EPIPELAGIC MESOZOOPLANKTON AND COPEPOD GRAZING ALONG AN EAST-WEST TRANSECT
IN THE MEDITERRANEAN SEA
Ioanna Siokou-Frangou
Hellenic Centre for Marine Research, Anavyssos, Greece
Abstract
An increasing east-west trend of mesozooplankton biomass and abundance was detected in the Mediterranean Sea during June 1999.
However, species composition did not show any significant differences. Samples collected from cyclonic gyres were distinguished for their
increased biomass, abundance and grazing impact on autotrophic production.
Key-words: mesozooplankton, grazing, Mediterranean
Rapp. Comm. int. Mer Médit., 37,2004
439
Introduction
A eastwards gradient of increasing oligotrophy is well established
in the Mediterranean Sea (1). Mesozooplankton abundance, biomass,
species composition and copepod grazing on autotrophs were studied
in the epipelagial during trans- Mediterranean cruise (EU-project
MATER).
Methods
Mesozooplankton samples were collected during June 1999 at 9
stations along an east-west transect in the epipelagial (to depth of
100m) by a WP-2 net (200
µ
m). Samples were collected at midday and
midnight for the estimation of copepod grazing. The gut ?uorescence
of two size fractions (<500
µ
m and >500
µ
m) of copepods was
measured and ingestion rates were estimated (2).
Results and discussion
Total zooplankton abundance values increased from east (M1=93
ind m
-3
)to west (M9=898 ind m
-3
, Figure 1). Biomass values (dry
weight) presented the same trend, varying from 1.09 mg m
-3
(M1) to
4.16 mg m
-3
(M9). Samples collected within the Cretan (M4) and
Rhodos (M2) cyclonic gyres were characterized by high
mesozooplankton abundance and biomass compared with other
samples in the eastern Mediterranean Sea; increased abundance
values were observed during previous studies in the Rhodos gyre (3,
4). Copepods were the dominant group, representing 56% (M8) to
91% (M9) of the zooplanktonic community. Clausocalanus
copepodites, Farranula rostrata, Oithona copepodites, Oncaea
media, Oithona plumifera, Mecynocera clausi, Calocalanusspp.,
Corycaeusspp., Oithona setigerawere the most abundant taxa over
all stations. Some species exhibited differentiated distribution
patterns: F. rostrataand Corycaeusspp. presented an east-west
decreasing trend; Oithona copepodites, O. setigeraand M. clausi
were more abundant in the central part. Clausocalanus pergens
dominated station M9, a fact probably related to the lower
temperature compared to the eastern basin (Theocharis, pers.comm.).
Similar zooplankton abundance and species composition were found
in the Ionian Sea in April-May 1999 by Mazzocchi et al.(5) The
abundance of appendicularians and of Oncaea spp was significant at
stations M2 and M4, suggesting feeding relationships among these
taxa (5). Station M8 was characterized by the significant presence of
chaetognaths, siphonophors and the low relative abundance of
copepods, implying a grazing control by the former taxa. The
copepods’gut pigment concentrations varied between 0.042 ng chl-
a
cop
-1
(<500
µ
m at M7-day) and 1.39 ng chl-
a
cop
-1
(>500
µ
m at M8-
night). Ingestion rates were calculated to be 1.8-35.9 ng chl-
a
cop
-1
d
-1
.
Although primary production values (measured during the same
cruise) have revealed an increasing east-west trend (O.Gotsis-
Skretas and K.Pagou, pers.comm.), the potential grazing pressure of
copepods over the >1.2
µ
m fraction of primary production did not
reveal any clear trend. The estimated consumed portion of the
autotrophic production was very low at stations M6 and M7 (less than
10%) and high at station M4 (35%). This strong grazing impact is
related both to high ingestion rates and to high copepod abundance
relatively to the available primary production. It is worth noting that
the abundance of ciliates at stations M2 and M4 was lower than that
of the neighbouring stations (6) probably due to a top-down control by
copepods.
Our results confirm that the mesozooplankton abundance and
biomass differ among the basins of the Meditterranean Sea. The
hydrological features seem to play an important role for the pelagic
food web within the very oligotrophic eastern basin.
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Fig. 1. Mesozooplankton abundance distribution in the 0-100 m layer
during June 1999.
