VOLUM OMAGIAL - Facultatea de Ştiinţe ale Naturii şi Ştiinţe Agricole

Laura Boicenco / Ovidius University Annals, Biology-Ecology Series 14: 163-169
Nitzschia’ species (N. tenuirostris and N.
delicatissima) had occurred and had low densities.
Apart from the N. tenuirostris’ single bloom
produced in July 2006 in Mamaia Bay (15.5∙10 6
cells∙l -1 ), the two species had densities higher than
10 6 cells∙l -1 only in five and six years, respectively.
N. tenuirostris started to vegetate intensely after
1981, and reached its amplest bloom in the summer
of 1989 - 74.8·10 6 cells·l -1 [6].
Dinoflagellates
With a long-term mean of 68.5∙10 3 cells∙l -1 ,
the dinoflagellates comprised small percentages of
the total phytoplankton, with a maximum of 17%
in 2007; the highest mean density was almost
225.6·10 3 cells·l -1 in 1997. However, during two
springs (1998 and 2007) the populations of
dinoflagellates were denser, with a density mean of
455.9·10 3 cells·l -1 . Between 1996 and 2007 a few
species had concentrations higher than 10 millions
cells per liter (Table 4) in different areas and years.
Table 4. The highest densities produced by
dinoflagellates (10 6 cells∙l -1 )
1996- 2001-
Species
2000 2007
Scrippsiella trochoidea 0.3 25.3
Heterocapsa triquetra 13.6 16.0
Gymnodinium cf. aureolum - 10.7
Prorocentrum minimum 10.5 9.0
Mass growth of the Prorocentrum minimum,
causing the water to turn red, was recorded for the
first time in the summer of 1974 along the
Romanian littoral; the phenomenon was repeated in
summers 1975 and 1976. Prorocentrum was the
first dinoflagellate species reacting to the sudden
decrease in salinity (monthly average reached 13
PSU, at Constanta) and huge increase in the
concentrations of phosphates and nitrates (18 and
11 times respectively higher than the period 1959-
1960). Presence of such extraordinary blooms had
never been noticed before: 181.5 (1974), 78.7
(1975) and 111.6∙10 6 cells∙l -1 (1976), in the
southern coastal waters, from Navodari to
Mangalia [8]. During the following decades, when
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eutrophication got stronger and stronger, up to a
climax from 1981-1990, the species attained even
more prodigious proliferations, up to the value of
807.6∙10 6 cells∙l -1 in July 1987. In fact, no other
species would ever attain such densities as the
Prorocentrum between 1971 and 1990. In the
following years, the amplitude of Prorocentrum’s
blooms decreased, but in July 1995 it reached a
density of 93.7∙10 6 cells∙l -1 (Fig. 5), 8.6 times lower
than its overwhelming density in July 1987 [1].
Fig. 5. Long-term evolution of P.minimum blooms.
Beside the diatom Skeletonema, P.minimum is
the second most common species in the whole Pontic
basin giving some of the highest blooms, especially in
the NW sector. In our study period, P. minimum
continued to have massive developments, but much
lower than the previous ones: in June 1999 –10.4∙10 6
cells∙l -1 and July 2001 – 8.93∙10 6 cells∙l -1 , both of them
in Mamaia Bay. Here, up to 2001, during warm
months, the species’ populations frequently exceeded
1 million cells per litre; then, the densities were lower
and lower, sometimes disappearing from samples.
Three other dinoflagellates reached
concentrations higher than 10∙10 6 cells∙l -1 , namely
Heterocapsa triquetra, Scrippsiella trochoidea and
Gymnodinium cf. aureolum (Table 2). After
developments, reaching a few or ten thousands cells
per litre in the 1970s, H. triquetra and S. trochoidea
came to the list of the bloom-forming species, the first
with a value of 97.6 ∙10 6 cells∙l -1 in the period 1971-
1980, and the second one with a value of 25.8 ∙10 6
cells∙l -1 in the period 1981-1990. After a period (1991-
1996) of insignificant concentrations (highest value of
1.9 ∙10 6 cells∙l -1 ) [1], Heterocapsa again reached high
concentrations: 13.6∙10 6 cells∙l -1 , in May 1998 (at Mila
9) and 10.3∙10 6 cells∙l -1 , in April 2000 (in Mamaia
Bay) (Fig. 6). All along Romanian littoral, but
especially in the northern sector, Heterocapsa
produced substantial densities, ranging from 2.0 to