Kromme River Estuary, St Francis Bay, Eastern Cape, South Africa
Dan Baird
Study area description
The Kromme River estuary is a permanently
open system, discharging through a constricted inlet into St Francis Bay, on the
south-east coast of South Africa (34.15°S, 24.85°E; see Figure 1).
The 95 km long Kromme River originates in
a coastal mountain range (the Tsitsikamma Mountains) and drains a catchment of about 936
km2 (Reddering and
Esterhuysen 1983), which is partly vegetated by fynbos vegetation and natural forest. Limited areas in the catchment are utilized for
stock raising and grain cultivation. No
industrial activities occur in the catchment or in the estuarine floodplain (Baird, Marais
and Bate 1992), so that this estuary is considered to be one of the few relatively
pristine systems in the country. Rainfall
occurs throughout the year, with lowest precipitation during summer (December to February)
(Bickerton and
Pierce 1988) and peaks during the austral spring and autumn. Annual rainfall varies between 700 mm and 1,200
mm with a mean annual runoff (MAR) of about 106x106 m3 (Reddering and
Esterhuysen 1983). The high MAR is a
consequence of the geomorphological characteristics of the catchment, i.e. high relief,
rocky slopes and sparsely vegetated areas. Alterations
to the river flow in the Kromme River as a result of two dams have severely reduced
freshwater input into the estuary. The dams
have the combined capacity of storing ca 133% of the MAR of the Kromme River catchment. The runoff into the estuary prior to the
construction of the second dam in 1982 was about 117x106 m3 yr-1
was subsequently drastically reduced to 1x106 m3 yr-1. Freshwater inflow into the Kromme estuary is low
and irregular with a mean annual flow rate of about 11x103 m3 d-1. The flow rate during the rainy months increases to
about 13x103 m3 d-1 and decreases to about 6x103
m3 d-1 during the dry months. The
system is effectively freshwater-starved, with relatively low concentrations of DIN and
DIP reaching the estuary from the catchment (Scharler et al. 1998), despite the fact that, in
addition to the natural freshwater runoff, about 2x106 m3 yr-1
are released from the upstream impoundments to compensate for evaporation in the estuary (Jezewski and Roberts
1986) (EMATEK (CSIR) 1994).
Figure 1. Map
and location of the Kromme River estuary.
The estuary is about 14 km long, with a
surface area of about 3 km2 (Bickerton and Pierce
1988). The estuary is shallow (average
depth at low water spring 3 m), with tidal amplitude of 2 m at the mouth. The tidal prism during spring tides is on average
2x106 m3 and during neap tides about 1x106 m3
in volume. The average flushing time at
spring tide is about 27 h. The water
temperature fluctuates between 12°C in winter and 27°C in summer (Baird and Pereyra-Lago 1992). The salinity has rarely dropped below 30 psu since
the completion of the dams in 1984. Salinity
in the Kromme estuary ranges from 32.8 psu in the lower reaches to 31.1 psu in the upper
reaches. Salinity stratification may occur in
the upper and middle reaches of the estuary during low to moderate freshwater inflow (Scharler et al. 1998).
The data on the salinity structures of the estuary, and on the concentrations of
dissolved inorganic nutrients in both the freshwater reaches and the estuary considered in
this study, were obtained from various theses and published information. Most of this information was collected during the
years 1996-1999 (cf. Baird and Pereyra-Lago 1992,
Scharler et al. 1998, Allanson and Baird
1999, Scharler 2000, Scharler and Baird 2000). The physical characteristics of the Kromme estuary
are given in Table 1.
| Table 1.
Physical characteristics of the Kromme River estuary. |
|
| Characteristic |
Value |
| Catchment
area (km2) |
936 |
| Length
of estuary (km) |
14 |
| Surface
area (km2) |
3 |
| Average
depth (m) |
3 |
| Average
system volume (106 m3) |
9 |
Water and salt balance
Two water and salt budgets are provided
for this estuary: one each for the dry and wet seasons.
Rainfall patterns are rather variable, but, in general, most of the
precipitation occurs along the Eastern Cape coastal region during the months March to
June, and again from August to November. Data
used are given in Table 2.
The system is fairly pristine with no
wastewater or industrial discharges into the river and estuary. The evaporation rate exceeds precipitation during
the austral summer months, when hypersaline conditions may occur from time to time in the
upper reaches of the estuary. Precipitation
barely exceeds evaporation on an annual scale.
The results of the water and salt balance
are illustrated in Figure 2. The residual
water (VR) and salt fluxes (VRSR), as well as the
exchange flows of salt water (VX)
and salinity [VX(Socn-Ssyst)] are highest during the wet
season. The water exchange time [t] of water in the system ranges from 66
days during the wet season to 130 days in the dry season.
The long water exchange time during the dry periods is probably due to the
low rate of fresh water inflows and low precipitation.
The major input and output terms show that the residual flows are from the
system.
Table 2. Variations
of physical properties, water budgets and water exchange times in the Kromme River estuary
and adjacent St. Francis Bay.
Season |
Freshwater input (103 m3d-1) |
Residual flow (103m3d-1) |
River salinity (psu) |
Ocean salinity (psu) |
Lagoon
salinity (psu) |
Exchange volume (103m3d-1) |
t (day) |
||
VQ |
VP |
VE |
|
|
|
|
|
||
Dry |
6 |
5 |
6 |
5 |
0.6 |
35.3 |
32.7 |
64 |
130 |
Wet |
13 |
9 |
6 |
16 |
0.6 |
35.3 |
31.0 |
121 |
66 |
Annual Mean |
11 |
8 |
6 |
12 |
0.6 |
35.3 |
31.6 |
102 |
87 |
Water flux data from Table 2 and Figure 2
were used to construct the DIP budget under well-mixed conditions. The mean annual nonconservative flux (DDIP) of +56 mol d–1,
exceeds the river input of 7 mol d–1, which suggests that the estuary
behaves as a source for DIP (see Table 3 and Figure 3).
Budgeting results show that the
estuary is also a net source of DIN on an annually averaged basis (see Table 4 and Figure
4).
Table 3. Nonconservative
fluxes of C, N and P in the Kromme 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 |
+35 |
+817 |
-3,710 |
+257 |
-1 |
+0.1 |
Wet |
+67 |
+1,552 |
-7,102 |
+480 |
-2 |
+0.2 |
Annual mean |
+56 |
+1,307 |
-5,971 |
+406 |
-2 |
+0.2 |
The net ecosystem metabolism (p-r), estimated from Redfield stoichiometric ratios
and DDIP
is -2 mmol m-2 d-1. These
negative values indicate that the estuary is net heterotrophic. Nitrogen fixation minus denitrification (nfix-denit), calculated as the difference between
the observed and expected DDIN, amounts to +0.1 mmole m-2 d-1. These results show that the estuary is, on annual
average, a net nitrogen fixing system.
Figure 2. Water
and salt budgets for Kromme 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 Kromme River estuary in the dry (a) and wet (b) seasons.
Figure
4. DIN budget for Kromme River estuary in the
dry (a) and wet (b) seasons.
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Last Updated 21 May 2006 by DPS