3D SPATIO-TEMPORAL VERTICAL DISTRIBUTION OF ZOOPLANKTON
AS ACOUSTICALLY INFERRED IN THE TURKISH SEAS
Erhan Mutlu
Institute of Marine Sciences, Middle East Technical University, Erdemli, Turkey -mutlu@ims.metu.edu.tr
Abstract
Sagitta setosaand Calanus euxinusshowed spatio-temporal distribution in the Black Sea thereafter they were acoustically identified.
C.euxinusoverwintered from March to May, August. S. setosawere completely adult during January-June whereas their juveniles
predominated in warm seasons. Their generation’s time was July and September. Zooplankton community was restricted in upper water
above the interface in Marmara Sea. Aurelia auritaaggregated just above the interface (evening), were dispersed towards the surface
(midnight) to form community during the daylight. The Mediterranean zooplankton was composed of two scattering layers: one migrated
between surface and thermocline and other below thermocline.
Keywords: Zooplankton, distribution, bioacoustics, Black Sea
Rapp. Comm. int. Mer Médit., 37,2004
407
About 150 zooplankton species are reported for the Black Sea,
including numerous brackish-water and freshwater organisms
restricted to coastal areas. Only about half of the Black Sea species
occur in the Mediterranean. All taxonomic groups of planktonic
animals are presented in the Eastern Mediterranean (EMED), but few
have penetrated into in the Black Sea. This seems to be mainly due to
differences in salinity between the two basins. Typical stenohaline
organisms such as radiolarians, siphonophores, pteropods, and salps,
common in the Mediterranean, do not occur in the Black Sea.
Abundant groups, such as copepods, chaetognaths, and medusae, are
present in much reduced numbers in the EMED. Appendicularia,
Chaetognatha, Cladocera, Copepoda, Polychaeta, Gastropoda and
Bivalvia larvae, Scyphozoa, and Ctenophora were found in the Sea of
Marmara (1-6).
Monthly acoustical data from different years were collected with a
scientific echosounder and Acoustic-Doppler-Current-Profiler.
Sagitta setosawas acoustically identified by looking at their diel
migratory pattern in different months in the Black Sea. In cold-water
season when population consisted of adults, they concentration layer
coexisted with Calanus euxinusin Oxygen-Minimum-Zone (OMZ).
In warm-water season when juveniles comprised more 60% of the
population, they stayed in the oxycline. In July and September,
individuals of new generation did not migrate and stayed in
subsurface water. S. setosaspeeded up in well-oxygenated subsurface
water. S. setosacompleted migration within 4 hrs at 0.38 cm s
-1
(7).
C. euxinuswere acoustically discriminated with respect to vertical
migration and swimming speed, according to dissolved oxygen (DO)
concentration and timing of migrations. Species became torpid in
water with DO values <0.5 mg l
–1
. Time spent swimming under DO
conditions between 2 and 5 mg l
–1
was insignificant, and varied from
the 10% to 25% of total time spent swimming (5 h) under normoxic
conditions (5–10 mg l
–1
). C. euxinusformed a concentration layer in
the water of 1–3 m thickness. Upward migration was completed in
about 3.5 h, starting 2.5 h before and ending 1 h after sunset (average
rate: 0.95 cm s
–1
) in summer. Species ascended discretely from the
suboxic to the lower boundary of cold intermediate layer (CIL) at
0.82cm s
–1
, and passed up the CIL and thermocline fast (2.3 cm s
–1
).
Downward migration took less time (2 h), starting ~1 h before and
ending ~1 h after sunrise. Swimming speed within the thermocline
and CIL was 2.7 cm s
–1
; copepods subsequently returned to daylight
depth at 0.57 cm s
–1
(8).
The noise was 4 dB higher in the Sea of Marmara than in the other
two Seas. This could have changed the “hardness” of the interface due
to the biological variations as Korneliussen (9) suggested that bottom
variations. Hardness of the interface could be associated with density
of jellyfish, Aurelia aurita. Their swimming rhythms showed that they
could be jellyfish (10). A variation of about 10-15 dB and occurrence
of a peak every 25-30 s due to swimming of the jellyfish were
observed in individual scatterers rising from the interface. 200 kHz
data showed significant correlation between measured and calculated
volume backscattering (Sv), and density of the taxa. Large-sized
copepods and chaetognaths in the Black Sea, Aurelia, Beroeand
chaetognaths and large sized and abundant appedicularians in Sea of
Marmara and fish larvae in the Mediterranean Sea contributed most to
the Sv. Biological scattering was vertically distributed between
surface and suboxic zone in the Black Sea, whereas it was confined
between surface and interface formed between waters of the Black
Sea and Mediterranean Sea in the Sea of Marmara. The Mediterranean
Sea was very different in terms of the volume backscattering due to
absence of shallow interface. Acoustical scattering was layered in and
above the suboxic zone during the daytime in offshore waters of the
Black Sea, while it was aggregated in the mixed layer at night hours.
As the bottom depth was shoaled, the volume backscattering strength
became homogenous. In the Sea of Marmara, the scattering was much
intensified and layered just above the interface during the daytime,
whereas it was homogenously distributed within the mixed water
characterized with the Black Sea above the interface. During daytime,
the layer between interface and transducer depth was deserted by the
plankton. Moderately high scattering was observed in the upper 100
m in the Mediterranean Sea at night, while the scattering observed in
the upper 60 m layer during the day was less intense since vertical
migratory species deserted the upper layers.
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