Aquatic Microhabitats

Hello everyone!  It’s my first real blog post in a while, so I hope it proves to be worth the wait.  I feel that it’s time to talk about a subject that I find fascinating: microhabitats in aquatic environments!

A stream might look like a fairly monotonous environment, with water traveling inexorably downstream.  However, if you really take the time to look, you’ll notice that there are lots of little pockets of space in a body of water that have slightly different sets of conditions compared to the little pockets of space around them.  These are microhabitats, small areas that have a particular set of conditions unique to that area, and insects are incredibly good at exploiting them.

Take, for example, the stream in this photo:

Prairie Ridge stream

Prairie Ridge stream

That’s the stream at Prairie Ridge Ecostation, where I work.  I think it’s a fascinating stream largely because it has very few insects in it, much lower diversity than one might expect for a reasonably clean stream in this part of the world.  I’ve got a group of high schoolers who are investigating the reasons why there are so few insects in the stream, but the few insects we find are found only in very specific places.  I draw your attention to the location I’ve indicated with the arrow:

Deep pool

Deep pool

That is a pool, an area of deeper water with lower flow.  Pools like that tend to have a lower oxygen concentration than adjacent areas of the stream because the water is deep (remember that oxygen moves very slowly in water and the deeper the water, the longer it takes for oxygen to reach the bottom) and comparatively still (lower flow often = lower oxygen).  In that particular area of the stream you would normally find swimmers, things like the predaceous diving beetles and backswimmers, things that like deep, calm water so they don’t have to fight the current to swim when they go to the surface to breathe.  Curiously, that is one group of insects that is conspicuously missing from this stream and I’m trying to figure out why.  But that’s a subject for another time!  Let’s contrast that deep pool with this area right here:

Stream rooty pool

At first glance, it might look like those two areas are very similar, and in some ways they are.  You’ll find relatively low flow and low oxygen levels in both areas, but the area indicated here is deeper, has a small sandbar that protects a little pocket of water behind it, and contains a lot of roots and other substrates on the steep bank that are absent in the adjacent pool.  This area is well out-of-the-way of the main flow, so swimming insects that rely on surface oxygen could easily live in this spot if this stream had them (e.g., it would be a good place to find whirligig beetles or those diving beetle, if we could find them anywhere in the stream).  However, you do find one thing clinging to the roots on the banks: jewelwing damselfly nymphs from the family Calopterygidae.  You find lots of them there!  Damselflies rely on oxygen dissolved in the water to breathe, have gills, and are rather inefficient swimmers.  The flow is so low in this area that they are at a low risk of being washed downstream if they become dislodged, but the oxygen levels are also rather low.  Thankfully, the roots give the damselflies something to hold onto if they need to move closer to the surface to get more oxygen, and you will almost always find them clinging to those roots.

This stream is so shallow and the flow is so low that the riffles, the areas where rocks and other objects introduce turbulence into the system, are pretty wimpy and the turbulence they generate is mild.  However, this little riffle is where you find most of the caddisflies in this section of the stream:



They are mostly caddisflies in the family Glossosomatidae and you’ll find them clinging to the underside of rocks where they build cases out of rocks.  Like the damselflies, the caddisfly larvae have gills and rely on dissolved oxygen to breathe.  Riffle areas are often the areas of highest oxygen in a stream because the rocks break up the flow and stir up the water, bringing oxygen rich water to the bottom of the stream far more quickly than oxygen could move there on its own.  However, there’s a trade-off: living in the area of highest oxygen often means that you’re out in the strongest flow in the stream.  Insects that live in this part of the stream typically sport a variety of adaptations that help them stay in place.  In the case of the caddisflies in this stream, they have both a set of hooks at the end of their abdomens and build themselves little harnesses of silk and rocks that keep them pressed firmly against the surface of the rock.  They might be right out in the middle with water constantly slamming into them as it flows downstream, but if you’ve ever had a chance to try to pry one of these insects off a rock you know that they are really well attached.  They’re not going anywhere!

Moving just a few inches downstream can mean a big change in the local conditions.  Take this log, for example:

log in stream

Log in stream

On the upstream side, there’s a small pool where the flow is relatively slow, the water is a little deeper, and the oxygen is a little low.  Just on the other side of the log is an area of turbulence and higher flow.  There’s a more oxygen there, but also more flow.  On the upstream side of the log you’d expect to find swimmers (except not in this stream) and downstream you’d expect to find the kinds of things that cling to rocks.  They are literally five inches apart, but the habitat is completely different!  Another foot downstream  is the start of a very deep pool that contains a lot of fish, but virtually no insects.  Considering the number of these microhabitats that are present in this one little stream (and I have only shown you 10 feet of the total length of the stream), you can see how aquatic insect diversity might go up as the stream becomes more complex and contains a greater variety of microhabitats.

When I go collecting with people who haven’t ever done it before, they typically comment on how I find all sorts of different things when they are finding the same thing over and over again.  It’s not that I’m better at catching things, that my technique is better, because it’s not.  It’s because I know that the odd little pool under the undercut bank has different insects than the rocks out in the middle of the stream and both have different insects than those lurking in the leaves that the pooled area downstream.  It takes a little practice, but with time anyone will start to see the huge variety of microhabitats.  It’s just a matter of looking, of not assuming you’ve collected everything in a stream or a pond because you’ve collected in one place.  Nature is far too clever and complex for that!  Keep looking and poke around in the places you might not expect to find things.  You might be very glad you did!


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