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Invertebrates occupying extremely nearshore (0.5-2 m depth) hard substrate (boulders, cobble, gravel) are important components of the nearshore community. Included in the nearshore, hard substrate assemblage are midges (good indicator taxa), as well as a stonefly (Utacapnia tahoensis) that is endemic to Lake Tahoe. The presence or absence of certain taxa, as well as their distribution and abundance provide an important and integral indicator of the present state of nearshore Lake Tahoe. Changes in the composition, distribution, and/or abundance (CDA) of certain taxa over time indicate changes occurring in the nearshore environment.
Invertebrates in sandy areas without boulder/cobble and in deeper silt and sand dominated nearshore habitats (soft substrate) are also important components of nearshore communities. Midge communities that are found in these habitats are excellent indicators of water quality conditions and sensitive endemic species can also be found in nearshore soft substrate-dominated areas of Lake Tahoe. Change over time in the CDA of certain taxa will, as described above, provide important indication of changes to the nearshore condition of Lake Tahoe.
Invertebrates in marina environments are typically composed of taxa that can tolerate conditions that are relatively eutrophic and rich in organic matter. Macroinvertebrate densities in marina environments are influenced by macrophyte communities and drive warmwater fish production in marinas. Marinas contain high numbers of midges, which can be important in determining of the relative trophic condition of marinas. Marina environments are also at high risk of non-native invertebrate invasion and thus monitoring of these environments could allow for early detection of non-native taxa.
Benthic macroinvertebrates have long been used as indicators of ecosystem health because of their relatively long life spans, ubiquitous distribution, diversity in sensitivity to stress, and position in food webs (Metcalfe 1989, Barton and Anholt 1997). Benthic macroinvertebrates can also be extremely useful in documenting change over time in systems where historical macroinvertebrate samples are available. For example, benthic macroinvertebrate communities in the Great Lakes have been used to reveal benthic responses to changes in the physical, chemical, and biological character of the lakes (Robertson and Alley 1966; Nalepa 1991; Stewart and Haynes 1994; Barton and Anholt 1997; Nalepa et al., 1998; Nalepa et al., 2000; Lozano et al., 2001; Nalepa et al., 2003; Nalepa et al., 2007). It is particularly attractive to use macroinvertebrates in Lake Tahoe as indicators of ecosystem health because of the presence of several unique endemic species that have experienced severe declines over the past four decades (Caires et al., in review).
The composition, distribution, and abundance (CDA) of macroinvertebrates collected from soft substrate in nearshore Lake Tahoe was documented in 1962-63 (Frantz and Cordone 1996) and in 2008-09 as part of a larger survey of benthic invertebrates in the lake. Macroinvertebrates were also collected from hard substrate at several locations around the lake in 2009. Macroinvertebrates were also collected from marinas around Lake Tahoe in 2008-09. Apart from these collections, macroinvertebrates have not been quantified in the nearshore zone of Lake Tahoe. The existing data from these collections provide a rough baseline for macroinvertebrate CDA.
One group of macroinvertebrates, the non-biting midges (Chironomidae), could be particularly useful in monitoring nearshore conditions over time. Midges have been commonly used as an environmental indicator in lake assessments (Charvet et al., 1998). The presence and relative quantity of certain midge species can indicate the trophic status of lakes (Weiderholm 1980, Saether 1979) and provide an easy way of monitoring human impacts on lentic systems. Although the use of midges as indicators of trophic condition has not been developed in the Lake Tahoe region, midge collections from the 1962-63 and 2008-09 benthic surveys are available. Midges from these collections have been identified to genus or species level and are available as a baseline for macroinvertebrate CDA.
Macroinvertebrate densities in 1962-63 and 2008-09 collections from nearshore soft substrate were significantly higher in the southern and western regions of Lake Tahoe in 1962-63 (Figure 17-1a; one-way ANOVA, F3,77 = 20.70, p < 0.0001; Tukey HSD, p < 0.05) and, while densities were also higher in southern and western regions of the lake in 2008-09, these differences were not significant (Figure 17-1a; one-way ANOVA, F3,34 = 1.96, p = 0.14). Macroinvertebrate densities from hard substrate collections in 2009 were substantially higher around Sunnyside (northwestern region of the lake); however distribution differences between sites could not be tested due to low sample sizes (Figure 17-1b). High densities of macroinvertebrates in soft substrate in southern Lake Tahoe appeared to be driven by (in order of dominance) worms, midges, and amphipods in the 1960s and by midges, worms, and Asian clams (Corbicula fluminea) today. It is likely that Asian clam densities are even higher in the nearshore of southern Lake Tahoe since the time of the 2008-09 survey. Dominant taxa in nearshore hard substrate collections at the site that had the greatest macroinvertebrate densities (Sunnyside) were midges, mayflies, and stoneflies, which are taxa that could serve as important invertebrate indicators (Figure 17-1b). Regular monitoring of these macroinvertebrate communities would reveal more detailed spatial and temporal patterns in macroinvertebrate CDA.
Comparisons of past vs. present midge communities in Lake Tahoe suggest that midge assemblages in the lake have changed dramatically. For example, soft substrate collections of midges in Lake Tahoe show that an ultra-oligotrophic to oligotrophic midge assemblage dominated in 1962-63, whereas an oligotrophic to mesotrophic assemblage dominated in 2008-09 (Table 17-1). Similarly, midges have been useful in characterizing the trophic state of Lake Tahoe marinas as compared to the main lake (Figure 17-2). These findings suggest that midges collected in the nearshore environment of Lake Tahoe could be excellent indicators of lake trophic status and change over time.
Macroinvertebrates should be collected from hard substrate at various locations around the lake biannually (spring, fall). Recommended collection locations are: Sand Harbor, CrystalBay, outside of Tahoe City Marina, outside of Sunnyside Marina, Cave Rock, Sugar Pine Point, and Emerald Bay (n = 7) at depths between 0.5 and 2 m. Samples from cobble and boulder substrates can be obtained with a modified lake vacuum, as described by Vander Zanden et al. (2006). Most samples can be collected by wading at the sample site, although deeper substrates may require snorkeling to collect samples. A minimum of three replicate samples (0.25 m2) each should be taken at each site. In the laboratory, macroinvertebrates can be separated from each sample using a sugar flotation (Anderson 1959) and visual inspection method. Upon preservation in 70 percent ethanol, macroinvertebrates can be enumerated and identified. Head capsules of midges should be separated from their bodies and slide mounted in Euparol for further identification.
Macroinvertebrates should be collected from soft substrate around the lake using a benthic dredge biannually (spring, fall). Recommended monitoring sites are: McKinney Bay at Homewood, Camp Richardson, Cave Rock, and Crystal Bay (n = 4). Short transects at each site should consist of a minimum of three replicate samples each collected from 1, 5, and 10 m depths (it may be necessary to reconsider sampling depths if the suggested depths do not fall within the defined nearshore zone at certain sites). Nearshore samples collected in 1962-63 were collected with a standard Ekman dredge, while nearshore samples collected in 2008-09 were collected with a Petite Ponar and Shipek grab. A sampler recommended for all-purpose macroinvertebrate sampling in nearshore Lake Tahoe is the Petite Ponar. Conversion factors are available for all three of these samplers in Lake Tahoe (Caires and Chandra 2012); however, it is recommended that the type of sampler used for regular monitoring remains the same. Once collected, samples can be processed in the laboratory as described for hard substrate collections above. Potential monitoring metrics for both hard and soft substrate collections would include midge assemblage structure, other dominant taxa, taxa richness and diversity, and presence or absence of special status and/or invasive taxa.
Macroinvertebrates in marinas should also be collected using a benthic dredge (Petite Ponar recommended) biannually. Suggested marinas for regular monitoring are: Tahoe City Marina, Tahoe Keys Marina, and Ski Run Marina (n = 3). A minimum of five replicate samples should be collected from each marina from the dock. In addition to collections with a Ponar, visual inspection of docks should occur to determine the presence or absence of non-native attached taxa (e.g., quagga or zebra mussels). Samples should be processed as described for other collection types above.