can be adversely affected
by consuming contaminated shellfish (shellfish filter
algae and accumulate toxins in their body tissues) or
via inhalation of organisms contained in ocean spray.
These blooms develop when environmental conditions
are perfect, that is, warm surface temperatures, stagnant and stratified water masses, low turbulence, and
high light intensities are critical factors in bloom
establishment and duration. An example of a bloom
considered of fundamental importance in nitrogen
cycling in tropical waters is produced by the cyanobacterium Trichodesmium. It fixes large amounts of atmospheric nitrogen and thus contributes significantly to
the availability of nitrogen in these otherwise oligotrophic waters (see below).
An amazing variety of dinoflagellates, centric and
pennate diatoms exist within the central GBR region
in this size class of phytoplankton (Fig. 14.9). Compared to the nanoplankton, their surface to volume
ratio is reduced due to their larger size and their
rates of cell division are slower. Also, their nutritional strategies differ from those of nanoplankton in
that these organisms can exploit nutrient patches by
storing these nutrients inside their cells (e.g.
phosphates). Thus, the presence and abundance of
microplankton may be ‘decoupled’ from nutrient
availability. Unlike the much smaller nanoplankton,
sinking out of the photic zone is possible for larger
phytoplankton. There are many means that phytoplankton use to avoid sinking. Buoyancy may be increased by the presence of gas vesicles or the storage
of light oils, surface area increased by the production
of elaborate cell surface extensions as observed in
the centric diatoms Bacteriastrum and Chaetoceros,
and mobility increased by powerful flagellation
in the dinoflagellates. Susceptibility to grazing is
size- dependent, with grazing pressure inversely
related to cell size. The evolution of spines (setae)
and the ability to form buoyant mats (Rhizosolenia) is
a valuable anti-sinking strategy and may deter some
herbivores.
Net plankton (organisms 0.1–1.00 mm)
Net plankton are often the focus of studies because they
represent a critical link between microbes and larger organisms such as fish. These organisms include a broad
range of zooplankton including nauplii and juveniles
(e.g. copepodites) as well as adult holoplankton and
many larval forms. Copepods (Subclass Copepoda)
often dominate zooplankton samples (up to 80% of
catches and 193 species on the GBR) and common genera in waters of the GBR include: small sized Parvocalanus and Bestiolina; medium-sized Acartia, Paracalanus,
and Temora (~ 0.7 mm to 1.1 mm adults), and larger
(~ 1.0 mm to 3.0 mm) Candacia, Undinula, Eucalanus, Centropages, Labidocera, and Pontella of the Order Calanoida;
small to medium-sized Oithona and Oncaea (~ 0.7 mm to
1.1 mm adults), of the Orders Cyclopoida; and Euterpina
and Microsetella of the Order Harpacticoida (Fig. 14.10).
Other important members of the Phylum Crustacea that
are found in net plankton include Penilia (Order
Ctenopoda).
Common elongate plankton include Oikopleura,
Fritillaria (larvaceans or appendicularians; eight species
on GBR) and small arrow worms (Phylum Chaetognatha, 12 species on GBR). Larvaceans feed on picoplankton, while chaetognaths are voracious predators
on meso- and megaplankton. The abundance of larval
forms varies with the timing of spawning of different
taxa, but common crustacean larvae include barnacle
nauplii (usually too small for mesoplankton nets) and
cyprids as well as crab zoea, prawns (e.g. Acetes),
stomatopod larvae (Squilla) and Lucifer. Fish larvae can
be very abundant especially after the mass release of
larvae from demersal eggs around new and full moons.
Coral spawn can be abundant in nets at times of mass
spawnings, causing plankton nets to go pink. Other
common larvae include polychaete worms and gastropod larvae. Although the cells of Trichodesmium are tiny
masses of cells they can clog mesoplankton nets.