Biological field and laboratory methods for measuring the quality of ...
Biological field and laboratory methods for measuring the quality of ...
Biological field and laboratory methods for measuring the quality of ...
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Because <strong>of</strong> <strong>the</strong> extreme spatial <strong>and</strong> temporal<br />
fluctuations <strong>of</strong> salinity in estuaries, simple, rapid<br />
instrumental <strong>methods</strong> <strong>of</strong> measurement are more<br />
desirable than slower, more precise chemical<br />
<strong>methods</strong> (38).<br />
Wide-range, temperature-compensated conductivity<br />
salinometers are recommended <strong>for</strong><br />
determining both horizontal <strong>and</strong> vertical salinity<br />
pr<strong>of</strong>iles at high-slack <strong>and</strong> low-slack tide levels in<br />
<strong>the</strong> area <strong>of</strong> estuary or reach <strong>of</strong> river being<br />
studied.<br />
3.0 SAMPLING METHODS<br />
3.1 QUANTITATIVE<br />
3.1.1 Definitions <strong>and</strong> purpose<br />
Although <strong>the</strong> data may be evaluated in various<br />
ways, a quantitative method essentially involves<br />
an estimation <strong>of</strong> <strong>the</strong> numbers or biomass<br />
(st<strong>and</strong>ing crop) <strong>of</strong> <strong>the</strong> various components <strong>of</strong><br />
<strong>the</strong> macroinvertebrate community per unit area<br />
in all or a portion <strong>of</strong> <strong>the</strong> available habitats<br />
(including artificially introduced habitats) in <strong>the</strong><br />
ecosystem being studied, <strong>and</strong> provides in<strong>for</strong>mation<br />
on <strong>the</strong> species composition, richness <strong>of</strong><br />
species <strong>and</strong> distribution <strong>of</strong> individuals among <strong>the</strong><br />
species.<br />
3.1.2 Requirements<br />
Obtain quantitative estimates by using devices<br />
that sample a unit area or volume <strong>of</strong> habitat,<br />
such as a Surber square-foot sampler, which in<br />
use presumably collects all organisms enclosed<br />
within <strong>the</strong> frame <strong>of</strong> <strong>the</strong> sampler, or an artificial<br />
substrate sampler having a fixed volume or<br />
exposing a fixed amount <strong>of</strong> surface.<br />
In <strong>the</strong> study <strong>of</strong> macroinvertebrate populations,<br />
<strong>the</strong> sampling precision is affected by a<br />
number <strong>of</strong> factors, including: size, weight, <strong>and</strong><br />
construction <strong>of</strong> <strong>the</strong> sampling device, <strong>the</strong> type <strong>of</strong><br />
substrate, <strong>and</strong> <strong>the</strong> distribution <strong>of</strong> organisms in<br />
<strong>and</strong> on <strong>the</strong> substrate. For example, it is expected<br />
that <strong>the</strong> estimates <strong>of</strong> st<strong>and</strong>ing crop drawn from a<br />
series <strong>of</strong> samples will be more precise (have a<br />
lower coefficient <strong>of</strong> variation) when <strong>the</strong><br />
community consists <strong>of</strong> a few species represented<br />
5<br />
MACROINVERIEBRAIE SAMPLING<br />
by a large number <strong>of</strong> individuals, evenly distributed<br />
in <strong>the</strong> substrate. Conversely, a large coefficient<br />
<strong>of</strong> variation would be expected if <strong>the</strong><br />
fauna consists <strong>of</strong> a large nUlubcr cf species with<br />
a patchy distribution <strong>of</strong> individuals. To obtain<br />
<strong>the</strong> same level <strong>of</strong> precision at a given level <strong>of</strong><br />
probability, a larger number <strong>of</strong> replicates would<br />
be required in <strong>the</strong> latter case than in <strong>the</strong> <strong>for</strong>mer.<br />
In general, <strong>the</strong> smaller <strong>the</strong> surface area<br />
encompassed by a sampling device, <strong>the</strong> larger <strong>the</strong><br />
number <strong>of</strong> samples required to obtain a desired<br />
level <strong>of</strong> precision. Thus, precision can be<br />
increased by collecting larger samples, or by<br />
increasing <strong>the</strong> numbers <strong>of</strong> samples collected.<br />
An objective, quantitative approach necessitates<br />
that a measure <strong>of</strong> <strong>the</strong> precision <strong>of</strong> <strong>the</strong><br />
estimates be obtained - thus, replicate sampling<br />
in each habitat or stratum selected <strong>for</strong> study is<br />
an absolute requirement. For measurement <strong>of</strong><br />
precision, three replicates are an absolute<br />
minimum. (A series <strong>of</strong> single samples taken at<br />
discrete points along a transect do not represent<br />
replicate samples <strong>of</strong> benthic organisms unless it<br />
can be demonstrated that <strong>the</strong> physical characteristics<br />
<strong>of</strong> <strong>the</strong> habitat do not change along <strong>the</strong><br />
transect.)<br />
It is preferable, if data are available (or can be<br />
obtained by reconniassance or exploratory<br />
studies), to determine <strong>the</strong> number <strong>of</strong> replicates<br />
on <strong>the</strong> basis <strong>of</strong> <strong>the</strong> desired level <strong>of</strong> precision as<br />
discussed in <strong>the</strong> Biometrics Section.<br />
3.1.3 Advantages<br />
In addition to providing <strong>the</strong> same data<br />
obtained from a qualitative study, <strong>the</strong> st<strong>and</strong>ing<br />
crop data generated by a quantitative study provide<br />
a means <strong>of</strong> comparing <strong>the</strong> productivity <strong>of</strong><br />
different environments; <strong>and</strong> if a measure <strong>of</strong><br />
turnover is available, <strong>the</strong> actual production can<br />
be computed.<br />
The use <strong>of</strong> quantitative sampling devices in<br />
carefully chosen habitats is recommended<br />
because <strong>the</strong>y reduce sampling bias resulting from<br />
differences in expertise <strong>of</strong> <strong>the</strong> sample collector.<br />
The data from properly designed quantitative<br />
studies are amenable to <strong>the</strong> use <strong>of</strong> simple but