62
dominated by diatoms and Phaeocystis sp., could be observed
(Goldman et al. 2015).
Within the Eastern English Channel, diatoms,
Chrysophyceae, Raphidophyceae and Prymnesiophyceae
contribute most to carbon biomass. 40 species of diatoms,
and two species for Chrysophyceae and Raphidophyceae can
be found, respectively. A yearly occurring Phaeocystis sp.
spring bloom represents the group Prymnesiophyceae.
During summer, mostly large diatoms (>100 μm) dominated
the community, whereas during the rest of the year mostly
small cells could be found. Furthermore, Cryptophyceae
(seven genera) could be found in early spring and autumn,
Dinophyceae (26 genera or species) were found with the
highest abundance in summer as well as Chlorophyceae and
Prasinophyceae (Breton et al. 2000).
Not et al. (2004) found that in the eukaryotic picoplankton the Prasinophyceae Micromonas pusilla was the dominating species in the Western English Channel. In contrast to
bigger size classes, picoplankton shows a high abundance
throughout the year. The microphytoplankton bloom was
dominated by a few diatom species like Guinardia delicatula, Chaetoceros socialis, Pseudo-nitzschia spp. and
Thalassiosira spp. during late spring and had maximum
abundances during late summer (Ward et al. 2011).
Long-term data from Helgoland in the German Bight suggested interactions of different environmental conditions
with phytoplankton seasonality. Increase in sunshine hours
correlates with increasing Secchi depths (measure of water
transparency) and water temperature. Less turbulence in the
water body leads to increasing Secchi depths. Higher temperatures improve growth rates of phytoplankton, but cause
lower abundances in early spring. Increased river discharge
causes a decrease in salinity in spring, which negatively correlates with Secchi depth. Increasing Secchi depth and thus a
bigger euphotic zone benefits the growth of phytoplankton.
Concentrations of nutrients such as nitrate, phosphate, and
silicate decline rapidly during spring, when the phytoplankton bloom starts and stay at low levels until autumn, when
another phytoplankton bloom occurs. Depletion of nutrients
causes inhibition of phytoplankton growth. In autumn and
winter, new nutrients are released, causing concentrations to
increase again. High zooplankton abundances cause belated
phytoplankton blooms during spring. Higher grazing pressure during winter decreases phytoplankton abundances,
which then need a longer recovery time (Wiltshire et al.
2015). The phytoplankton community is dominated by diatoms in spring and early summer according to daily counts
(Wiltshire et al. 2008). Dinoflagellate abundance rose from
spring and reached maximum values during summer, where
Noctiluca scintillans, Gyrodinium spp., and Protoperidinium
spp. dominated. Mixotrophic dinoflagellates occurred in
lower abundances than heterotrophs, which correlate with
phytoplankton availability. However, during summer 2007, a
bloom could be observed, in which several dinoflagellates
such as Lepidodinium chlorophorum, Scrippsiella/
Pentapharsodinium spp., and Akashiwo sanguinea occurred
(Löder et al. 2012). Cryptophytes could be found throughout
the year with decline during diatom dominated times (Metfies
et al. 2010).
As a sub-tropical region, the estuaries of the Gulf of
Mexico are representing a warmer temperate region with
long periods of warm temperatures as well as tropical storms
(Georgiou et al. 2005; D’sa et al. 2011; Turner et al. 2017).
In a study in the Pensacola Bay (Florida, USA) from 1999 to
2001, Murrell and Lores (2004) investigated the role of cyanobacteria on the seasonal dynamics. The three most abundant taxa were belonging to diatoms (Thalassiosira sp.,
Pennales, and Cyclotella sp.), and diatoms represented over
50% of total abundance of phytoplankton counts. During
December and January, dinoflagellates had high abundances
(Prorocentrum minimum, Gymnodinium sp.), whereas high
abundances of chlorophytes and cryptophytes were found
during the spring and summer months. Cyanobacteria
showed a strong correlation with high water temperatures
and had highest abundances in summer. Further characterization indicated that the cyanobacteria belonged to the
Synechococcus genus. In total, cyanobacteria made up of a
large percentage of total chlorophyll (on average 43%) and
dominated the chlorophyll biomass during their summer
peak (Murrell and Lores 2004).
Another study showing similar results was conducted by
Dorado et al. (2015) in Galveston Bay (Texas, USA) from
February 2008 to December 2009. North of Galveston Bay
high phytoplankton biomass could be observed, with diatoms
being the dominating phytoplankton group, followed by
dinoflagellates, cryptophytes, and green algae. In comparison, the phytoplankton biomass was lower in the southern
part of the bay and dominated by cyanobacteria. Cyanobacteria
and green algae correlated inter alia to temperature and chlorophyll a. Results of a multivariate analysis also showed that
dinoflagellates and cyanobacteria are more abundant in areas
where vertical mixing is limited (mid- and lower region of the
bay). Seasonal patterns showed that diatoms, dinoflagellates,
and cryptophyte abundances were highest during winter and
spring, whereas cyanobacteria were most abundant in summer. It was found that high freshwater discharge correlated
with diatom growth, indicating that a decrease of freshwater
is accompanied with lower nutrient concentrations. These
conditions coupled with the temperature changes are then
more favorable for cyanobacteria growth.
L. Käse and J. K. Geuer
dominated by diatoms and Phaeocystis sp., could be observed
(Goldman et al. 2015).
Within the Eastern English Channel, diatoms,
Chrysophyceae, Raphidophyceae and Prymnesiophyceae
contribute most to carbon biomass. 40 species of diatoms,
and two species for Chrysophyceae and Raphidophyceae can
be found, respectively. A yearly occurring Phaeocystis sp.
spring bloom represents the group Prymnesiophyceae.
During summer, mostly large diatoms (>100 μm) dominated
the community, whereas during the rest of the year mostly
small cells could be found. Furthermore, Cryptophyceae
(seven genera) could be found in early spring and autumn,
Dinophyceae (26 genera or species) were found with the
highest abundance in summer as well as Chlorophyceae and
Prasinophyceae (Breton et al. 2000).
Not et al. (2004) found that in the eukaryotic picoplankton the Prasinophyceae Micromonas pusilla was the dominating species in the Western English Channel. In contrast to
bigger size classes, picoplankton shows a high abundance
throughout the year. The microphytoplankton bloom was
dominated by a few diatom species like Guinardia delicatula, Chaetoceros socialis, Pseudo-nitzschia spp. and
Thalassiosira spp. during late spring and had maximum
abundances during late summer (Ward et al. 2011).
Long-term data from Helgoland in the German Bight suggested interactions of different environmental conditions
with phytoplankton seasonality. Increase in sunshine hours
correlates with increasing Secchi depths (measure of water
transparency) and water temperature. Less turbulence in the
water body leads to increasing Secchi depths. Higher temperatures improve growth rates of phytoplankton, but cause
lower abundances in early spring. Increased river discharge
causes a decrease in salinity in spring, which negatively correlates with Secchi depth. Increasing Secchi depth and thus a
bigger euphotic zone benefits the growth of phytoplankton.
Concentrations of nutrients such as nitrate, phosphate, and
silicate decline rapidly during spring, when the phytoplankton bloom starts and stay at low levels until autumn, when
another phytoplankton bloom occurs. Depletion of nutrients
causes inhibition of phytoplankton growth. In autumn and
winter, new nutrients are released, causing concentrations to
increase again. High zooplankton abundances cause belated
phytoplankton blooms during spring. Higher grazing pressure during winter decreases phytoplankton abundances,
which then need a longer recovery time (Wiltshire et al.
2015). The phytoplankton community is dominated by diatoms in spring and early summer according to daily counts
(Wiltshire et al. 2008). Dinoflagellate abundance rose from
spring and reached maximum values during summer, where
Noctiluca scintillans, Gyrodinium spp., and Protoperidinium
spp. dominated. Mixotrophic dinoflagellates occurred in
lower abundances than heterotrophs, which correlate with
phytoplankton availability. However, during summer 2007, a
bloom could be observed, in which several dinoflagellates
such as Lepidodinium chlorophorum, Scrippsiella/
Pentapharsodinium spp., and Akashiwo sanguinea occurred
(Löder et al. 2012). Cryptophytes could be found throughout
the year with decline during diatom dominated times (Metfies
et al. 2010).
As a sub-tropical region, the estuaries of the Gulf of
Mexico are representing a warmer temperate region with
long periods of warm temperatures as well as tropical storms
(Georgiou et al. 2005; D’sa et al. 2011; Turner et al. 2017).
In a study in the Pensacola Bay (Florida, USA) from 1999 to
2001, Murrell and Lores (2004) investigated the role of cyanobacteria on the seasonal dynamics. The three most abundant taxa were belonging to diatoms (Thalassiosira sp.,
Pennales, and Cyclotella sp.), and diatoms represented over
50% of total abundance of phytoplankton counts. During
December and January, dinoflagellates had high abundances
(Prorocentrum minimum, Gymnodinium sp.), whereas high
abundances of chlorophytes and cryptophytes were found
during the spring and summer months. Cyanobacteria
showed a strong correlation with high water temperatures
and had highest abundances in summer. Further characterization indicated that the cyanobacteria belonged to the
Synechococcus genus. In total, cyanobacteria made up of a
large percentage of total chlorophyll (on average 43%) and
dominated the chlorophyll biomass during their summer
peak (Murrell and Lores 2004).
Another study showing similar results was conducted by
Dorado et al. (2015) in Galveston Bay (Texas, USA) from
February 2008 to December 2009. North of Galveston Bay
high phytoplankton biomass could be observed, with diatoms
being the dominating phytoplankton group, followed by
dinoflagellates, cryptophytes, and green algae. In comparison, the phytoplankton biomass was lower in the southern
part of the bay and dominated by cyanobacteria. Cyanobacteria
and green algae correlated inter alia to temperature and chlorophyll a. Results of a multivariate analysis also showed that
dinoflagellates and cyanobacteria are more abundant in areas
where vertical mixing is limited (mid- and lower region of the
bay). Seasonal patterns showed that diatoms, dinoflagellates,
and cryptophyte abundances were highest during winter and
spring, whereas cyanobacteria were most abundant in summer. It was found that high freshwater discharge correlated
with diatom growth, indicating that a decrease of freshwater
is accompanied with lower nutrient concentrations. These
conditions coupled with the temperature changes are then
more favorable for cyanobacteria growth.
L. Käse and J. K. Geuer