Underestimation of rotifer abundance a much greater problem than previously appreciated

Abstract

Although rotifers are important components of aquatic food webs and suitable sampling methods have been described and tested in the peer-reviewed literature, they are frequently overlooked or quantified with improper methods (e.g., mesh sizes ≥ 63 µm) in freshwater ecology studies. As a result, we believe that the role of rotifers in aquatic food webs and ecosystem processes remains underappreciated, and this conceptual shortfall is exacerbated by the continued use of improper sampling methodology. We examined density and biomass estimates of metazoan zooplankton in the Upper Mississippi and Missouri rivers from two sampling methods. The macrozooplankton method was designed to target cladocerans, adult and juvenile copepods, by filtering 180 L water through a 63-µm mesh. The microzooplankton method was designed to target rotifers and copepod nauplii by filtering 18 L water through a 20-µm mesh. The macrozooplankton method underestimated the density and biomass of common rotifers by two to three orders of magnitude, a far greater amount of error than reported in previous studies. This bias in density estimates for rotifers decreased with increasing mean length of rotifers. The microzooplankton method proved to be ineffective for quantifying cladoceran species richness or cladoceran density at the species level. We urge aquatic ecologists to match their sampling methodology with the specific goals of their study. An accurate understanding of the role of rotifers in freshwater ecosystems will only be possible when the use of appropriate methodology becomes the rule rather than the exception.

Spring rotifer community structure in the Alcantara River (Sicily, Italy), using different mesh size nets: relation to environmental factors

“…Moreover, different mesh size nets result in different ratios between zooplankton groups, as evident from the data of the present study, with no significant effect on the less abundant groups but a significant underestimate of the almost all dominant rotifers species and a different ratio between the overwhelming two rotifer subclasses. More remarkable differences in densities were estimated when entire zooplankton and rotifer communities were sampled by 63 and 20 mm mesh size nets, respectively with two or three magnitude order abundance variations (Chick et al, 2010).…”

“…The abundance found in 55 mm mesh size net community are similar to those recorded in that collected by 35 mm mesh size one from the Aliakmon River (Greece) in spring (Zarfdjian et al, 2000). According to this study, maximum densities are lower than those of large rivers which often exceed 10 6 ind.m x3 (Klimowicz, 1981;Ferrari et al, 1989;Van Dijk and Van Zanten, 1995;Chick et al, 2010). Alcantara River with only 60 m 3 .s x1 of water discharge, as well as rivers Aliakmon (Zarfdjian et al, 2000) and Illinois (Brown et al, 1989), has a lower water discharge and so a more reduced water volume than other large rivers (e.g., the river Rhine 1000-800 m 3 .s x1 , Van Dijk and Van Zanten, 1995).…”

“…Their importance in aquatic food webs (Porter, 1995;Jurgens et al, 1999;Miracle et al, 2007) is explained also at the higher trophic levels, as prey items for fishes (Bass et al, 1997;Chick and Van Den Avyle, 1999;Sampson et al, 2008). This group has been far less studied relative to crustacean zooplankton (Chick et al, 2010). Chick et al (2010) explained this tendency to overlook rotifers by methodological roots that caused an underestimation of rotifer abundances.…”

“…This group has been far less studied relative to crustacean zooplankton (Chick et al, 2010). Chick et al (2010) explained this tendency to overlook rotifers by methodological roots that caused an underestimation of rotifer abundances. Net mesh size variations have a significant effect on the organism capture and prey selection and thus the choice of the mesh size depends on the taxa to be sampled and it varies mainly in relation to the size of the organisms and their ability to avoid the sampler.…”

A comparative analysis of zooplankton field collection and sample enumeration methods

“…For example, investigators should assure that their mesh size adequately targets the size of the zooplankton of interest (e.g., rotifers versus Daphnia spp. ; Chick et al 2010). Similarly, in choosing a zooplankton counting method, one desires a fast, affordable, and replicable technique that also achieves an acceptable level of precision and accuracy (Gannon 1980).…”