92
pulations showed a single abundance maximum (0.4-0.6 mg ChI a m- 3 ) at
the beginning of March mainly due to diatoms, in particular AsterioneZZa
gZaaiaZis, ThaZassiothrix frauenfeZdii, RhizosoZenia aZata f.graaiZZima,
Nitzsahia aZosterium and several species of Chaetoaeros (c. affine, C.
aurvisetum, C. deaipiens, C. breve). An increased ve~tical stability of
the water column, a depletion of nutrients and a presumed intense grazing activity of zooplankton after the spring diatom maximum resulted in
-3
lower phytoplankton biomass levels (about 0.2 mg ChI am) as well as
a change in species composition. During this period the bulk of the population consisted of ama.ll flagellates «10 ~) and naked dinoflagellates. The annual minimum (0.01-0.15 mg ChI a m- 3 ) occurred in summer and
characterized aless diversified phytoplankton community dominated by
small flagellates. Coccolithophorids and dinoflagellates were also consistently present during this period, each of which comprised about 20%
of the total population. EmiZiania huxZeyi, CaZaidisaus Zeptoporus, Proroaentrum baZtiaum and Oxytoxum variabiZe were frequently recorded.
.
-3
Relatively stable values of chlorophyll a (0.1-0.3 mg m ) were recorded
during the autumn-winter period. Phytoplankton populations primarily
consisted of coccolithophorids (EmiZiania huxZeyi, Anthosphaera quadriaornu, Syraaosphaera puZahra, UmbeZZosphaera tenuis, ecc.) and various
diatoms. The latter group increased in abundance during winter when
water column was well mixed.
Notwithstanding the low phytoplankton standing crops of these regions, the coccolithophorids were consistently abundant (up to 60%)
throughout the year. In particular, the ubiquitous, opportunistic species EmiZiania huxZeyi was often dominant presumably due to its relatively high growth rate under a wide range of temperature-light conditions.
In winter the depth of the euphotic zone (about 2 ~E m- 2 sec- 1 ) was
70-80 m which extended to more than 100 m (about 10 ~E m- 2 sec- 1 ) in summer, suggesting that viable phytoplankton populations could occur in
deep waters. This phenomenon, already recorded in the Western Mediterranean (Cahet et al., 1972; Estrada, 1981; Velasquez, 1981), was often
observed in offshore waters of both Gulfs. Chlorophyll a and oxygen
maxima were recorded below the thermo~~inein late su~mer and in autumn
(fig. 2). Further studies are in progress to evaluate the contribution
pulations showed a single abundance maximum (0.4-0.6 mg ChI a m- 3 ) at
the beginning of March mainly due to diatoms, in particular AsterioneZZa
gZaaiaZis, ThaZassiothrix frauenfeZdii, RhizosoZenia aZata f.graaiZZima,
Nitzsahia aZosterium and several species of Chaetoaeros (c. affine, C.
aurvisetum, C. deaipiens, C. breve). An increased ve~tical stability of
the water column, a depletion of nutrients and a presumed intense grazing activity of zooplankton after the spring diatom maximum resulted in
-3
lower phytoplankton biomass levels (about 0.2 mg ChI am) as well as
a change in species composition. During this period the bulk of the population consisted of ama.ll flagellates «10 ~) and naked dinoflagellates. The annual minimum (0.01-0.15 mg ChI a m- 3 ) occurred in summer and
characterized aless diversified phytoplankton community dominated by
small flagellates. Coccolithophorids and dinoflagellates were also consistently present during this period, each of which comprised about 20%
of the total population. EmiZiania huxZeyi, CaZaidisaus Zeptoporus, Proroaentrum baZtiaum and Oxytoxum variabiZe were frequently recorded.
.
-3
Relatively stable values of chlorophyll a (0.1-0.3 mg m ) were recorded
during the autumn-winter period. Phytoplankton populations primarily
consisted of coccolithophorids (EmiZiania huxZeyi, Anthosphaera quadriaornu, Syraaosphaera puZahra, UmbeZZosphaera tenuis, ecc.) and various
diatoms. The latter group increased in abundance during winter when
water column was well mixed.
Notwithstanding the low phytoplankton standing crops of these regions, the coccolithophorids were consistently abundant (up to 60%)
throughout the year. In particular, the ubiquitous, opportunistic species EmiZiania huxZeyi was often dominant presumably due to its relatively high growth rate under a wide range of temperature-light conditions.
In winter the depth of the euphotic zone (about 2 ~E m- 2 sec- 1 ) was
70-80 m which extended to more than 100 m (about 10 ~E m- 2 sec- 1 ) in summer, suggesting that viable phytoplankton populations could occur in
deep waters. This phenomenon, already recorded in the Western Mediterranean (Cahet et al., 1972; Estrada, 1981; Velasquez, 1981), was often
observed in offshore waters of both Gulfs. Chlorophyll a and oxygen
maxima were recorded below the thermo~~inein late su~mer and in autumn
(fig. 2). Further studies are in progress to evaluate the contribution