Tuesday, March 24, 2009

Science of the Seasons: Little insects have huge role in Kenai River ecosystem

Over the past couple weeks, the Kenai River channel has been slowly opening.

Initially there was only a patch of thin ice at the outlet of Skilak Lake and then a few small areas of open water about a half-mile downstream. A number of overwintering swans and a few mergansers used these open-water areas to forage for food and find a little protection from predators.

Then the river channel opened up about a mile farther downstream. This slow downstream opening of the river will continue as the days lengthen and temperatures rise. Usually in April, the entire river will have an open channel with slabs of ice along each bank.

As the river opens and spring seems to be arriving, a number of aquatic insects will start to appear along the shore. Some of the very first insects will be a number of species of the dipteran family called chironomidae or “midges.”

These insects start emerging from the water when it reaches about 0.5 degrees Celsius. A large number of species of midges will emerge throughout the summer and fall until the water temperatures fall back down toward freezing again in late October or November.

We know at least 88 different species of midges live in the Kenai River. They are certainly the most abundant group of insects in this river, and probably the most abundant group in any freshwater river of the world. Not only are there more species of chironomidae than any other aquatic insect in the river, they are also going to be the group of insects with the largest number of individuals. It would surprise most people to know that there are probably around 100,000 midges that grow, develop and emerge from every square meter of river surface area. That equates to some staggeringly large numbers of insects emerging from the river each year.

Anyone who has traveled along the river in summertime has probably seen swarms of tiny bugs along the shoreline on a sunny day. These swarms of miniature, mosquitolike insects are mating swarms of midges. The males form the swarm and use a species-specific pitched sound to attract females. The females enter the swarm, mate and fly off to lay their eggs. Females can lay thousands of eggs in masses that stick to twigs, rocks or debris along the shore.

The eggs may hatch immediately into tiny larvae or may delay their development until many months later. These young larvae feed on algae, diatoms or fine detritus in the stream or river. Once they reach a certain size, they become pupae. After only a few days as pupae, they emerge and fly away as aerial insects. The adults usually do not feed, although a few are known to take in some sugar-rich fluids from flowers. The adults quickly mate and die within a week or two.

When the pupa becomes an adult, the pupal skin is left floating on the water surface. These pupal skins, called exuviae, remain on the water surface for a couple days and can be collected and identified to species. By collecting these exuviae, I was able to identify which midges were found in the Kenai River and when they emerged.

Even though these insects are only a couple millimeters long, they are a very important component of the aquatic community. They are a favorite food for virtually every species of fish. Young salmon fry feed heavily on midge larvae, pupae and adults. Many of the aquatic insect predators, like stoneflies, frequently feed on midges, too.

If you have ever seen swallows flying back and forth over the river or along most streams, you have seen another predator feeding heavily on midges. Because the adult midges are so small, we rarely recognize what the swallows are capturing. Dragonflies are well-known for feeding on mosquitoes, but they are also a major predator of adult midges.

While midges are certainly some of the smallest aquatic insects in the river, they play a disproportionately large role in the food web because of their high diversity and enormous numbers.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River watershed.

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