1.2 Study Design
19
years but may not represent the conditions of greatest interest. For example,
surveys to assess the effects of nutrient input or acidic deposition on surface
water chemistry can substantially underestimate impacts if low-flow periods
are used for sampling (Lawrence et al. 2008). Some studies endeavor to collect
one or more samples during high-flow periods (heavy rain or snowmelt), perhaps to augment index chemistry sampling. During high-flow periods, hydrologic conditions tend to cause
• relatively high NO 3
− , Al i , and P concentrations,
• relatively low pH and ANC, and
• variable concentrations of SO 4 and BC (depending on local
characteristics).
Therefore, the most stressful conditions (to aquatic biota) often occur
during high-flow periods. The range of difference between high-flow and
low-flow chemistry varies by region, with drought cycles, and by individual
water body. In general, we recommend that characterization of lakes and
streams be represented by one or more index samples for a given year, plus
at least two additional samples during snowmelt or rainstorm to partially
characterize variability. It is preferable to collect characterization data for
at least 2 years and to document interannual variability associated with wet/
drought cycles.
1.2.2 Synoptic Survey
Synoptic surveys of lake or stream chemistry within a designated study area
are usually collected at times expected to exhibit fairly stable water chemistry. For acidic and nutrient deposition monitoring, this is usually spring base
flow for streams in the southeastern United States and is the summer or fall
index period * for lakes or streams in regions that typically develop substantial snowpack. Typically, one sample, sometimes with replicates, is collected
for each lake or stream that was selected for sampling. Synoptic surveys are
ideally (but not always) statistically based, allowing extrapolation of sample
results for individual water bodies to the regional population of interest. It is
desirable to standardize conditions at each site at the time of sample collection
to generally consistent weather and runoff conditions. For that reason, periods of high temporal variability such as heavy rain and periods of rapid snowmelt, or periods with heavy smoke from wildfires, are typically avoided to the
extent possible during a synoptic survey. However, if assessment of acidic or
* An index period is a relatively narrow period of time for synoptic sampling (often a 2-month
window during spring, summer, or fall) intended to represent the lake or stream chemistry for
that year. Typically, rain or snowmelt conditions are avoided when collecting index samples.
19
years but may not represent the conditions of greatest interest. For example,
surveys to assess the effects of nutrient input or acidic deposition on surface
water chemistry can substantially underestimate impacts if low-flow periods
are used for sampling (Lawrence et al. 2008). Some studies endeavor to collect
one or more samples during high-flow periods (heavy rain or snowmelt), perhaps to augment index chemistry sampling. During high-flow periods, hydrologic conditions tend to cause
• relatively high NO 3
− , Al i , and P concentrations,
• relatively low pH and ANC, and
• variable concentrations of SO 4 and BC (depending on local
characteristics).
Therefore, the most stressful conditions (to aquatic biota) often occur
during high-flow periods. The range of difference between high-flow and
low-flow chemistry varies by region, with drought cycles, and by individual
water body. In general, we recommend that characterization of lakes and
streams be represented by one or more index samples for a given year, plus
at least two additional samples during snowmelt or rainstorm to partially
characterize variability. It is preferable to collect characterization data for
at least 2 years and to document interannual variability associated with wet/
drought cycles.
1.2.2 Synoptic Survey
Synoptic surveys of lake or stream chemistry within a designated study area
are usually collected at times expected to exhibit fairly stable water chemistry. For acidic and nutrient deposition monitoring, this is usually spring base
flow for streams in the southeastern United States and is the summer or fall
index period * for lakes or streams in regions that typically develop substantial snowpack. Typically, one sample, sometimes with replicates, is collected
for each lake or stream that was selected for sampling. Synoptic surveys are
ideally (but not always) statistically based, allowing extrapolation of sample
results for individual water bodies to the regional population of interest. It is
desirable to standardize conditions at each site at the time of sample collection
to generally consistent weather and runoff conditions. For that reason, periods of high temporal variability such as heavy rain and periods of rapid snowmelt, or periods with heavy smoke from wildfires, are typically avoided to the
extent possible during a synoptic survey. However, if assessment of acidic or
* An index period is a relatively narrow period of time for synoptic sampling (often a 2-month
window during spring, summer, or fall) intended to represent the lake or stream chemistry for
that year. Typically, rain or snowmelt conditions are avoided when collecting index samples.