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Plankton
are midwater dwellers unable to swim against currents.
They don't need to, since their world is a small volume
of water around them, a little drifting world inhabited
by tiny plants - unicellular microalgae, microscopic
"herbivorous" and predatory animals - infusoria,
amoebas, various crustaceans, invertebrate and fish
larvae; their gametes and eggs; algal spores...
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Dinophycean
microalga Dinophysis caudata has large sails
to sail in water column.
This dinoflagellate
is common in spring to autumn phytoplankton in Black
Sea . It produces diarrhetic toxins which can be accumulated
by bivalve molluscs, including mussels.
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Planos - is soaring, or sailing
in Greek. They sail through the water, they struggle
not to sink, since they're not accustomed to benthic
life, they're eaten very fast if they approach bottom.
When even the unusually large Black Sea plankton animal,
jellyfish Rhizostoma pulmo nears bottom, it is
immediately gets attacked: the small crab Macropipus
holsatus able to swim with its flattened rear legs,
shoots up from the bottom to the jellyfish, settles
on it, and travels on it while eating its flesh.
Planktonic algae need to keep in the sunlit upper layer
of seawater - the photic zone, since the light
gives them life: it's a fuel for their photosynthetic
wonder-machine.
Planktonic organisms have different adaptations for
staying in midwater: most of them have a body density
close to that of water, e.g. jellyfish.
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To slow down sinking many plankters also have an increased
body surface area as related to the body volume, like small
parachutes. Planktonic algae possess various kinds of cell
surface projections: dinoflagellates of genus Dinophysis
have wide sails, diatoms of genus Chaetoceros have
long chaeta, and Dictyochophycean algae have long silicon
spicules. Another way of increasin sailing surface is the
formation of colonies by dividing cells of algae and bacteria.
Usually they build chain colonies, some species make globular
colony held together by mucous excretion of the cells like
in Chaetoceros socialis.
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Most plankters can swim. Although this mobility is
insufficient to withstand currents, it allows microscopic
prey avoid microscopic predators or in other cases allows
predator - to reach a prey.
Many planktonic algae have flagella to propel them
through the water; planktonic crustaceans row with their
legs; infusoria, larvae of molluscs, echinoderms, and
many worms have cilia to move them. Medusas project
themselves upward pushing water out of their canopies,
while ctenophores use thousands of rowing plates made
of cilia.
The locomotion provides some of the plankton an ability
to move up and down the water column; scyphozoan jellyfish,
ctenophores, and most planktonic crustaceans can regulate
their depth by active movement.
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Aurelia aurita and Mnemiopsis
leidyi - scyphozoans and ctenophores - large masses
of gelatinous plankton at South Crimean coast.
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Not all plankton is of microscopic size. Large
jellyfish and comb jelly are plankton too. For the large gelatinous
plankton it is as dangerous to move close to the sea surface
as to the bottom, because even small waves disrupt their soft
bodies. They go deeper when waves grow higher. To help regulate
the right depth of submergence, jellyfish have statocysts,
organs of equilibrium that indicate both the direction to
bottom and report animal of wave disturbance. Scyphozoan jellyfish
also possess simple eyes, informing animals of how close they
are to the sea surface.
Sometimes currents and waves drive innumerable masses of
jellyfish to the Black Sea shores. The portion of gelatinous
plankton is unusually high in the Black Sea. The biomass
of scyphozoans and ctenophores account for more than 90% of
total zooplankton biomass in coastal Black Sea waters during
the warm period of year, sometimes reaching over 1000 g m-3
of seawater (why it is so - Evolution
of the Black Sea Ecosystem).
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blenny
larva
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A few other plankters can be seen without the microscope.
They include the elongated, glassy-transparent, agile
Chetognate predator Sagitta; errant Polychaeta
which become particularly conspicuous when gathering
for spawning - most common plankton polychaeta in the
Black Sea is Plathynereis dumerilii; a semi-transparent
5mm long larva of blenny - it's already similar to an
adult fish.
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Still the great majority of plankton species, in all
their stunning diversity are creatures so small that
we just can not see them, but, they
do exist! They are in any drop of seawater we swim
in, and in any little splash of sea wave breaking ashore.
We enter the world of these small plants and animals
of Black Sea using a light microscope. To do that we
first need to concentrate plankton: we sample it with
plankton net made of fine 10 um mesh. Only the smallest
unicellular algae and bacteria are able to pass trough
it, the majority of plankton species is detained by
the mesh. The easiest way to study coastal plankton
is to filter large volume of surface, near-shore sea
water through the net, e.g. we pass 100 liters of seawater
trough the net, collecting the sample in a 0.5 L jar,
concentrating it 200 times.
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That's what we got in our sample. Unicellular microalgae,
phytoplankton, and microscopic animals grazing phytoplankton
- zooplankton: crustaceans, hydroid medusa...
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A typical
sample of late summer plankton in the Black Sea: a hydroid
medusa Sarsia sp., copepod crustacean Oithona sp.; large
Dinophycean microalgae Ceratium fusus (like
cutlasses), smaller
Prorocentrum sp., some
of which are swallowed by Sarsia
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