2.2 Where, What, and When to Sample
31
be considered. Such a survey could range, depending on resource availability, from a sampling of a few variables to a study of full water chemistry.
A screening survey to identify candidate sites for further study could be
focused mainly on such parameters as specific conductance and perhaps
field pH, with the possibility of full laboratory chemistry for only a subset
of the sites.
2.2.1.1.3 Candidates for Inclusion
Prior to conducting either a random or a nonrandom survey of lakes or
streams, it is important to determine what kinds (classes) of lakes or streams
should be included as candidates for sampling. Preselection of classes of water
bodies to include or exclude may change the population frame in statistically
based sampling or simply change the waters that are candidates for selection
in a nonrandom design. Candidate lakes might be restricted by hydrologic
type (to drainage lakes or seepage lakes, for example), lake size, topographic
position, ease of access, or depth. Candidate streams might be restricted to
NUTRIENT STATUS
A rough evaluation of the nutrient status of a lake or stream can
be made on the basis of the molar ratio of total N to total P in solution. This determination was formerly based on the Redfield N:P
ratio equal to 16. More recent compilations of experimental data
(cf. Elser et al. 2009) suggest a cutoff near 44 for N- versus P-limited
freshwater lakes. If the ratio is above about 44, the water body is
presumed to be P limited, and further addition of N would not be
expected to have a large effect on primary productivity. If the ratio
is below about 44, the water body is presumed to be N limited and
therefore may be sensitive to nutrient enrichment effects from N
addition. Such a rough evaluation should be based on multiple samples (at least 10) collected across multiple seasons (ideally spring
through fall), as the nutrient status can change with season or with
short-term changes in flow or other conditions. Because this is an
area of active research, such interpretation should be considered
uncertain and subject to change.
A more complete evaluation of nutrient status should be based on
laboratory, and perhaps in situ, nutrient addition experiments. This
might add N and P, individually and combined, to laboratory flasks or
in situ enclosures, with measurement of nutrient concentrations and
chlorophyll a or some other measure of primary productivity.
31
be considered. Such a survey could range, depending on resource availability, from a sampling of a few variables to a study of full water chemistry.
A screening survey to identify candidate sites for further study could be
focused mainly on such parameters as specific conductance and perhaps
field pH, with the possibility of full laboratory chemistry for only a subset
of the sites.
2.2.1.1.3 Candidates for Inclusion
Prior to conducting either a random or a nonrandom survey of lakes or
streams, it is important to determine what kinds (classes) of lakes or streams
should be included as candidates for sampling. Preselection of classes of water
bodies to include or exclude may change the population frame in statistically
based sampling or simply change the waters that are candidates for selection
in a nonrandom design. Candidate lakes might be restricted by hydrologic
type (to drainage lakes or seepage lakes, for example), lake size, topographic
position, ease of access, or depth. Candidate streams might be restricted to
NUTRIENT STATUS
A rough evaluation of the nutrient status of a lake or stream can
be made on the basis of the molar ratio of total N to total P in solution. This determination was formerly based on the Redfield N:P
ratio equal to 16. More recent compilations of experimental data
(cf. Elser et al. 2009) suggest a cutoff near 44 for N- versus P-limited
freshwater lakes. If the ratio is above about 44, the water body is
presumed to be P limited, and further addition of N would not be
expected to have a large effect on primary productivity. If the ratio
is below about 44, the water body is presumed to be N limited and
therefore may be sensitive to nutrient enrichment effects from N
addition. Such a rough evaluation should be based on multiple samples (at least 10) collected across multiple seasons (ideally spring
through fall), as the nutrient status can change with season or with
short-term changes in flow or other conditions. Because this is an
area of active research, such interpretation should be considered
uncertain and subject to change.
A more complete evaluation of nutrient status should be based on
laboratory, and perhaps in situ, nutrient addition experiments. This
might add N and P, individually and combined, to laboratory flasks or
in situ enclosures, with measurement of nutrient concentrations and
chlorophyll a or some other measure of primary productivity.