Evaluating alternative macroinvertebrate sampling methodologies in wetlands: Influence of sieve mesh size on relationships between environmental and assemblage variables
Michael J. Weber
By definition, a macroinvertebrate is any invertebrate that is large enough to be retained on a 500-Ã Âµm mesh sieve. Although macroinvertebrates (hereafter invertebrates) are important in food webs and sensitive to environmental conditions, they are often omitted from wetland condition monitoring programs because use of this fine-mesh sieve makes sample collection and processing time consuming and expensive. The objective of this study was to identify a more cost-effective approach for obtaining invertebrate-based data that can be used to evaluate wetland condition. In 2014 and 2015, invertebrates and associated particulate matter (PM; living and dead plants, sediment) were collected from 27 wetlands in the Iowa Prairie Pothole Region using a stovepipe sampler. Sample material retained on a 500-Ã Âµm mesh sieve was preserved. In the laboratory, samples were washed through a series of sieves that separated invertebrates and particulate matter into four size fractions. The 6-mm fraction included material retained on the 6-mm sieve, the 4-mm fraction included material retained on both 4- and 6-mm sieves, the 2-mm size fraction included material retained on 2-, 4-, and 6-mm sieves, and the 500-Ã Âµm fraction was the sum total of material retained on 2-, 4-, 6-, and 500-Ã Âµm sieves (i.e., the entire sample). Volume of each sample size fraction and time required to separate invertebrates from PM were recorded. Invertebrates were identified to family (mollusks, insects, isopods), order (amphipods, decapods), or class (annelids), and numbers of individuals were recorded. Subsequently, invertebrates were again washed through the sieve column in the absence of PM, and organisms in each size fraction were identified and counted. Values for invertebrate numerical density and taxon richness variables were quantified for each of the four invertebrate size fractions and when PM was present (+ PM) and absent (- PM). Invertebrate assemblage variables were related to environmental variables indicative of wetland condition (e.g., turbidity, chloride concentration, fish biomass, tiger salamander abundance, plant cover). Relationship strengths were affected by sieve mesh size, invertebrate variable measured, and presence or absence of PM in samples. Across the entire gradient of sieve mesh sizes and PM abundance, invertebrate taxon richness variables were more consistently and strongly related to environmental variables than invertebrate density variables. Regardless of sieve mesh size and PM presence, invertebrate taxon richness exclusive of four taxa recorded in every wetland (planorbid snails, oligochaetes, leeches, chironomid midges; TTR-PEOC) was positively correlated with plant cover and negatively correlated with turbidity and fish biomass. Use of a 2-mm, 4-mm, or 6-mm mesh sieve reduced sample volume by 19-35% and time required to separate invertebrates from PM by 36-54%, relative to use of a 500-Ã Âµm mesh sieve. Results presented here indicate that use of an invertebrate taxon richness metric in which ubiquitous taxa are eliminated from analysis, and using a sieve mesh size of 6 mm, will generate cost savings in wetland monitoring while still producing data that accurately reflect wetland condition.