Environmental Engineering Reference
In-Depth Information
6.6.3 Temporal and Spatial Considerations
As described above, wetland hydroperiod could influence water quality and lead to
divergence between sites due to differences in water loss and associated effects of
evapoconcentration. Ideally, water quality would best be compared between
wetlands at comparable stages of their hydroperiod. Jackson (
2006
) provides a
discussion of approaches that could be used to characterize wetland hydroperiod,
including regular site visits, and use of devices such as staff gauges, automatic
water level monitors and piezometers. Unfortunately, this type of intensive
surveying may not be possible for studies with limited resources or that have the
goal of sampling a large number of wetlands. Species composition of resident plant
assemblages has also been used to provide a general indication of the duration of
wetland hydroperiod (Babbitt et al.
2003
; Sharitz and Pennings
2006
) and this may
assist in either developing finer-scale groupings of wetlands or as ancillary data to
help with interpretation of observed patterns in water quality data.
If the objective of a wetland monitoring study is to obtain water quality data that
represent the system as a whole, potential spatial differences between different habitat
types should be considered (Fig.
6.1
). This could be addressed by first mapping the
wetland to identify major habitat types and tributaries running into the system, and
then using these areas as strata in a stratified random sampling approach. Vertical
stratification of open water zones of the wetland may also need to be considered,
particularly if functional parameters such as carbon processing are being assessed
(e.g. Ryder and Horwitz
1996
). Representative values for the wetland as a whole could
then be generated by simply calculating the arithmetic mean of the data derived from
each habitat type or by using weighted averages to provide proportional representation
for each habitat. Another alternative is to develop composite samples of water derived
from different wetland strata and conduct all chemical analyses on those samples. If
representative sampling of different habitats in the wetland is not possible, limiting
sampling to only the major habitat type may be the next best option. Some effort
should also be made to record water quality measurements from comparable habitat
types in each wetland visited. Unfortunately, published wetland studies that include
collection of water quality data often fail to indicate the type of habitat the data were
derived from or if potential habitat variability in the parameters being measured was
considered in the study design.
Diurnal variation in water quality driven by changing temperatures, photosyn-
thetic activity and respiration may best be addressed by sampling wetlands within a
defined time period (e.g., 10 AM to 2 PM) and some published studies do indicate
the time interval during which water quality measurements were made in wetlands
(e.g., Trebitz et al.
2007
; deCatanzaro and Chow-Fraser
2011
). However, this may
again pose a challenge for studies aimed at sampling a large number of wetlands or
in cases where travel time between sampling sites is significant. This is a good
reason to include the time of day the wetland was sampled on field data sheets since
this information could be used in later data analysis to determine if any diurnal
trends in the data are apparent. Routine water quality monitoring of wetlands often
