Sundays River Estuary, Algoa Bay, Eastern Cape
Dan Baird
Study area description The Sundays River estuary (33.72°S, 25.42°E) is about 20 km
long (MacKay and
Schumann 1990) and discharges through a permanently open mouth into Algoa Bay (Indian
Ocean) about 30 km north-east of the city of Port Elizabeth, South Africa (Figure1). It receives freshwater enriched by residues of
fertilizers and pesticides used in citrus orchards in the catchment.
The Sundays River has a catchment area of
22,000 km2. Sheep farming and
citrus cultivation are the main activities in the catchment and along the entire river,
which is about 310 km in length. The mean
annual precipitation in the region is about 320 mm, categorizing the catchment as
semi-arid. The mean annual runoff (MAR) is
about 186x106 m3 while the two dams in the catchment retain
collectively about 140% of the MAR (Reddering and
Esterhuysen 1981). The river receives
water from one of the largest rivers in South Africa, the Orange River, through an
inter-basin water transfer scheme to provide irrigation water for the extensive citrus
farming industry in the Sundays River catchment area.
Because of the inter-basin water transfer scheme, the Sundays River estuary has a
rather regular pattern of freshwater input, leading to a dilution of the natural saline
regime of the estuary (Emmerson 1989). Recreation (sailing, fishing) is the main activity
on the estuary, with limited farming practised in the flood plain. There are no industrial activities, and very low
human habitation along the river and estuary. It
is thus a relatively “pristine” estuary, although elevated levels of N and P
have been measured from time to time due to agricultural practices within the catchment (Emmerson 1989).
Figure 1. Map
of Sundays River estuary, Algoa Bay.
The Sundays River estuary receives
freshwater at an annual mean inflow rate of about 59x106 m3 yr-1
or 162x103 m3 d-1. Rates
of freshwater inflow are available for most months of the year (Scharler et al.
1998), but the estuary receives about 237x103 m3 d-1
during the rainy season and 86x103 m3 d-1 during the dry
season. The salinity range in this estuary
from 28 psu in the lower reaches to 5.3 psu at the head.
The salinity structure does not vary drastically between summer and winter
within the various reaches of the estuary. The
mean annual axial salinity is 16.4 psu, varying from 15.1 to 17.6 psu during the wet and
dry seasons, respectively. Stratification of
the water column occurs rarely, so that the water in the system is well-mixed throughout
the year. Temperatures fluctuate from 17° C
in winter to 24°C in summer (Scharler et al.
1998). The physical characteristics of
the Sundays River estuary are summarized in Table 1.
| Table 1.
Physical characteristics of the Sundays River estuary. |
|
| Characteristic |
Value |
| Catchment
area (km2) |
22000 |
| Length
of estuary (km) |
20 |
| Surface
area (km2) |
3 |
| Average
depth (m) |
4 |
| Average
system volume (106 m3) |
12 |
Water and salt balance
Water and salt fluxes for the Sundays
River estuary are illustrated in Figure2, and some results are also given in Table 2. The water exchange time during the rainy season is
about 23 days, 61 days during the dry season, and average at about 42 days over a year.
Table 2. Variations
of physical properties, water budgets and water exchange times in the Sundays River
estuary and adjacent Algoa Bay.
Season |
Freshwater |
Residual (103m3d-1) |
River (psu) |
Ocean salinity (psu) |
System (psu) |
Mixing
volume (103m3d-1) |
(day) |
||
VQ |
VP |
VE |
|||||||
Dry |
86 |
2 |
8 |
80 |
0.5 |
35.3 |
17.6 |
117 |
61 |
Wet |
237 |
2 |
8 |
231 |
0.5 |
35.3 |
15.1 |
282 |
23 |
Annual mean |
162 |
2 |
8 |
156 |
0.5 |
35.3 |
16.4 |
200 |
42 |
The nonconservative nutrient budgets
indicate that the system is a source of DIP (DDIP = +39 mol d-1), but a net
sink of DIN at a rate of -1,858 mol d-1 (see Table 3 and Figures 3 and 4).
Net ecosystem metabolism (p-r) estimated from Redfield stoichiometrics is
-4,134 mol d-1, or -1 mmol m-2 d-1. Results also show that the difference between
nitrogen fixation and denitrification (nfix-denit)
is -2,482 mol d-1 (or -0.9 mmol m-2 d-1). The net system metabolism is thus
heterotrophic, and it is also net denitrifying (see Table 3).
Table 3. Nonconservative fluxes of C, N and P in the
Sundays River estuary.
Time |
DDIP
(mol d-1) |
DDIN
(mol d-1) |
(p-r) (mol d-1) |
(nfix-denit) (mol d-1) |
(p-r) (mmol m-2 d-1) |
(nfix-denit) (mmol m-2 d-1) |
Dry |
+25 |
-657 |
-2,650 |
-1,057 |
-0.8 |
-0.4 |
Wet |
+53 |
-3,058 |
-5,618 |
-3,906 |
-1.9 |
-1.3 |
Annual mean |
+39 |
-1,858 |
-4,134 |
-2,482 |
-1 |
-0.9 |
Figure
2. Water and salt budgets for Sundays River
estuary in the dry (a) and wet (b) seasons. Water flux in 103 m3
d-1 and salt flux in 103 psu-m3 d-1.
Figure 3. DIP
budget for Sundays River estuary in the dry (a) and wet (b) seasons.
Figure 4.
DIN budget for Sundays River estuary in the dry (a) and wet (b) seasons.
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Last Updated 21 May 2006 by DPS