State of the Bay Report 2010-Final - Anchor Environmental

Anchor Environmental
coast, result from the decay of large plankton blooms under calm conditions. Once all the oxygen in
the water is depleted, anaerobic bacteria (not requiring oxygen) continue the decay process, causing
the characteristic sulphurous smell. Apparent oxygen utilization (AOU - a measure of the potential
available oxygen in the water that has been used by biological processes) values for Small and Big
Bay over the period April 1974 - October 1982 and July 1988 are given in Monteiro et al. (1990).
AOU is defined as the difference between the saturated oxygen concentration (the highest oxygen
concentration that could occur at a given water temperature e.g. 5 ml/l) and the measured value
(e.g. 1 ml/l) – hence positive AOU (5 ml/l – 1 ml/l = 4 ml/l) values indicate an oxygen deficit
(indicated in red in Figure 5.4). More recent data on oxygen concentration in Small Bay (covering
the period September 1999-February 2000) were provided by Monteiro et al. (2000). During this
study, oxygen concentration at 10 m depth was recorded hourly by an instrument moored in Small
Bay, these values were converted to AOU and the monthly average plotted in Figure 5.3.
Chlorophyll concentration (ug/l )
Nitrate concentration ( uM )
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
Apr-74
Aug-74
Jun-74
Oct-74
Oct-74
Aug-74
Jun-74
Apr-74
Feb-75
Dec-74
Feb-75
Dec-74
Apr-75
Jun-75
Jun-75
Apr-75
Oct-75
Aug-75
Dec-75
Feb-76
Dec-75
Oct-75
Aug-75
Apr-76
Feb-76
Jun-76
Apr-76
Aug-76
Jun-76
Oct-76
Aug-76
Oct-76
Feb-77
Dec-76
Feb-77
Dec-76
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 89
Apr-77
Jun-77
Oct-77
Aug-77
Jun-77
Apr-77
Aug-77
Feb-78
Dec-77
Dec-77
Oct-77
Jun-78
Apr-78
Apr-78
Feb-78
Aug-78
Jun-78
Feb-79
Dec-78
Oct-78
Aug-78
Oct-78
Jun-79
Apr-79
Feb-79
Dec-78
Apr-79
Oct-79
Aug-79
Figure 5.3. Time series of chlorophyll and nitrate concentration measurements for Saldanha Bay.