Conversion factors as determined by relative macroinvertebrate sampling efficiencies of four common benthic grab samplers

Figures & data

Figure 1. Box plots of dominant macroinvertebrate and total macroinvertebrate densities (No./m²) from collections with each of the tested samplers in silt substrate at site 1. The top, middle, and bottom lines of each box represent the 75th percentile, median, and 25th percentile, respectively. The top and bottom bars represent the 90th and 10th percentiles, respectively, and the points represent outliers.

Table 1. Total benthos and dominant taxon mean (±SE) densities (No./m²) collected with different samplers at three sites during sampler efficiency testing. Logistical constraints prevented the collection of benthos with all samplers at all sites.

Taxon	Shipek	Petite Ponar	Standard Ekman	Large Ekman
Site 1 (silt substrate)	n = 11	n = 10	n = 9	n = 10
Total benthos	2340 ± 373	4604 ± 571	4845 ± 526	7537 ± 620
Oligochaeta	2228 ± 363	4252 ± 535	4343 ± 490	6958 ± 611
Chironomidae	112 ± 25	352 ± 97	502 ± 67	579 ± 61
Site 2 (sand substrate)		n = 10	n = 10	n = 10
Total benthos	Not obtained	2670 ± 276	2161 ± 346	1079 ± 151
Oligochaeta	Not obtained	2335 ± 239	1861 ± 324	787 ± 155
Chironomidae	Not obtained	217 ± 53	239 ± 33	262 ± 21
Bivalvia	Not obtained	117 ± 35	61 ± 24	31 ± 11
Site 3 (sand substrate)	n = 10		n = 10
Total benthos	787 ± 115	Not obtained	1516 ± 218	Not obtained
Oligochaeta	667 ± 97	Not obtained	1426 ± 213	Not obtained
Chironomidae	121 ± 28	Not obtained	90 ± 25	Not obtained

Table 2. Conversion factors determined from mean (±SE) densities collected with each sampler in silt substrate for all invertebrates combined and dominant taxa. A conversion factor was assumed to be 1.0 when there was no statistical difference (p ≤ 0.05) in mean density (No./m²) between samplers.

	Total benthos CF	Oligochaeta CF	Chironomidae CF
Petite Ponar/Shipek = CF	1.97 ± 0.06	1.91 ± 0.06	3.14 ± 0.14
Standard Ekman/Shipek = CF	2.07 ± 0.09	1.95 ± 0.08	4.48 ± 0.33
Large Ekman/Shipek = CF	3.22 ± 0.21	3.12 ± 0.20	5.17 ± 0.50
Standard Ekman/petite Ponar = CF	1.0	1.0	1.0
Large Ekman/petite Ponar = CF	1.64 ± 0.06	1.64 ± 0.06	1.65 ± 0.22
Large Ekman/standard Ekman = CF	1.56 ± 0.04	1.60 ± 0.03	1.0
Note: CF = conversion factor.

Figure 2. Box plots of dominant macroinvertebrate and total macroinvertebrate densities (No./m²) from collections with each of the tested samplers in sand substrate at site 2. The top, middle, and bottom lines of each box represent the 75th percentile, median, and 25th percentile, respectively. The top and bottom bars represent the 90th and 10th percentiles, respectively, and the points represent outliers.

Figure 3. Box plots of dominant macroinvertebrate and total macroinvertebrate densities (No./m²) from collections with each of the tested samplers in sand substrate at site 3. The top, middle, and bottom lines of each box represent the 75th percentile, median, and 25th percentile, respectively. The top and bottom bars represent the 90th and 10th percentiles, respectively, and the points represent outliers.

Table 3. Conversion factors determined from mean (±SE) densities collected with each sampler in sand substrate for all invertebrates combined and dominant taxa. A conversion factor was assumed to be 1.0 when there was no statistical difference (p ≤ 0.05) in mean density (No./m²) between samplers. Conversion factors could not be calculated for all sampler comparisons because benthos were not collected with all samplers at all sites (see Table 1).

	Total benthos CF	Oligochaeta CF	Bivalvia CF	Chironomidae CF
Standard Ekman/Shipek = CF	1.93 ± 0.01	2.14 ± 0.01		1.0
Standard Ekman/petite Ponar = CF	1.0	1.0	1.0	1.0
Large Ekman/petite Ponar = CF	0.40 ± 0.02	0.34 ± 0.03	1.0	1.0
Large Ekman/Standard Ekman = CF	0.50 ± 0.01	0.42 ± 0.01	1.0	1.0
Note: CF = conversion factor; blank cell = conversion factor not calculated.

Table 4. Studies that have tested the efficiency of benthic samplers of interest in this study. Although other samplers may have been tested in the listed studies, only the Shipek, petite Ponar, standard Ekman, and large Ekman are included in the summary. Conversion factors were calculated from significantly different (p ≤ 0.05) sample means when not provided directly by the author(s).

Study	Samplers tested	Target organism(s)	Replicates per sampler	Substrate description	Conversion factor
Flannagan (1970)	Double Shipek, standard Ekman	Total invertebrate density	n = 10–12	Mud	Standard Ekman/double Shipek = 3.0
Flannagan (1970)	Single Shipek, standard Ekman	Total invertebrate density	n = 3 for each substrate type	Mud, sand, gravel	No significant difference due to small sample size
Schloesser and Nalepa (2002)	Petite Ponar, large Ekman	Mayfly (Hexagenia spp.) density	n = 3 for each sampling date	Mud	Petite Ponar/large Ekman = 1.0
Shostell and Williams (2005)	Petite Ponar, standard Ekman	Total invertebrate density	n = 5–10 for each substrate type	Mud, clay/gravel/mud	Mud: petite Ponar/standard Ekman = 1.0; clay/gravel/mud: petite Ponar/standard Ekman = 2.2
This study	Single Shipek, petite Ponar, standard Ekman, large Ekman	Total invertebrate, Oligochaeta, Chironomidae, and Bivalvia densities	n = 9–11 for each substrate type	Mud, sand	See Tables 2 and 3

Flannagan , JF . 1970 . Efficiencies of various grabs and corers in sampling freshwater benthos . Journal of the Fisheries Research Board of Canada , 27 : 1691 – 1700 .

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Schloesser , DW and Nalepa , TF . 2002 . Comparison of 5 benthic samplers to collect burrowing mayfly nymphs (Hexagenia spp.:Ephemeroptera:Ephemeridae) in sediments of the Laurentian Great Lakes . Journal of the North American Benthological Society , 21 : 487 – 501 .

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Shostell , JM and Williams , BS . 2005 . A new benthic sampling device for soft sediments in shallow habitats . Journal of Freshwater Ecology , 20 : 595 – 602 .

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