Methods Manual for Salt Lake Studies/Macroinvertebrates
Authors: BV Timms, PSJ Coleman,
Salt lakes host both aquatic and terrestrial invertebrates. When the lakes contain water a range of insect larvae, molluscs and crustaceans inhabit the water column and benthic zone. When the waters evaporate the dry playas host wolf spiders and other terrestrial invertebrates (Hudson???)
Methods for sampling aquatic macroinvertebrates
Aquatic macroinvertebrates may be collected using pond nets. The collected specimens are emptied into a white tray for sorting and voucher specimens may be relaxed with soda water before being preserved in alcohol.
When surveying a single lake, sampling should occur within the entire range of habitats available (open waters, shallow water over hard and soft bare benthos or over submerged aquatic macrophytes, rocky edhes, within areas of emergent vegetation). To ensure you have sampled representatively, construct a species accumulation curve (Smith, 1996). Collect in 5 minute units from a variety of locations within each habitat area until the number of new species encountered with each collection flattens out.
When surveying groups of lakes within a bioregion, two 10-minute collections are sufficient in lakes with a relatively homogenous habitat. Those lakes with several distinct types of habitat (e.g. rock edges, aquatic weeds, mud) should be sampled using two 10-minute collections of each major habitat type. The construction of an accumulation curve, based on 5-minute sub samples of the two 10-minute samples plus another 40 minutes (i.e. 1 hr of collecting divided into 5 minute units) will enable the researcher to determine how representative the 10-minute collections are. A reasonably diverse lake from the group of surveyed lakes should be chosen to conduct the accumulation curve.
Specimens should be listed, and representatives preserved for later identification. Species abundance may be noted on a log scale (r =1, o =5 individuals, x = 10 individuals, xo =50 individuals, xx = 100 individuals, xxo =500 individuals, etc.). This method is rapid, and the accuracy is sufficient to allow the data to be used for community analysis in PRIMER multivariate analysis.
Quantitative sampling by methods such as benthic cores provide poor species richness results and require large numbers of replicates to obtain accurate abundances. This is due to benthic habitat heterogeneity. Such methods require a very large investment in time and sample handling effort, making them more suitable for in-depth studies than for simple characterisation of lakes.
A voucher collection should be maintained of aquatic macroinvertebrates collected. Vouchers may be used to ensure 'between survey' consistency where different personnel conduct surveys, and also allow later researchers to ensure which species were present at a point in time, regardless of taxonomic revisions that may have occurred in the interim. Macroinvertebrate species considered new to a region may be separated for identity confirmation by the appropriate taxonomic expert for that group at a time convenient to the expert.
To ensure the invertebrate fauna are relaxed prior to death and preservation, samples of invertebrates should be deprived of oxygen. This may be done by adding soda water to the samples. Once all fauna have ceased movement, the vouchers may be transferred to alcohol for preservation. While methylated spirits is sufficient for most purposes, it is not appropriate if the vouchers may need to have DNA analysis at some time in the future, or for species new to science. In these cases pure ethyl alcohol is used for preservation.
Methods for collecting terrestrial invertebrates on dry playa lakes or adjacent to wet lakes
Terrestrial invertebrates utilising the emergent macrophyte habitats of the salt lakes may be sampled by using a sweep net (2 x 10 min sweeps) and by observation. "Pollard walks" are timed observational walks where the observer passes through all the habitat types adjacent to the lake and across the playa surface. "Quadrats along transect" observations are similar to pollard walks, but used for playa flat surveys to determine the presence of fauna that burrow or stay close to the soil. Wet micro-pitfalls are usefull for collecting invertebrates, as are light trapping, tanglefoot bands on trees, sticky cockroach lures and whitefly/aphid traps. Active searching for ants and soil invertebrates using leaf litter sieving and soil core sieving are time consuming but can provide a surprising quantity of creatures.
When sampling terrestrial invertebrates, the following advice holds:
- wet micropitfalls and various sticky lures should be spaced at least 9 per 100 square metres;
- construct a new species accumulation curve over the trapping period, and
- cease trapping when the curve starts to flatten, or when four days have passed.
Invertebrates are poorly known and recorded in many parts of the world and voucher specimens from surveys may be lodged with the regional museum.