Presence of long-lived invertebrate taxa and hydrologic permanence

Abstract

The presence of long-lived invertebrate taxa has been used as a criterion for determining hydrologic status in streams based on the assumption that water is present for a sufficient period of time to allow the organisms to complete their life cycle. However, analysis of life-history strategies and age distribution of several long-lived taxa (Anchytarsus bicolor, Oulimnius latiusculus, Ameletus, Tipula, and Cambaridae) from headwater streams in West Virginia suggest that the presence of long-lived taxa is an insufficient indicator of hydrologic status. Anchytarsus bicolor, O. latiusculus, and Ameletus exhibited distinct cohort differences between ephemeral and perennial locations. These three taxa do not have life-history strategies for remaining on-site when the stream dries, suggesting that their presence in ephemeral sites was due to recent colonization. Tipula and Cambaridae did not show different size ranges between ephemeral and intermittent locations, possibly because of their ability to burrow into the substrate. The upper reaches of these headwaters can provide short-term habitat for opportunistic colonizers and relatively long-term habitat for invertebrates with specialized life-history strategies, such as burrowing. Life-history strategies and age distribution should be carefully considered before using the mere presence of long-lived taxa as an indicator of hydrologic permanence.

Keywords:

• ephemeral
• intermittent
• macroinvertebrates
• life cycle
• burrowing
• streambed drying

Introduction

Ephemeral, intermittent, and perennial streams are generally defined by flow permanence (Bain and Stevenson 1999; Fritz et al. 2006). Perennial streams flow all year, with the streambed always below the water table. Intermittent streams flow in response to precipitation and when the water table is raised over the streambed for some portion of the year. Ephemeral streams flow only in response to precipitation, with the bed at all times above the water table (Bain and Stevenson 1999). Definitive, continuous monitoring of flow is impractical in most streams, so identification of hydrologic permanence in headwater sites can be uncertain, even though it may be necessary for determining if a stream meets certain jurisdictional criteria (Nadeau and Rains 2007).

In lieu of continuous monitoring, various geomorphic, hydrologic, and biologic criteria have been derived for assessments of hydrologic permanence (Fritz et al. 2006). One biological criterion is the presence of ‘long-lived taxa’, (NCDWQ 2010; TDEC 2011). Long-lived taxa usually have a life cycle of one year or more, although shorter time frames (e.g., three to six months of aquatic existence) have also been proposed. It is assumed that their presence indicates that water (or at least a damp condition) is present for a long enough time period that the individuals can carry out the aquatic portion of their life cycle.

However, if these taxa are represented only by young instars or vagile adults, then it cannot be assumed that they are indicative of extended water presence. Very young larvae may indicate recent colonization and adults may be able to leave the water through aerial dispersal (AWWQRP 2006). Similarly, some long-lived taxa have life-history strategies that could allow them to survive periods of no flow. A closer examination of the age and/or life-history strategies of long-lived taxa is warranted, especially if the assessment is for jurisdictional purposes.

Methods

We used samples collected at the headwaters of 12 streams in protected natural areas of West Virginia (Figure 1) to test the hypothesis that long-lived taxa can be indicative of perennial status. Two sites on each stream, an ‘ephemeral’ site and an ‘intermittent’ site, were sampled once in May 2010 after precipitation events within the previous five days. The ephemeral site was located at the upstream terminus of flowing water at or near the channel initiation point. The intermittent site was located downstream where wetted width equaled 0.3 m. Due to lack of continuous hydrology data, the designation of sites as ephemeral or intermittent is necessarily tenuous. However, observations of other geomorphic (e.g., defined channel formation), hydrologic (e.g., springs), and biologic (e.g., vegetation) characters during sampling suggest that these designations are reasonable. The intermittent site was typically no more than 200 m downstream from the ephemeral site.

Figure 1. Protected natural areas and stream sites in southern West Virginia where benthic macroinvertebrate samples were collected in May 2010. Six streams were in the Big Ugly Wildlife Management Area, four streams were in the Kanawha State Forest, and two streams were in the Laurel Lake Wildlife Management Area.

Invertebrate samples were collected in a single, comprehensive sample by kicking through all habitats within a 100 m reach using a kick net with mesh size of 500 µm. Samples were preserved in 95% ethanol and processed in the laboratory. Long-lived taxa were identified based on their inclusion in official stream permanence criteria documents (NCDWQ 2010; TDEC 2011) and verified with voltinism data in Hobbs (1976), Brigham et al. (1982), and Merritt et al. (2008).

We measured maximum head capsule width of all intact immature individuals of the ptilodactylid beetle Anchytarsus bicolor (n = 15), the elmid beetle Oulimnius latiusculus (n = 6), and the mayfly genus Ameletus (n = 377), total length of all individuals of the crane fly genus Tipula (n = 12), and carapace length (tip of cephalic spine to middorsal posterior margin) of all intact cambarid crayfish (n = 69) collected across all samples. In each group, the body parts that were measured grow reliably and predictably over time (Dyar 1890) and were used as indicators of the relative ages of the individuals through development of a frequency distribution analysis. Data were analyzed using two-tailed t-tests in NCSS (Hintze 2009) to determine if significant differences existed between ephemeral and intermittent sites for each taxon.

For reference, Ameletus is represented by five species in West Virginia, but nymphs are not all separable to species (Kondratieff and Meyer 2010). Ameletus has a univoltine life cycle, with spring and fall cohorts in West Virginia (Matthews and Tarter 1989; Merritt et al. 2008). Anchytarsus bicolor and O. latiusculus each have a two-year life cycle (White and Roughley 2008). Tipula can have life cycles ranging from six weeks to five years (Byers and Gelhaus 2008), but they are still listed as long-lived taxa (NCDWQ 2010; TDEC 2011). Crayfish also have multi-year life cycles (Hobbs 2001). All crayfish large enough to permit further identification were Cambarus, represented by at least 10 species in West Virginia (Hobbs 1976). Other long-lived taxa were too infrequently collected (n < 6) to allow for statistical analysis.

Results and discussion

Across all 12 streams, there was no overlap in head capsule widths in A. bicolor (t = −6.419, df = 13, p < 0.001) and O. latiusculus (t = −9.323, df = 4, p < 0.007) between the ephemeral and intermittent sites (Figure 2). Although there was overlap in the head capsule width measurements in Ameletus (Figure 2), there was a statistically significant difference between ephemeral and intermittent site size ranges (t = −9.447, df = 375, p < 0.001). Tipula and Cambaridae did not show statistically different size ranges between ephemeral and intermittent sites (Figure 2; t = 0.905, df = 10, p = 0.387; and t = −1.704, df = 55, p = 0.094, respectively).

Figure 2. Box plots (including 90th, 75th, 50th, 25th, and 10th percentiles) of measurements of selected long-lived organisms from headwaters of streams in southern West Virginia, May 2010. Ephemeral sites were located at the upstream terminus of flowing water and intermittent sites were located where stream width first reached 0.3 m.

Upstream ephemeral reaches are primarily colonized opportunistically as adults fly in from perennial reaches further downstream or from overland sources and lay eggs (Williams and Hynes 1976; Williams 1977, 2001). In our sites, where recent rains provided ephemeral habitat by moving the upstream terminus of contiguous flow further upgradient, only recent colonization could occur for A. bicolor, O. latiusculus, and Ameletus, which do not have life-history strategies allowing them to persist in dry conditions. Even though these three taxa were present in the ephemeral sites, the previous year's cohort was absent, suggesting lack of permanent flow to support a full life cycle for these organisms.

Some life-history strategies, particularly burrowing into the substrate, may allow some taxa to remain in ephemeral sites and to tolerate drying streams better than others. Stout and Wallace (2003) found long-lived taxa in similar habitats in West Virginia and Kentucky, reasoning that they avoided dry conditions by burrowing or moving to intermittent pools. Many crayfish burrow into wet substrate or to the hyporheic zone to survive no-flow stream conditions (Hobbs 1976; Hobbs 2001) and many species of Tipula are semiterrestrial (Byers and Gelhaus 2008). Since the previous year's cohort of Tipula and Cambaridae could be present in our ephemeral sites, our results also support the notion that some long-lived taxa can survive no-flow conditions, possibly by burrowing. Unfortunately, we were unable to observe these behaviors and it is also possible that the species of Tipula and Cambaridae that we collected in the ephemeral sites were not the same as those collected in the intermittent sites.

This study suggests that the mere presence or absence of long-lived taxa as a criterion in protocols for assessment of flow status of stream segments may be inaccurate without examining the actual age and/or life history of the specimens. Either taxa lists should be revised or organism age and life history should be considered. For example, NCDWQ (2010) has amended their protocols to reflect these limitations by disregarding burrowing crayfish and by making the presence of long-lived taxa a secondary criterion in the assessment of stream flow permanence. Other agencies should follow that lead.

Acknowledgments

We thank Jason Mullen, Shai Kamin, and other GEI field and laboratory personnel who made this study possible. Funding was provided by the National Mining Association through the tireless efforts of Karen Bennett. Sampling was conducted under West Virginia Division of Natural Resources Collecting Permit Number 2010.116. Erin Smith, Norka Paden, Jason Mullen, and two anonymous reviewers made helpful comments on this manuscript.