Gosford City Council Historical Water Quality Review & Analysis

RESULTS OF WATER QUALITY DATA ANALYSIS 3-17
Turbidity in Avoca Lagoon is also highly variable, with measurements up to 160 NTU. As discussed
for the Koolewong site, turbidity records can be influenced by fouling of the probe, and there appears
to be some evidence of this at Avoca Lagoon. Also, given its relatively shallow water, stirring of fine
bed sediments during periods of lower water levels would also be a notable cause of elevated
turbidity levels in Avoca Lagoon. Higher turbidity concentrations are observed during and flowing
entrance breakout events, and also periods of catchment runoff (which do not result in entrance
breakout).
3.2.3 Cockrone Lagoon
The Cockrone Lagoon entrance also broke out 14 times during the period March 1996 to Jun 2002,
which included a period of 20 months (Sept 1999 to May 2001) when no breakouts occurred. The
hydrology of Cockrone Lagoon is very similar to that described for Avoca Lagoon, with sudden
responses to catchment runoff, gradual evaporation effects, and small periods of tidal influence
following entrance breakout.
The salinity of Cockrone Lagoon is variable, falling to levels of about 6ppt when the lagoon is ‘full’,
and peaking at levels well in excess of oceanic salt concentrations (i.e. Hypersaline conditions)
following breakout, recharge with ocean waters, and subsequent evaporation of ocean waters from the
closed waterway.
Temperature within Cockrone Lagoon varied on a seasonal basis, from approximately 25 – 30 °C in
the summer to about 10 – 15 °C in the winter.
pH levels generally ranged between 7 and 10. Similar to Avoca Lagoon, the pH levels in Cockrone
Lagoon generally followed a seasonal trend, with higher levels during the later Autumn / early
Summer months, and lower levels during the Autumn / Winter months. This trend was somewhat
disrupted by the predominantly dry period of 2000 / 2001, which resulted in higher than expected pH
levels for the majority of the time. pH is likely to be affected by the amount of primary productivity
(photosynthesis) occurring within the water.
Dissolved Oxygen (DO) in Cockrone Lagoon is highly variable ranging between 16 mg/L and zero.
Although not completely evident, there appears to be a trend in DO concentrations that is also
seasonal. Higher DO concentrations are recorded during the late autumn / early summer period,
coinciding with the increase in primary productivity (as suggested by the higher pH levels). Lower
DO levels occur during mid to late summer, extending through autumn. It is possible that the higher
primary production rates in early summer are unsustainable, resulting in a massive die-off of the algae
colonies. The decay (reduction) of this organic matter would generate a significant oxygen demand
on the water column, thus depleting DO levels.
Turbidity in Cockrone Lagoon is mostly less than about 10 NTU, however, some periods of heighten
hydrologic activity result in significantly elevated turbidity levels (up to 160 NTU). Higher turbidity
is recorded coinciding with catchment runoff events and entrance breakouts. However, there remains
a considerable amount of data with higher turbidity concentrations that cannot be explained by such
mass hydrologic processes.
D:\R.N0754.002.01.DOC 7/11/03 16:11