Observations of Water Quality - Gosford City Council

Observation of Water Quality in Cockrone Lagoon on 13 March 2002
Dr Joanne Wilson and Peter Evans
Centre for Natural Resources
Dept Land and Water Conservation
Introduction
In early February 2001 Cockrone Lagoon was closed to the ocean with a
water level of about 1.9m above mean sea level (see Figure 3). Heavy rainfall
on the 4 th and the 5 th resulted in a rapid rise in lake level. The lagoon was
opened by breaching the berm mechanically on 8 th February 2002 to prevent
flooding of low-lying property. The opening was undertaken in accordance
with Gosford Council's Lagoons Opening Policy, (Gosford Council's Coastal
Lagoons Management Plan). The lake remained open to the ocean for about
six tidal cycles before the beach berm reformed, once again closing the
lagoon to the ocean on the 12 th February. The lake level then stabilised at
about 1.2m above mean sea level (AHD). Four days later it was reported that
the water became black and odorous and that a fish kill occurred.
An inspection of the lagoon on the 13 th March 2002 was made to determine of
Draft
the cause of this event, and to observe the general characteristics of the
lagoon. It has been concluded that the likely cause of the fish kill was low
dissolved oxygen levels due to the decomposition of extensive amounts of
macroalgae which were growing in the lagoon and which were killed when the
lagoon emptied. The results of water quality testing carried out on
13 th March 2002, and that recorded prior to, during, and after the opening
event, are presented in this report.
Methods
On 13 th March 2002, the lagoon was surveyed from a canoe for water quality
and to make general observations. Water quality profiles (from the surface to
the bottom) were recorded at five sites in the lake (entrance channel,
stormwater channel, mid lake, algal mat and creek) (Figure 1). The following
parameters were recorded using a Hydrolab Quanta multiparameter probe –
pH, dissolved oxygen, temperature, and salinity.
Water samples were taken from two sites in the lake (mid lake and creek) for
identification and counts of phytoplankton, and total and inorganic nutrients
(analysed by AWT Sydney). Samples for inorganic nutrient analysis were
passed through a 0.45mm filter and frozen. Samples for algal identification and
counts were kept cool and fixed with Lugol’s iodine.
The Manly Hydraulics Laboratory, under contract to Gosford City Council
maintains a logging water level and water quality instrument at the seaward
end of the Lagoon (see Figure 1). Information on lake level, pH, temperature,
dissolved oxygen concentration and salinity was available, and data from 1
October 2001 to 28 Feb 2002 (which includes the opening event and
subsequent fish kill) was examined.
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Creek
Algal Mat
Mid Lake
Stormwater
Channel
Draft
Figure 1: Cockrone Lagoon showing sites of water quality sampling and
testing on 13 March 2002.
Results
General Observations
The deepest site recorded was close to the site of the water quality logger at
the entrance channel with the ocean. The main body of the Lagoon was
between 0.5-1.0m. A large amount of filamentous macroalgae identified as
Cladophora sp (?) covered the bottom of the western portion of the Lagoon.
These algae formed a floating mat approximately 4ha and 30-50cm thick in
the far south western corner (Figure 1).
Water Quality
The results of water quality profiles at each site are shown in Figure 2. The
salinity was ~22 ppt at all sites except at the algal mat which had trapped a
layer of freshwater on the surface. Dissolved oxygen was very low in the lake
with values less than 4mg/l recorded at all sites. The lowest dissolved oxygen
was recorded in the creek (0.2-1.4 mg/l) and on the bottom at the entrance
channel (0.3 mg/l). Temperature of the water was between around 23C in the
lake body and around 25C in the creek. There was some stratification
(layering) of the water column with lower of dissolved oxygen and higher
temperature (approx 1C) in bottom waters at the entrance channel and the
creek. Surface pH throughout the Lagoon ranged between 7.3-7.9, which is
normal for these types of waters. However, the pH of bottom waters at the
channel and creek was slightly acidic (Figure 2).
The results of nutrient and algal analyses are shown in Table 1. The levels of
inorganic nutrients measured from both the mid lake and the creek are
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elatively low. However, the values for Total Nitrogen and Total Phosphorus
at the mid lake site are very high compared to ANZECC guidelines (TN –
0.35mg/l, TP – 0.035mg/l).
Algal counts showed the phytoplankton consisted only of diatoms at this site
and no blue green algal species were recorded.
Table 1: Results of nutrient analysis of surface waters from Cockrone
Lagoon 13 March 2002. (na = not applicable).
Mid lake
(unfiltered)
Mid lake
(filtered)
Cockrone Creek
(filtered)
Nutrients
Ammonia (NH3) (mg/l) na 0.015 0.02
Nox (mg/l) na

values less than 2mg/l, which are not high enough to sustain life. These very
low oxygen levels are presumably due to the decomposition of organic matter
from the dead filamentous algae, which would consume most of the oxygen
from the water.
Draft
4

0.0
0 10 20 30 40 50
Conductivity (mS/cm)
0.5
Depth (m)
1.0
1.5
channel
stormwater
mid lake
algal mat
creek
2.0
0.0
0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Dissolved Oxygen (mg/L)
0.5
Depth (m) Depth (m) Depth (m)
1.0
1.5
channel
stormwater
midlake
algal mat
creek
2.0
22.5 23.0 23.5 24.0 24.5 25.0 25.5 26.0
0.0
Temperature ( o C)
0.5
1.0
1.5
2.0
channel
Stormwater
midlake
algal mat
creek
6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0
0.0
0.5
1.0
pH
1.5
2.0
channel
stormwater
midlake
algal mat
creek
Figure 2: Water quality profiles from 5 sites in Cockrone Lagoon,
13 th March 2002.
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3.0
2.5
water height
Depth (m)
2.0
1.5
1.0
0.5
0.0
-0.5
40
35
Oct 01 Nov 01 Dec 01 Jan 02 Feb 02 Mar 02
salinity
Salinity (ppt)
30
25
20
15
DO (mg/L)
Temperature ( o C)
10
5
18
16
14
12
10
8
6
4
2
0
32
30
28
26
24
22
20
18
16
11
10
Oct 01 Nov 01 Dec 01 Jan 02 Feb 02 Mar 02
dissolved oxygen
Draft
Oct 01 Nov 01 Dec 01 Jan 02 Feb 02 Mar 02
temperature
Oct 01 Nov 01 Dec 01 Jan 02 Feb 02 Mar 02
pH
pH
9
8
7
6
Oct 01 Nov 01 Dec 01 Jan 02 Feb 02 Mar 02
Figure 3: Surface water quality data from Cockrone Lagoon 1 st October 2001
until 28 th February 2002. Logger located at entrance channel. Probe
calibration dates are shown on x axis.
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Discussion
Information gathered from both field observations and water quality testing
indicate that Cockrone Lagoon is likely to be eutrophic ie there is an
unsustainable load of nutrients entering the waterway. This conclusion is
based on the excessive and persistent growth of filamentous algae, the
occurrence of blue green algal blooms (Chris Carloss (GCC), pers comm)
and, prior to the entrance opening, high dissolved oxygen values, which had a
large diurnal range, and high pH values. The high values of oxygen and large
diurnal fluctuations are due to the high biomass of algae fuelled by excess
nutrients. During the day, the algae are photosynthesising producing large
quantities of oxygen. During the night, the algal tissue is respiring and
consuming oxygen. This results in very high values in late afternoon and very
low values in the early morning. The pH in the lagoon was consistently very
high at least since the beginning of October. High pH results from a complex
chemical reaction that occurs in high oxygen environments. Thus both the
dissolved oxygen and pH values indicate that the lagoon contained large
biomass of algae since at least beginning of October 2001. Examination and
analysis of logged data prior to this date is recommended so that the timing of
any changes in the water
Draft
quality parameters in the system can be identified
After the entrance opened, the death and decomposition of much of the
filamentous algae caused a low dissolved oxygen event and subsequent fish
kill. Values for dissolved oxygen were still very low on 13 th March almost one
month after the fill kill. Low dissolved oxygen will create conditions suitable
for release and recycling of nutrients stored in the sediments, thus
exacerbating the eutrophic condition of the lagoon. While there are many
signs of high nutrient load to the system, values of inorganic nutrients
measured in the water column are low. This is not unusual as inorganic
nutrients are taken up and incorporated very quickly by algal cells. Thus
these values are not a good indicator of nutrient status of the lake. In
contrast, values for total nitrogen and total phosphorus are very high
indicating a high turnover of nutrients in the system.
Management implications
Increasing the flushing of the lake by opening it more frequently may seem
like a simple option to improving water quality. However, this will expose
existing filamentous algae causing further low dissolved oxygen events.
Reducing the water levels in the lagoon is likely to kill surrounding wetland
vegetation, which is acting as a filter for nutrients from the catchment. In
addition, the duration of opening is likely to decrease with increased opening
frequency at a reduced berm height.
Therefore the most effective way to improve the health of the lagoon, and
reduce filamentous and blue green algae growth, is to reduce the load of
nutrients to the lagoon from the catchment.
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