Environmental Engineering Reference
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such as temperature and light availability have on these processes. This is clearly
illustrated by the differences in wetland water quality that can exist between
different habitat types in a single wetland. For example, due to the link between
CO 2 and the carbonic acid buffering system in water (Wetzel and Likens 2000 , also
see Table 6.1 ), uptake of CO 2 by photosynthesizing aquatic plants increases pH
while release of CO 2 by respiring organisms decreases it. The competing effects of
photosynthesis and respiration similarly influence dissolved oxygen levels which
can have effects on other important parameters such as the oxidation and reduction
(redox) potential in water and sediments.
Stands of aquatic macrophytes in particular can have a major influence on
commonly measured water quality variables. Dense mats of floating plants and
high levels of algal biomass have both been associated with localized increases
in water temperature (in some cases by as much as 11 C, see Reeder ( 2011 ) for
discussion), while shading from emergent vegetation may locally reduce water
temperature (Rose and Crumpton 1996 ). High microbial respiration associated
with decaying plant biomass and reduced diffusion of atmospheric oxygen often
leads to near anoxic conditions and reduced pH of the water surrounding beds
of emergent and submergent plants as compared to open water zones (Chimney
et al. 2006 ; Rose and Crumpton 2006 ).
Distinct vertical profiles in water quality variables due to thermal stratification of the
water column are well described for deeper ponds and lakes, and may also be observed
in the wetland water column. Ryder and Horwitz ( 1995 ) report significant differences
in temperature, pH, conductivity, dissolved oxygen and redox potential between the
surface and bottom of the permanently inundated zone of a depressional wetland in
Australia. This stratification was observed in water less than 1.5 m deep, was most
pronounced near stands of macrophytes, and exhibited a diel pattern of formation from
early afternoon to early evening. Diel cycles of thermal stratification and associated
vertical profiles of dissolved oxygen and dissolved methane were also observed in a
shallow Australian floodplain wetland (Ford et al. 2002 ). In this study, surface water
oxygen levels were highest in late afternoon and would sometimes be near zero by
morning due to high respiratory demand. Boeckman and Bidwell ( 2007 ) also observed
summertime thermal stratification across a maximum depth of 30 cm in an Oklahoma
depressional wetland that resulted in vertical profiles of dissolved oxygen, pH, and
suspended solids. The shallow nature of wetlands can make stratification of the water
column quite transient as it is easily disrupted by wind (Boeckman and Bidwell 2007 ).
However, stratification may still have an influence on wetland functional processes as
indicated by Ryder and Horwitz ( 1996 ) who attributed reduced leaf processing in
certain areas of the wetland they studied to limitations on microorganisms and
invertebrates imposed by the diurnal stratification of the water column.
6.4.4 Temporal Influences
Temporal changes in wetland water quality can be driven by changing hydrologic
conditions (see previous discussion of wetland water sources and hydroperiod) and
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