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3.2 Water chemistry
The distribution of saline water in an estuary (the longitudinal salinity distribution) is of
fundamental importance as it affects the distribution of all biota in the system due to their
differing salinity tolerances. River inflow and sea level together determine the penetration of
seawater into the system, thereby determining the salinity profile of the estuary.
In 1979 and 1981 surveys showed that salinities below the bridge ranged from 35.5‰ at the
mouth to 26‰ at the bridge. The main channel had a salinity of 20‰ 2 km from the mouth
and 0‰ only 500 m further upstream (Heydorn & Bickerton 1982). Harrison (2004) measured
salinities at six stations along the estuary and reported a mean value of 15.43‰ (SE ± 2.11).
Surface water temperature was recorded as 24.5˚C to 25.5˚C at 400 m and 500 m from the
mouth respectively in the 1979 survey (Heydorn & Bickerton 1982). At the same sampling sites
dissolved oxygen concentrations of 9.8 mg/l and 10.8 mg/l were measured, with a higher
measurement of 13.0 mg/l near algae at the latter (Heydorn & Bickerton 1982). Harrison
(2004) measured a mean temperature of 17.33˚C (SE ± 0.27), a mean dissolved oxygen
concentration of 8.49 (SE ± 0.11) mg/l and a mean turbidity of 5.33 (SE ± 0.88) NTU. Extended
mouth closure events will affect the water chemistry of the estuary. The estuary is no longer
flushed by the sea or freshwater as frequently as it was in the past and this could result either
in hypersaline conditions or fresh conditions developing within the estuary, depending on the
amount of freshwater inflows and the amount of evaporation.
3.3 Microalgae
Microalgae in estuaries comprise unicellular algae that live either suspended in the water
column (termed phytoplankton) or benthically on rocks or sediments in the estuary (termed
microphytobenthos or benthic microalgae). These microalgae (i.e. phytoplankton and
microphytobenthos) are very important in estuarine systems as they are generally the main
source of primary production in the estuary.
Phytoplankton communities in estuaries are influenced by salinity, generally dominated by
flagellates where river flow dominates and by diatoms in marine dominated areas. Diatoms
are most common in the area of the estuary where the salinity is in the region of 10-15‰,
often referred to as the River Estuary Interface (REI) zone. Phytoplankton biomass in an
estuary is also generally at its maximum in this region. Biomass of phytoplankton in estuaries
varies widely and may range from 0-210 µgChla/l (Adams et al. 1999). If nutrient
concentrations in an estuary are high (particularly in the case of nitrogen) then phytoplankton
biomass in the estuary is generally high too. Under extreme conditions, when nutrient levels
are very high, certain toxic dinoflagellate species may form dense blooms known as red tides.
Less is known about benthic microalgae (microphytobenthos) in estuaries than phytoplankton.
Values for benthic microalgae biomass are often reported in different units which makes
comparisons between estuaries difficult. Currently there is no available information on
microalgae in the Uilkraals Estuary.
Uilkraals Estuary Situation Assessment
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