The Gulf of Guayaquil and the Guayas River Estuary, Ecuador
249
from 15 to 25m during the dry season and is <10m during the wet season.
Seasonal salinity patterns in the Guayas (0.08-33, mean 18), Churute
(2-29, mean 16), and Salado (19-36, mean 28) subestuaries reflect freshwater discharge (Cardenas 1995; Fig. 17.2). Average pH values in the subestuaries range from 7.5 to 7.7 at high and low river flow, respectively.
However, pH increases significantly between the upper (7 .3±0.1) and lower
(8.01±0.2) Guayas River estuary (Cardenas 1995). During the dry season,
pH values tend to increase (0.2) from the inner to the outer Gulf. Changes
of up to 0.5 during the wet season are attributed to higher productivity and
detritus concentration in the inner region.
17.3 Biogeochemistry
Except during initial stages of high river flow, mean total suspended solid
(TSS) concentrations in Guayas (362 mg 1- 1 ), Churute (175 mg 1- 1 ), and
Salado (127 mg 1- 1 ) subestuaries are higher at low flow. Concentrations
tend to reflect the relative importance of freshwater input as a source of
suspended particulate matter (Cardenas 1995), although increased TSS
may be associated with resuspension of bottom sediments during flood
tides, particularly during low river flow (Meade 1972). Episodic or seasonal
flow of the Guayas River may temporarily cause high concentrations of
TSS, with monthly loads from 2.62 x 10 6 to 0.12 x 10 6 tons during high and
low river flow, respectively. Water turbidity (Secchi depth) relates inversely
with TSS (0.51-0.57 m in the Guayas and Churute, 1.20 m in the Salado)
and varies between about 1m in the inner and 13m in the outer Gulf (Stevenson 1981).
During the wet season, dissolved oxygen (DO) concentrations in the
Gulf are lower than 4.6 mll- 1 and are rarely saturated. During the dry season, DO ( 4.8-5.0 mll- 1 ) is saturated or supersaturated and pronounced DO
stratification (5.0 mll- 1 at surface, <2.0 mll- 1 at 70 m depth) occurs in the
outer Gulf region (Stevenson 1981). Surface DO concentrations in the
inner Gulf are 55-60% saturated. Average DO concentrations in the subestuaries are similar (3.2-4.2 mll- 1 ) and display little seasonal variation
(Cardenas 1995). Despite high total oxygen demand, owing to urban wastewater input and shrimp pond effluents (Solorzano 1989; Twilley 1989),
tidal exchange and water turnover appear to maintain DO above biological
stress levels and prevent development of hypoxia in the Guayas River
estuary.
Nitrate, dissolved organic nitrogen (DON), and particulate nitrogen
represent about 30% each of the total nitrogen (TN) pool and nitrate
249
from 15 to 25m during the dry season and is <10m during the wet season.
Seasonal salinity patterns in the Guayas (0.08-33, mean 18), Churute
(2-29, mean 16), and Salado (19-36, mean 28) subestuaries reflect freshwater discharge (Cardenas 1995; Fig. 17.2). Average pH values in the subestuaries range from 7.5 to 7.7 at high and low river flow, respectively.
However, pH increases significantly between the upper (7 .3±0.1) and lower
(8.01±0.2) Guayas River estuary (Cardenas 1995). During the dry season,
pH values tend to increase (0.2) from the inner to the outer Gulf. Changes
of up to 0.5 during the wet season are attributed to higher productivity and
detritus concentration in the inner region.
17.3 Biogeochemistry
Except during initial stages of high river flow, mean total suspended solid
(TSS) concentrations in Guayas (362 mg 1- 1 ), Churute (175 mg 1- 1 ), and
Salado (127 mg 1- 1 ) subestuaries are higher at low flow. Concentrations
tend to reflect the relative importance of freshwater input as a source of
suspended particulate matter (Cardenas 1995), although increased TSS
may be associated with resuspension of bottom sediments during flood
tides, particularly during low river flow (Meade 1972). Episodic or seasonal
flow of the Guayas River may temporarily cause high concentrations of
TSS, with monthly loads from 2.62 x 10 6 to 0.12 x 10 6 tons during high and
low river flow, respectively. Water turbidity (Secchi depth) relates inversely
with TSS (0.51-0.57 m in the Guayas and Churute, 1.20 m in the Salado)
and varies between about 1m in the inner and 13m in the outer Gulf (Stevenson 1981).
During the wet season, dissolved oxygen (DO) concentrations in the
Gulf are lower than 4.6 mll- 1 and are rarely saturated. During the dry season, DO ( 4.8-5.0 mll- 1 ) is saturated or supersaturated and pronounced DO
stratification (5.0 mll- 1 at surface, <2.0 mll- 1 at 70 m depth) occurs in the
outer Gulf region (Stevenson 1981). Surface DO concentrations in the
inner Gulf are 55-60% saturated. Average DO concentrations in the subestuaries are similar (3.2-4.2 mll- 1 ) and display little seasonal variation
(Cardenas 1995). Despite high total oxygen demand, owing to urban wastewater input and shrimp pond effluents (Solorzano 1989; Twilley 1989),
tidal exchange and water turnover appear to maintain DO above biological
stress levels and prevent development of hypoxia in the Guayas River
estuary.
Nitrate, dissolved organic nitrogen (DON), and particulate nitrogen
represent about 30% each of the total nitrogen (TN) pool and nitrate