3.3 Light traps in the Grand Canyon
The health of a river or stream can be assessed from the diversity of aquatic insects and by the species that are present. For example, some insects can tolerate low oxygen levels, which might arise as a consequence of pollution, whereas others require very clean water so would disappear quickly from fresh water as pollutants arrived.
The United States Geological Survey (USGS) wants to monitor the health of the Colorado River, which flows through the Grand Canyon (Figure 15). Aquatic insects are a key component of this ecosystem, but the small number of professional scientists in the area cannot hope to monitor a 250-mile length of river over long periods of time. So they have recruited people who work on the river to take samples for them all year round (Figure 16).
There is a standard procedure to be followed. At dusk, the citizen scientists set out a light trap on the edge of the river. The trap has a fluorescent black lamp which attracts flying insects. Below the lamp is a container of alcohol into which the insects fall. The trap is run for one hour and then the sample is poured into a bottle that can be sent to the USGS labs, where staff can identify and record the insects collected.
From the thousands of samples already collected (and millions of insects identified), it has been possible to show that the abundance and diversity of aquatic insects is influenced by hydro-electric power generation on the river. There are hourly changes in the amount of water discharged from the Glen Canyon Dam, which means that the water level in the river fluctuates, with an intertidal zone between the highest and lowest water levels. The data provided by citizen scientists show that, in regions where low water occurs daily at dusk, the abundance of aquatic insects is three times that of areas where water levels at dusk are high. This difference is attributed to differential mortality of the insect eggs.
But just how important is the survey, which is very labour intensive? The answer to this question lies in the food webs in the ecosystem. Aquatic insects support the entire ecosystem, providing a food source for predators in the water such as fish, terrestrial predators such as lizards and flying predators such as birds and bats. You can find more information about the study on.