Environmental Engineering Reference
In-Depth Information
1992, after which a decreasing trend was apparent until low and stable
conditions were reached and maintained post-2002. A population of Dreissena
polymorpha (zebra mussel) has been present within the enclosed basins since the
early 1990s; however it is unknown whether the filtering properties of this
species played a part in reducing the phytoplankton biomass. Over the past few
years, a diverse phytoplankton assemblage has been recorded composed of
several groups, though dominance by chlorophytes has been maintained since
2000. The blue-green species Microcystis sp. has been recorded in the Quays
during autumn since 2004, and the propensity for the cells to aggregate at
the leeward end of the basins has caused densities to exceed the Environment
Agency guideline thresholds of 20 coloniesml
1
(90
m
m diameter) and
1.5 coloniesml
1
(200
m
m diameter) (EA
2004
) in these locations. However the
densities are low and often undetectable at the routine sampling sites. There is
therefore no evidence that current levels signify a reversion to the situation
observed during the mid 1990s. Maintenance of the oxic sediment layer
through Helixor mixing is crucial in retaining the 'trapped' nutrients and thus
the continued successful control of algal biomass and protection of the water
sports amenity within Salford Quays.
The rapid improvement in water quality, in particular the reduction in
ammonia and suspended solids plus increased DO, facilitated diversification
of other components of the Quays ecosystem. Zooplankton abundance initially
increased in the enclosed basins from around 1 L
1
in Basin 9 in May 1988 to
500 L
1
by July due to a rise in copepod and to a lesser extent cladoceran popula-
tions (White et al.
1993
). Similar seasonal changes occurred in the following
2 years, although the peak in 1990 was much lower. Total zooplankton density
in all basins was positively correlated (stepwise multiple regression, p
0.05)
with algal biomass (as chlorophyll-a concentration; Hendry
1991
). Although total
zooplankton were significantly correlated with some algal groups (p
<
0.05),
no correlation were found with the blue-green algae Oscillatoria and Anabaena.
Cladocerans, in particular Daphnia longispina, were however significantly correl-
ated with blue-green algae (p
<
0.05). The initial increase and subsequent rela-
tive decline in zooplankton abundance may have been a response to the shift
in algal group dominance (White et al.
1993
). It is widely recognised that blue--
green species often form large inedible colonies, and this can be promoted by
a reduction in other algal species by the grazing of large zooplankton (Lynch &
Shapiro
1981
;Williamset al.
2002
). Therefore, 'top-down' control on the algal
biomass from zooplankton was absent in the Quays.
Overpredation from stickleback also affected the zooplankton population
(Hendry et al.
1997
). Depletion of this resource and lack of an alternative food
source caused the condition of stickleback to deteriorate, rendering them
susceptible to secondary fungal infection by Saprolegnia spp. Periodic mass
mortality of stickleback adversely affected the aesthetics of the water as fish
<
