10 Seasonal and Spatial Nutrient Dynamics in Saronikos Gulf: The Impact of Sewage Effluents …
115
10.3 Characteristics of the Inner Saronikos
Gulf
10.3.1 Field, Laboratory Analysis and Data
Assessment
Data were obtained from four to 12 cruises per year from
1987 to 2010 (total 170 cruises), at Saronikos Gulf, using
small boats and the research vessels Aegaeo and Filia of the
Hellenic Center for Marine Research (HCMR). Five sampling stations in the inner Saronikos Gulf were selected: station S7 at the outfall of the Psittalia sewage pipe; station S8
located 6.81 km southwest of Psittalia Island; station S13
located 15.3 km southwest of Psittalia Island; station S11
located 7.44 km southeast of Psittalia Island; and S16 located 18.1 km southeast of Psittalia Island; Fig. 10.2). At
the selected stations seawater samples were collected from
surface, 10 m, 20 m, 50 m, 75 m, and near bottom with Niskin bottles, either mounted on a rosette or individually on
a hydrowire.
Temperature, salinity, and density in the water column
were measured with a CTD profiler (Sea Bird Electronics),
which was equipped with pressure, temperature, and conductivity sensors. Before the 1990s measurements were performed with reversing thermometers attached to NIO bottles,
whereas, the salinity was determined with an auto lab inductive salinometer.
For oxygen content, samples were taken in sampling
bottles with precautionary measures to prevent any biological activity and gas exchanges with the atmosphere (Riley
1975). Samples were analyzed immediately after the collection on board following Winkler method, according to Carpenter (1965a, b).
Water samples for nutrient analyses were collected in
100 mL polyethylene bottles aged with 10 % HCl and kept
deep-frozen (−20 °C) until their analysis for nitrate, nitrite,
silicate, dissolved organic nitrogen, and dissolved organic P
(DOP) in the laboratory. Filtered samples from 1987 to1995
analyzed with a TECHNIKON CSM-6 Autoanalyzer, from
between 1996 until 1999 with an ALPKEM autoanalyzer
and after 1999 to present with a BRAN + LUEBBE II autoanalyzer according to standard methods (Mullin and Riley
1955 for silicate; Strickland and Parsons 1977 for nitratenitrite and Murphy and Riley 1962 for SRP).
Ammonium and SRP were measured using Hitachi and
Perkin Elmer 20 Lambda and 25 Lambda spectrophotometer
(Koroleff 1970 for ammonium and Murphy and Riley 1962
for SRP). ANOVA was used to compare the nutrient concentrations among seasons and sampling stations using SPSS
(version 17) statistical program. For rest of the data collected
Ocean Data View (ODV) program was used (Schlitzer 2011).
Thematic maps indicating an eutrophication index were
produced on the basis of application of interpolation for each
parameter for three different periods year 1987–1994, 1995–
2004, and 2005–2010. The interpolation method used in this
work was Spline with Barriers and a pixel size of 120 m. The
analysis was performed in ArcGIS/ArcINFO environment.
10.3.2 Observations on Ecosystem Dynamics
According to Kontoyiannis (2010), the circulation of the Saronikos Gulf has a distinct two-layer structure in the period
from late spring to late fall, whereas it is barotropic during
the rest of the year (December–March). The treated effluent plume form Psittalia is highly buoyant owing to its large
content of freshwater, and it rapidly ascends toward the sea
surface. During the stratification period (May–November),
the effluent plume is trapped within the seasonal pycnocline whereas during the mixing period (December–April) it
reaches the sea surface. Throughout the year, the spreading
of the plume is governed by advection caused by the prevailing circulation pattern at the particular layer where the
plume floats. Winter temperatures fall to near 13–14 °C, with
a complete mixing of the water column in the inner Gulf.
The strong thermocline that appears during summer results
in a surface-to-bottom temperature change of ~ 27–15 °C.
Higher salinity values are observed in the lower layers owing
to Aegean water that spreads north through the southern end
of the Gulf where it communicates with the deeper sea. In
the presence of a well-developed thermocline (during August–September), the treated plume that is trapped within
the pyclnocline in the inner Saronikos Gulf is characterized
by relatively lower transparency and lower salinity values
(Fig. 10.3).
The sewage plume has relatively high nutrient concentrations (ammonium, SRP, and nitrite; Fig. 10.4). The highest
nutrient values during the period of operation of the sewage
treatment plant in Psittalia (1994–2010) were recorded at the
site of the plume outflow (station S7; Table 10.1). During
the warm period (June-October) ammonium concentrations
below the pycnocline reached the value of 20 μmol/L (average for the period 1994–2010: 3.26 ± 4.64 μmol/L), whereas,
the average SRP concentration in the warm period during
1994–2010 was 0.703 ± 0.658 μmol/L. According to the previous data, nutrient concentrations indicated a significant decreasing shift with distance from the sewageoutfall (station
S7), showing that the prevailing water circulation led to the
dilution of the effluents in the inner Saronikos Gulf (Pavlidou et al. 2008).
Sewage treatment, deep outflow of the sewage field, and
its entrapment below 50 m during the warm season do not
favor the development of nitrophilous phytobenthic Chlorophycae in the upper infralittoral zone. As a result the ecological quality of the phytobenthic communities improved
since 1994 (primary treatment) and even more after 2004
115
10.3 Characteristics of the Inner Saronikos
Gulf
10.3.1 Field, Laboratory Analysis and Data
Assessment
Data were obtained from four to 12 cruises per year from
1987 to 2010 (total 170 cruises), at Saronikos Gulf, using
small boats and the research vessels Aegaeo and Filia of the
Hellenic Center for Marine Research (HCMR). Five sampling stations in the inner Saronikos Gulf were selected: station S7 at the outfall of the Psittalia sewage pipe; station S8
located 6.81 km southwest of Psittalia Island; station S13
located 15.3 km southwest of Psittalia Island; station S11
located 7.44 km southeast of Psittalia Island; and S16 located 18.1 km southeast of Psittalia Island; Fig. 10.2). At
the selected stations seawater samples were collected from
surface, 10 m, 20 m, 50 m, 75 m, and near bottom with Niskin bottles, either mounted on a rosette or individually on
a hydrowire.
Temperature, salinity, and density in the water column
were measured with a CTD profiler (Sea Bird Electronics),
which was equipped with pressure, temperature, and conductivity sensors. Before the 1990s measurements were performed with reversing thermometers attached to NIO bottles,
whereas, the salinity was determined with an auto lab inductive salinometer.
For oxygen content, samples were taken in sampling
bottles with precautionary measures to prevent any biological activity and gas exchanges with the atmosphere (Riley
1975). Samples were analyzed immediately after the collection on board following Winkler method, according to Carpenter (1965a, b).
Water samples for nutrient analyses were collected in
100 mL polyethylene bottles aged with 10 % HCl and kept
deep-frozen (−20 °C) until their analysis for nitrate, nitrite,
silicate, dissolved organic nitrogen, and dissolved organic P
(DOP) in the laboratory. Filtered samples from 1987 to1995
analyzed with a TECHNIKON CSM-6 Autoanalyzer, from
between 1996 until 1999 with an ALPKEM autoanalyzer
and after 1999 to present with a BRAN + LUEBBE II autoanalyzer according to standard methods (Mullin and Riley
1955 for silicate; Strickland and Parsons 1977 for nitratenitrite and Murphy and Riley 1962 for SRP).
Ammonium and SRP were measured using Hitachi and
Perkin Elmer 20 Lambda and 25 Lambda spectrophotometer
(Koroleff 1970 for ammonium and Murphy and Riley 1962
for SRP). ANOVA was used to compare the nutrient concentrations among seasons and sampling stations using SPSS
(version 17) statistical program. For rest of the data collected
Ocean Data View (ODV) program was used (Schlitzer 2011).
Thematic maps indicating an eutrophication index were
produced on the basis of application of interpolation for each
parameter for three different periods year 1987–1994, 1995–
2004, and 2005–2010. The interpolation method used in this
work was Spline with Barriers and a pixel size of 120 m. The
analysis was performed in ArcGIS/ArcINFO environment.
10.3.2 Observations on Ecosystem Dynamics
According to Kontoyiannis (2010), the circulation of the Saronikos Gulf has a distinct two-layer structure in the period
from late spring to late fall, whereas it is barotropic during
the rest of the year (December–March). The treated effluent plume form Psittalia is highly buoyant owing to its large
content of freshwater, and it rapidly ascends toward the sea
surface. During the stratification period (May–November),
the effluent plume is trapped within the seasonal pycnocline whereas during the mixing period (December–April) it
reaches the sea surface. Throughout the year, the spreading
of the plume is governed by advection caused by the prevailing circulation pattern at the particular layer where the
plume floats. Winter temperatures fall to near 13–14 °C, with
a complete mixing of the water column in the inner Gulf.
The strong thermocline that appears during summer results
in a surface-to-bottom temperature change of ~ 27–15 °C.
Higher salinity values are observed in the lower layers owing
to Aegean water that spreads north through the southern end
of the Gulf where it communicates with the deeper sea. In
the presence of a well-developed thermocline (during August–September), the treated plume that is trapped within
the pyclnocline in the inner Saronikos Gulf is characterized
by relatively lower transparency and lower salinity values
(Fig. 10.3).
The sewage plume has relatively high nutrient concentrations (ammonium, SRP, and nitrite; Fig. 10.4). The highest
nutrient values during the period of operation of the sewage
treatment plant in Psittalia (1994–2010) were recorded at the
site of the plume outflow (station S7; Table 10.1). During
the warm period (June-October) ammonium concentrations
below the pycnocline reached the value of 20 μmol/L (average for the period 1994–2010: 3.26 ± 4.64 μmol/L), whereas,
the average SRP concentration in the warm period during
1994–2010 was 0.703 ± 0.658 μmol/L. According to the previous data, nutrient concentrations indicated a significant decreasing shift with distance from the sewageoutfall (station
S7), showing that the prevailing water circulation led to the
dilution of the effluents in the inner Saronikos Gulf (Pavlidou et al. 2008).
Sewage treatment, deep outflow of the sewage field, and
its entrapment below 50 m during the warm season do not
favor the development of nitrophilous phytobenthic Chlorophycae in the upper infralittoral zone. As a result the ecological quality of the phytobenthic communities improved
since 1994 (primary treatment) and even more after 2004