The lack of cost-effective, reliable sampling methods for many wetland
characteristics hinders efforts to describe the structural and functional
properties of wetlands. This study evaluated techniques for sampling the
subsurface hydrology and invertebrates of freshwater wetlands. The depth of
rusting on mild steel rods was compared with water well measurements to
determine the reliability of rust depth as a predictor of subsurface water levels.
An emergence trap and a benthic coring device were compared to determine
the utility of each for sampling the invertebrate fauna of a wetland.
Accuracy of the rods in estimating different water table measurements
(average, lowest, most recent) and comparability of rod data (within sets of
five rods) were investigated for different reference points on the rods,
residence times, and wetland soils. The effect of the presence of vegetation in
a soil low in organic matter on rod accuracy also was evaluated. The depth of
lowest formation of a rust band on the rods predicted average and most recent
water table depths in peat soil (r² for regressions of rust band depth on water
table depth ranged from 0.71-0.95). Estimates of average water table depths
were most precise for peat soil. Accuracy and precision were considerably
lower in sand and clay soils, but significant relationships (P < 0.10) between
depth of rust band formation and water table depth were found for all soils (r²
values for sand and clay ranged from 0.13-0.55). The presence of vegetation
had no effect on rod accuracy in the sand soil. Differences in rod
performance between residence times were not apparent. However, a rod
residence time of 4-6 weeks is recommended to balance the time necessary for
adequate rust formation on the rods and to minimize the chance of exposure
to large changes in water levels. A decrease in water table depth of
approximately 40 cm in one month in the clay wetland caused a month lag
time in rust formation. Differences in depth of rust band formation between
the five rods within replicate sets were greatest for rods from clay (mean SD
= ±7.9 cm). Variability of rust band measurements within replicate sets was
lower in peat (mean SD = ±2.3 cm) and sand (mean SD = ±2.6 cm). The
results indicated that the rusty rod technique has serious limitations and should
be applied only in situations where the use of standard methods must be
restricted.
Emergence traps and a benthic coring device were used to sample the
invertebrates of a freshwater, emergent wetland during late spring and
summer, 1989. The fauna captured by each technique, disparities between the
techniques in sampling certain taxa, and factors potentially affecting abundance
estimates were examined. In addition, the efficiency of each technique,
expressed as the number of samples required to achieve a desired level of
precision, in estimating mean abundances of the dominant invertebrate group,
the Chironomidae, was evaluated.
Total and monthly estimates of insect family richness were higher for
continuous sampling of emergence than for monthly core samples of the
benthos. Emergence traps also caught a greater variety of the insect taxa
inhabiting the wetland. The precision and efficiency of each technique in
estimating abundances of the dominant group, the Chironomidae, varied
between months and habitats (open water; vegetation). The variation was
most likely due to the natural spatial and temporal variations inherent in
invertebrate populations. The number of samples required (n[subscript r]) to estimate
mean Chironomidae abundances for the entire summer, June-September, to a
precision of D= 0.20 (equivalent to a standard error equal to 20% of the
mean), varied between techniques. Fewer sampling stations would have been
required to estimate mean adult abundances using emergence traps than would
have been required for estimates of larval abundances using benthic core
samples. Large numbers of benthic cores (27-208 individual cores per habitat)
would have generally been required for both monthly and seasonal estimates of
non-insect invertebrate abundances. Labor costs for processing emergence
samples were about 30% of those for benthic samples. Subsampling of
dominant groups in the emergence samples would have further reduced costs.
Frequent sampling throughout a season, with several different techniques,
is required to completely characterize the invertebrate community of a
wetland. This study compared two quantitative techniques for sampling
wetland insects. Continuous sampling with emergence traps provided higher
estimates of insect family richness and more precise estimates of Chironomidae
abundances at a lower cost per sample than monthly core samples of the
benthos. / Graduation date: 1992
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/37134 |
| Date | 03 June 1991 |
| Creators | Harenda, Mary G. |
| Contributors | Kentula, Mary E. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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