Environmental Engineering Reference
In-Depth Information
Figure 14.4.
Typical dissolved oxygen profiles in Basin 7 shortly after impoundment
(1986), following installation of the Helixors (1989), during the mid 1990s (1996)
and recently (2007) (APEM unpublished data).
Dramatic reductions in ammonia have also occurred in the MSC, however,
with a recent 5-year average of 1.46mg L
1
(APEM unpublished data). Suspended
solids concentrations decreased initially to around 3mg L
1
from the previous
ton biomass disguised the reduction in suspended solids derived from the
Irwell catchment (White et al.
1993
). Owing to the low phytoplankton biomass
and lack of polluting load, current suspended solids concentrations in the
Quays are low, as indicated in the most recent 5-year average of 1.47mg L
1
compared to 7.14mg L
1
in the MSC during the same period (APEM unpublished
data). The figure for the MSC nonetheless represents a substantial reduction from
historic values provided above, albeit based on monthly sampling and, therefore,
neglecting storm events which raised levels to
50mg L
1
(APEM
2008a
).
Isolation from the MSC and the resultant rapid decrease in suspended solids
inevitably led to a concomitant increase in Secchi transparency. Initially, algal
biomass in the Quays was low but increased rapidly as photic depth increased
in the nutrient rich waters. The blue-green algae Oscillatoria dominated, increas-
ing with both frequency and intensity over the next 4 years. The most severe
blooms were recorded in the summer of 1992, when densities peaked at 5m
(filament length) ml
1
(Hendry et al.
1993
). Throughout the following 5 years,
the algal biomass remained high but showed a gradually decreasing trend, and
by 1998 a stable and low biomass was achieved. It was originally anticipated
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