Environmental Engineering Reference
In-Depth Information
winter and to desiccation during summer. The plans by
water managers in the last 20 years, to allow more
dynamic water levels in set-aside nature areas, espe-
cially, entail extending the upper and lower limits of
the permissible annual water level fl uctuations (Coops
& Hosper 2002). Near-natural water levels that allow
wider fl uctuations than the current 'fi xed ' levels are
considered the best option by most water managers.
A good case study on WLF management is that of
Lake Okeechobee (Florida, United States, headwaters of
the Everglades). As a subtropical lake ecosystem, it
exhibits large seasonal and interannual variation in
rainfall and high evapotranspiration. This can result in
large and unpredictable variations in water level that
can affect ecosystem dynamics. In response to this, as
well as to other environmental concerns, a 30-year
plan was initiated to construct structures to restore the
quantity, quality, timing and distribution of water
(Havens 2002). This overall programme is called Com-
prehensive Everglades Restoration Program (CERP).
CERP is expected to reduce the occurrence of damaging
high and low WLF and to increase the occurrence of
spring water level recessions that benefi t native biota.
Johnson et al . (2007) studied the response to hydrologi-
cal variations in Lake Okeechobee of both vegetation
and fi sheries. Their fi ndings indicate that lakes with a
high water level and prolonged fl ooding are likely to
show substantial losses of aquatic and wetland plants
as well as a negative impact on fi sh populations, as is
the case at Lake Okeechobee. In contrast, shallower
lakes with moderate water level variations were
reported to have widespread stands of emergent and
submerged aquatic vegetation in most years. Conse-
quently, habitat structure was improved and food
resources increased for largemouth bass ( Micropterus
salmoides ) and other fi sh species, alligators, wading
birds and snail kites ( Rostrhamus sociabilis ).
tions in the upper mixed layers, so that the dominant
phytoplankton shifts to noncyanobacterial forms. In
the Netherlands, artifi cial mixing is used to disrupt
stratifi cation of the water column (i.e. a water column
that is divided into an upper and a lower layer due to
the increase in spring and summer temperature) to
prevent bloom formation by cyanobacteria (Huisman
et al . 2004). Thus, mixing or circulation, unlike nutri-
ent reduction, produces instantaneous improvements
in water quality, and without decreasing the nutrient
concentrations or loading. However, the technique
seems to be more effective for lakes in which stratifi ca-
tion of the water column occurs.
18.4
PERSPECTIVES
Although in many European countries freshwater
lakes and reservoirs constitute only a small fraction
of the land area, their importance for human health,
recreation and national economies is considerable.
There is obviously an urgent need to further curtail,
divert and treat the unwarranted inputs into the lakes
and reservoirs of nutrients, organics, silt and contami-
nants from the watershed. Despite more than a quarter
century of nutrient reduction measures in runoff
waters, many lakes still have poor water quality. The
principal cause for the delay of responses to corrective
measures is the large stockpiles of P in lake sediments.
The slow release of this element allows eutrophication
symptoms to persist and even proliferate. Secondly,
inadequate nutrient reductions in the runoff waters
and the virtually unabated and diffuse inputs from the
agricultural grounds in the lake catchment areas act
as a major bottleneck to lake restoration. It is, there-
fore, diffi cult to predict the response of an ecosystem
under restoration. Thus, further manipulations of both
chemical and biological processes are needed to sustain
the positive effects of the corrective measures. However,
monitoring of water quality and restoration of the in-
lake processes alone will be a futile exercise if not
carried out in a watershed context. This latter involves
documenting the entire landscape setting, including
habitat type, hydrological regime, soil properties,
topography and invasive species, which all can hamper
restoration measures. Moreover, chances of improving
a lake are often better if the chosen measures comple-
ment each other.
It is diffi cult to draw any conclusions on the effects
of climate change on lake restoration. At present, we
Mixing of the w ater c olumn
Oxygen-poor water conditions can be counteracted by
artifi cial mixing, which involves aerating or oxygenat-
ing lakes using pumps, jets and bubbling air at the lake
bottom. The technique has been used in the United
States to prevent fi sh dying during ice cover (Cooke
et al . 2005). Aeration oxidizes substances in the water
column, allows removal of Fe and Mn from the water
into the sediments more rapidly and reduces internal P
loading from sediment. Aeration seems to prevent
cyanobacteria from exploiting the optimal light condi-
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