EXECUTIVE SUMMARY
Under the auspices of the Aquatic Effects Technology Evaluation (AETE) program, a critical review was undertaken on the use of artificial substrates for collection of benthic invertebrate samples, and on the utility and limitations of this method as a cost-effective environmental monitoring tool for the Canadian mining industry. The review included a survey of colonization dynamics as these affect performance of artificial substrates, an assessment of the strengths and weaknesses of artificial substrate sampling compared with conventional sampling techniques, and a detailed evaluation of four classes of artificial substrates that are potentially useful for environmental monitoring in the mining industry. The advantages and disadvantages of each device were compared using a consistent set of criteria including reliability of data, ease and practicality of use, and cost.
Artificial substrates do have a place in an efficient and cost-effective biomonitoring program for the Canadian mining industry. There is no advantage to be gained from using artificial substrates in shallow streams and rivers with cobble or gravel substrata, where conventional sampling techniques provide at least as reliable data without many of the drawbacks and difficulties of artificial substrates. Rather, artificial substrates should be reserved for those locations where conventional sampling is inefficient or unfeasible, including (1) water bodies with very deep or turbid water, (2) water bodies with soft or unstable bottoms of sand, mud or organic ooze, (3) water bodies with unbroken bedrock bottoms or bottoms of large boulders and (4) rivers with torrential currents. Use of artificial substrates is not justified in shallow, rocky-bottomed streams or rivers where the variation in habitat type within the study reach is relatively minor and an abundant and diverse indigenous fauna may be expected. An exception could be made to this rule if the study area includes both hard-bottomed and soft-bottomed habitats and consistency in the sampling method were desired.
Besides permitting sampling of habitats that would be otherwise difficult to sample effectively, artificial substrates allow greater flexibility in selection of sampling sites than conventional sampling, and allow comparison of environmental effects of effluents along a watercourse where the microhabitat is not constant, such as erosional zones upstream and depositional zones downstream. Artificial substrates provide samples with much greater numbers and diversity of organisms than conventional samples, especially in lentic or depositional habitats, but reduce variability in organism densities among samples, and thereby increase the sensitivity of the monitoring program because smaller site differences can be detected.
The key to successful application of artificial substrates is to have a clear and precise objective beforehand, and to understand exactly what the artificial substrates are capable of measuring. The invertebrate community on an artificial substrate is an indicator of water quality during only the period of exposure. These samplers do not (1) measure the composition of the native bottom fauna, (2) indicate habitat conditions other than water quality, (3) estimate availability of food organisms, or (4) integrate long-term effects of pollution. The samplers function essentially as an on-site, multi-species toxicity test that uses the colonization success of drifting and migrating organisms as the endpoint. Careful comparison of community composition of artificial substrate samples from above and below a point source such as mine effluent can provide information on the nature, degree and extent of potential environmental effects from the effluent, one of the objectives of a biomonitoring program.
Artificial substrates do not collect a representative sample of the indigenous benthos at the site where they are placed, but rather select for mobile, drift-prone species of hard substrata. Therefore they indicate the potential effect of an effluent or disturbance, not the real effect. Moreover, they do not effectively monitor the effects of sediments or sediment-bound toxicants on aquatic biota because sediment-dwelling taxa tend to be under-represented in artificial substrate samples. This is a potentially significant difficulty in using artificial substrates to monitor mining effects because metals tend to partition onto fine sediments, which are not effectively sampled by artificial substrates.
Other limitations of artificial substrates are:
- They may overestimate the real severity of an effluent or disturbance because vagile organisms colonizing the samplers are apt to re-enter the drift, lowering the species diversity and possibly interrupting the expected successional sequence;
- They require a long period for colonization, and colonization dynamics, and hence optimum exposure times, are incompletely known;
- They require two trips for each sample, effectively doubling the cost of field sampling compared with conventional sampling;
- They are prone to loss from accidents, high flows and vandalism, which creates irreparable gaps in the data and adds to the cost of field work;
- They may be bulky, heavy and difficult to handle and transport, and field deployment is often logistically complicated; and
- They may lose organisms while the sampler is being retrieved, especially in deep waters were it is not feasible to use a collecting net.
Four kinds of artificial substrate sampler are potentially useful for environmental monitoring in the Canadian mining industry: multiplate samplers, Beak trays, rock-filled baskets and rock-filled trays. Rock-filled baskets are recommended as the sampler of choice for most applications in mine effluent monitoring because (1) they closely mimic natural substrata yet (2) permit standardization of sampler area, (3) provide abundant microhabitat for colonization, (4) produce low replicate variability, (5) are reasonably stable in currents and (6) are easy and cheap to build. Beak trays are recommended for the particular application of sampling large, fast-flowing rivers with unstable substrata, where other sampling techniques would be ineffective, dangerous, or prone to failure. Though they collect less representative samples than rock-filled baskets, multiplate samplers have the advantages of small size and ease of use, and may be useful for sampling large, soft-bottomed rivers, where bottom sampling is difficult or impossible. Rock-filled trays hold considerable promise, but should be considered experimental for now.
Artificial substrates are best used as one component of a multi-part program, in which measurements of indigenous fauna, water or sediment quality, and possibly laboratory toxicity tests, are combined to provide a clear picture of the state of the system and the effects of mine effluents. Sampling efficiency would be greatly improved by using smaller samplers and increasing the number of replicates. We recommend using the smallest feasible sampler, which for rock-filled baskets is 2500 cm3 , and increasing the number of replicates to at least six, with an additional allowance for lost samplers. An exposure period of six weeks is recommended as optimal for artificial substrates used for biomonitoring. The low flow period from late summer to early fall is usually the best time for benthic invertebrate sampling with any artificial substrate. Where site conditions permit, the sampler should be placed on the bottom of the water body to take advantage of all possible sources of colonization. Samplers suspended in the water column can still be effective, but are more difficult to deploy.
Fine-mesh nets or other means should be used to minimize losses of invertebrates while the sampler is being removed. A number of environmental variables (pH, dissolved oxygen, conductivity, temperature, current velocity, depth) should be measured when the samplers are placed and again when they are retrieved. Measuring the amount of periphyton growth or detritus accumulation in the samplers can aid data interpretation and is strongly recommended.
Limited data suggest artificial substrates are promising tools for assessment of environmental impacts of mining on lakes, but there are too few data for a detailed assessment. This information deficiency should be remedied by undertaking a simple study comparing benthic invertebrate populations with populations colonizing artificial substrates in a lake or lakes with different substratum characteristics. The study should include a comparison of invertebrate populations in a lake or part of a lake receiving mine effluent.
