Gosford City Council Historical Water Quality Review & Analysis

RESULTS OF WATER QUALITY DATA ANALYSIS 3-16
The dissolved oxygen concentrations measured by the probe are difficult to interpret and are of
limited value. The reason for this is the fact that the probe appears to have been affected by
biological fouling. The results show a slow decline in dissolved oxygen concentrations between
changeover periods. At changeover, the levels increase suddenly, indicating that prior to changeover,
recorded levels were incorrect.
Turbidity levels measured at Koolewong were highly variable, with values up to 160 NTU recorded
regularly. A comparison of the turbidity and salinity plots indicated that higher turbidity levels were
not always corresponding to catchment runoff events (as depicted by a reduction in salinity levels).
Therefore, other factors also affect turbidity levels at the Koolewong datasonde. By way of example,
it may be possible that wind waves cause resuspension of finer bottom sediments in the vicinity of the
Koolewong probe. Also, the turbidity probe appears to be affected by biological fouling, with sudden
reductions in turbidity occurring at instrument changeover (and coinciding with sudden changes in
dissolved oxygen levels).
3.2.2 Avoca Lagoon
The results of the Avoca Lagoon datasonde show that the Avoca Lagoon entrance broke out 14 times
between March 1996 and June 2002. On most occasions tides returned to the lagoon following
breakout for a period of a few days before closing once again. When closed, water levels in the
lagoon respond to periods of catchment runoff (with sudden increases in water levels) and
evaporation from the water surface (with slow reductions in water level).
In most cases, salinity follows a reverse trend to water level, with dramatic increases in salt
concentration associated with entrance breakout, followed by a gradual reduction in salinity as the
lagoon hydrology is subsequently dominated by freshwater catchment inputs.
Temperature showed a strong seasonal trend, with temperatures peaking in January / February at
about 25 – 30 °C, and reaching a minimum in June / July at about 10 – 12 °C.
pH varied considerably within Avoca Lagoon, between about 7 and 9.5, although some values greater
than 10 were also recorded. In general, higher pH levels appeared to occur during the early summer
months (Oct – Dec) with levels generally about 9 or greater. Levels would then reduce to about 7.5
during late spring / winter months. It is suggested that the pH levels within Avoca Lagoon are
influenced by the seasonal cycles of phytoplankton and macroalgae growth. Although chlorophyll-a
monitoring has not been conducted to measure the amount of primary productivity within the lagoon,
it is likely that the increasing temperatures associated with the early summer months would initiate
higher amounts of productivity in the waterway, resulting in increasing pH levels. When pH levels
are elevated, entrance breakout can also have an impact on pH, as most of the phytoplankton and
macroalgae would be removed or detrimentally affected by the sudden change in lagoon hydrology.
Dissolved oxygen levels in Avoca Lagoon vary between 16 mg/L and zero. Oxygen levels can be
influenced by several different factors in mostly closed lagoons, including phytoplankton and
macroalgae growth (photosynthesis), organic decay (respiration), entrance breakout, and catchment
runoff. The DO probe also shows some signs of biological fouling of the sensor, so all information
derived from the data should be used with caution.
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