Efficiencies of Various Grabs and Corers in Sampling Freshwater Benthos

Abstract

None of several samplers compared in limited tests in Lake Ontario gave realistic estimates of benthos in all substrates encountered. The Ponar and Shipek grabs, the only samplers that functioned in gravel, came closest to all-sediment samplers. In sand, however, the Franklin–Anderson grab appeared to be much more efficient than either of these two samplers. In mud the Ekman grabs gave the highest mean numbers of animals per m2.In trials in Lake Winnipeg profundal mud, hand-taken diver's core samples being used as a quantitative standard, only the Fisheries Research Board multiple corer and the standard Ekman grab gave quantitative results for total macrobenthos. However, the multiple corer collected significantly fewer chironomids, and the standard Ekman grab significantly fewer oligochaetes, than did the diver's cores. There were indications that the tall Ekman grab was either not tall enough or too heavily weighted for use in soft sediments and that a small improvement in design could make both this and the standard Ekman more efficient. Most of the samplers apparently sampled some groups of organisms much better than others. Neither the Ponar grab nor the tall weighted Ekman grab sampled the chironomid or oligochaete populations satisfactorily. However, both grabs indicated densities of sphaeriids not significantly different from the diver's samples.

Quantitative sampling of soft-bottom sediments: problems and solutions

“…Otherwise, a hydraulic shock wave (bow wave) created in front of the orifice can wash away flocculent or easily resuspended surficial sediments before the corer reaches the sediment surface. This source of error has been reported for grabs (Wigley 1967, Flannagan 1970, Ankar 1977, openbarrel gravity corers (McIntyre 1971b, Elmgren 1973, Baxter et al 1981, Jensen 1983, Leonard 1990), box corers (Jumars 1975a, b, Thiel et al 1988/1989) and piston corers (McCoy & von Herzen 1971, McCoy 1972, Stowe & Aksu 1978.…”

“…The results of earlier studies reporting actual observations from sampllng trials (Wigley 1967, Flannagan 1970, McCoy & von Herzen 1971, McCoy 1972, Ankar 1977, Seyb et al 1977, Blomqvist 1985 illustrate the importance of visual evaluations of the samplers' performance. This suggests that it is advisable to document the performance photographically, to corroborate asserted sampling qualities, when describing novel devices.…”

“…Also, the sampler may sink down too deeply to provide a representative sample (e.g. Wundsch 1924, Lundbeck 1926, Valle 1927, Flannagan 1970, Blomqvist 1990.…”

Long-Term Decline in Freshwater Mussels (Bivalvia: Unionidae) of the Western Basin of Lake Erie

“…The greater "shockwave" of the Peterson, which blows away small organisms at the sediment surface just before impact, should not have any effect on these large, heavy-shelled forms. The effect is minimal even for the shelled but much smaller Sphaeriidae (Flannagan 1970, Nalepa et al 1988. While grab samplers generally underestimate unionid abundances (Haukioja and Hakala 1974, Isom and Gooch 1986, Nalepa and Gauvin 1988, any bias in abun- dance estimates should be reflected similarly in the three surveys.…”

“…Sampling methods and procedures were basically similar in the three surveys (Table 1). Carr and Hiltunen (1965) (Flannagan 1970), efficiencies of the two grabs are likely to be similar when collecting unionids. The greater "shockwave" of the Peterson, which blows away small organisms at the sediment surface just before impact, should not have any effect on these large, heavy-shelled forms.…”