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International Journal of Marine Science 2013, Vol.3, No.15, 121-127
http://ijms.sophiapublisher.com
125
maximums of phytoplankton biomass, ranging
between 80 and 170 mg С/m
3
, were recorded in June
2006 and 2007 and in October 2006. In these periods
125%~175% of the daily phytoplankton production
was removed by the microzooplankton grazing.
Interestingly, that in the beginning of blooms in
September 2006 and in May 2007, when
phytoplankton biomass was rising after a minimum
and the total abundance
Skeletonema costatum
and
Chaetoceros socialis
was 2~3×10
6
cells/L, the
phytoplankton mortality due to microzooplankton
predation did not exceed 50%~55%.
Compared to 2006~2007, over most part of 2010 the
phytoplankton biomass was noticeably less. In the
Sevastopol bay, the largest estimates in 150 and 300 mg
C/m
3
, were registered in February and in July,
correspondingly (Figure 2). In February, the dominant
taxa were the diatom
S. costatum
and in July, two small
dinoflagellates,
Gymnodinium simplex
and
P. cordatum
,
accounted up to 87% of the total phytoplankton biomass.
The loss of primary production due to microzooplankton
grazing during these two periods was assessed 33% and
76%, correspondingly. In February, June and September
the microalgal biomass in the Quarantine bay was
estimated as high as 120 mg C/m
3
, and the g/µ ratio was
50%~60%. In July, in the near-shore area adjoining the
Kruglaya bay, phytoplankton biomass increased
maximally to 100 mg C/m
3
; simultaneously, the g/µ ratio
was 80%. According to the year-round observations from
two sampling stations near village Katsiveli, the portion
of phytoplankton consumed by microzooplankton, or g/µ
ratio, was mainly below 100% (Figure 3). Frequent
upwelling – downwelling events induced by wind
prevent accumulation of phytoplankton biomass in this
region of the sea (Subbotin et al., 2007).
Based on the records for 2006~2007 and 2010, in the
examined locations of the Black Sea the specific rate
of phytoplankton mortality due to microzooplankton
grazing was evaluated, on the average, as 59% of the
primary production for the warm season and 70% for
the cold season, making the average of 65% for the
complete observation period.
At the periods of phytoplankton biomass maximums
the g/µ ratio mainly varied from 33% to 80% but
sometimes it was estimated zero; therefore the average
of 50%.
2 Discussion
The study conducted in the coastal seawater areas of
the Black Sea has shown that seasonal variability of
the specific grazing rate of the microzooplankton,
absolute values of which were comparable to the
estimates of the specific growth rate of the
phytoplankton, varied very broadly (Stelmakh et al.,
2009). The amount and quality of prey phytoplankton
have been the main factors which regulated the micro-
zooplankton predatory pressure on phytoplankton.
Quality, or prey selectivity, has been the principal
factor where the abundance of prey phytoplankton
approximated a saturation point. For instance, in
October 2006, when phytoplankton biomass in the
Sevastopol bay was as large as 350 mg С/m
3
, the
microzooplankton grazing impact was negligible. The
phytoplankton biomass then was predominantly due to
two large-celled species, the diatom
Pseudosolenia
calcar-avis
and the dinoflagellate
Ceratium furca
. As a
prey item, the former is not in favor with the
microzooplankton (Stelmakh et al., 2009), and the
latter episodically provides a prey to only a few
heterotrophic dinoflagellates, e.g.,
Protoperidinium
steinii
(Olseng et al., 2002). It was reported earlier
(Stelmakh et al., 2009) that in September~October
2005, during autumn diatom blooms in the western
Black Sea, as the portion of
Pseudosolenia calcar-avis
increased in the total phytoplankton biomass, the
specific rate of phytoplankton mortality due to
microzooplankton grazing gradually decreased.
Emiliania huxleyi
is also a prey of minor interest to
microzooplankton; in October~November 2010, the
observations related to the autumn bloom of this
species of coccolithophorids in the western Black Sea
have shown that in the localities where
E. huxleyi
had
the abundance as high as 1.5~3 million cell/L the
consumption of phytoplankton by microzooplankton
often ceased (Stelmakh and Babich, 2011).
The phytoplankton consumed by the micro-
zooplankton varies from long-chain diatoms (Sherr
and Sherr, 2007) to large dinoflagellates (Olseng et al.,
2002). According to our observations, the mortality of
phytoplankton due to microzooplankton grazing was
usually largest during the periods when diatoms of
Chaetoceros
spp. dominated in the plankton, therefore
a deduction about a favorite prey item for the
microzooplankton. Some proofs to this can be found
elsewhere (Shinada et al., 2000).