The Stream Whisperer

The Song of the Stream

Streams sing their own songs if you listen and look.  A stream's song, rather than being words, is composed of the living and non-living things in and around it.  A stream will whisper and talk to you if you stroke it in just the right way.  

Here is how a stream sings.  It is alive with the buzzing, wriggling, and crawling organisms at its bottom; it is buoyant with the gurgle and splash of its riffles and eddies; it vibrates with physical, chemical, and organic activity.  Walking along a stream and trying to say something about its labyrinthine ecology is like listening to a piece of obscure classical music for the first time: You can't really say much except that you either like it or you don't.  Measuring temperature, velocity and discharge, and looking up some land-use maps is a little like having a look at the musical score: You start to see structure and organization beneath the wash of sensation.  Collecting the macroinvertebrates from the stream and letting them talk to you themselves is a little like learning to play the piece itself: Now you really see and feel what this thing is, and why it looks and acts and sounds- even smells- the way it does.

Assessing the health of a stream is a little like playing a song on an instrument.  The twelve small notes aggregate and coalesce into something beautiful and substantial.  The disparate observations begin to sing and hum and whisper to you.  The Dakota Drain talks to me, and it is just now beginning to talk to my students.  They are starting to see what it means to be a Stream Whisperer.

Aquatic Biology: Not for Numskulls

The most important factors in determining the health and biological structure of a stream are 1.) The assemblages of macroinvertebrates in the lotic zone and 2.) the chemical and physical properties of the water itself.  The water's properties mostly determine the aquatic ecology, but the biological community will also communicate to you what mere chemistry cannot.  

Now river biologists have been studying macroinvertebrates for many decades and have amassed impressive volumes of information on the habitats, feeding habits, and aquatic preferences of hundreds and hundreds of organisms.  From leeches, to skimmer dragonfly larvae, to physid and viviparid snails, to midge and mosquito larvae, to various fly maggots, to giant water bugs and crawling water beetles, we know what they eat and where they live.  With this in mind, we can capture several hundred over the course of a few weeks, do a few simple calculations, and- astonishingly- discover an obscenely diverse array of patterns beneath the panoply of data.

The ratios between different niches, or what in the technical parlance are called "functional feeding groups," can tell us scads of information about whether food sources are plant- or detritis- (organic debris) based, whether the food is coarse particulate organic matter (CPOM) or fine (FPOM), whether the FPOM is up in the water column or sitting along the stream bed, whether the channel is stable, and whether energy is flowing through our ecosystem at a healthy rate.  We can of course also calculate the Shannon-Weiner diversity index described in this blog post to tell us whether the community is diverse.  A diverse community is a good thing because it is more stable and resistant to environmental changes.

One or two words on functional feeding groups (FFGs).  Scrapers, such as many snails, eat algae and require a stable substrate to do well.  Shredders eat CPOM and generally swim up in the water. Gatherers, such as midges, eat FPOM that is stuck in the river bed and crawl around rather than swim.  Filterers, such as fingernail clams, collect FPOM from up in the water and require a stable substrate and relatively clear water to be successful.  Lastly, predators eat the other FFGs and tend to swim rapidly and either engulf (swallow whole) their prey, like the narrow-winged damselfly larva, or pierce it (with a sharp straw called a stylet), such as the giant water bug.  (Watch both of these short clips immediately).

We can calculate the ratios between the numbers of organisms in different FFGs, and because we know their preferences as far as food and habit, we can say something meaningful about the stream they come from.

A few of the indices that we can calculate follow:

Autotrophy/Heterotrophy Index = Scrapers / (Shredders + Gatherers + Filterers)                       Greater than 0.75 means most food is algae.  Less means it is mostly detritus.

CPOM/FPOM Index = Shredders / (Gatherers + Filterers)                                                                   Greater than 0.50 means most detritus is CPOM.  Less means it is mostly FPOM.

FPOM_{in transport} to FPOM_{in sediment} Index = Filterers / Gatherers                                                     Greater than 0.50 means most FPOM is floating in water.  Less means it’s mostly on the riverbed.

Substrate Stability Index = (Scrapers + Filterers) / (Gatherers + Shredders)                                  Greater than 0.50 means channel is stable.  Less means it is shifting and unstable.

Predator Control Index = Predators / (Gatherers + Filterers + Shredders + Scrapers)                   0.10 to 0.20 means energy is flowing through community at a healthy rate. Less than 0.10 is too slow.

Here is what all this nonsense means.

Autotrophy simply refers to how much food is produced locally, within the stream ecosystem itself.  If a stream has an autotrophy/heterotrophy index of greater than 0.75, it means that the ecosystem is mostly dependent upon algae, or "autochthonous" input.  Less means that the stream is fed by fallen leaves and twigs, and any other detritus or junk that falls into it.  It is then referred to as "allochthonous."

Recall that CPOM is chunks of detritus while FPOM is very fine food particles.  We can use the CPOM/FPOM index to contrast them.   If more than 50% of the organisms we find are shredders (they eat CPOM), then the detritus of our stream is mostly coarse chunks, not fine pieces.  The food is probably coming from things like leaves and grass clippings.

Whatever FPOM that is in a stream will either flow with the water, in which case it is "FPOM in transport," or it will be lodged in the substratum, in which it will be "FPOM in sediment."  The two FPOM-eating functional feeding groups live in different places because they eat the FPOM in different ways.  Gathering collectors, such as midge larvae and many snails, crawl about in the muck and find FPOM.  Filtering collectors, such as clams and certain caddisflies, adhere to a hard surface and filter FPOM from the water.  Caddisflies have a peculiarly unappealing way of filtering: they spin a sticky net of rope or webbing out of mucus and let it waft in the current; then after a few minutes, they eat the mucus net, along with any FPOM that has stuck to it.  So don't go inviting any caddisflies to supper.

Our substrate stability index tells us if the stream is shifting or stable.  Scrapers and filterers prefer to have a lot of hard surfaces to stick to, and they can't do that in a mucky, unstable stream.  So if we find a lot of them (more than 50%), it indicates that the channel is stable.

Lastly, the predator control index indicates how rapidly food energy is flowing through our biological community.  Predators represent an active and dynamic step in matter and energy exchange.  So if we find that between 10% and 20% of our organisms are predators, then energy is flowing at a healthy rate.  If predators make up much less than 10%, then our community is sluggish and not very interactive.  This is generally a bad thing as far as stream health.

We can do many similar calculations with the vertebrate community: A wealth of vertebrate predators, such as piscivorous (fish-eating) birds, indicates that energy is not only flowing at a healthy rate, but that there is probably an exchange of energy between the benthic (river bottom) community and the vertebrate community.  This is what we would expect in a healthy stream.  We can also find the fraction of the vertebrates that are aquatic organisms.  If most of our stream vertebrates are actually terrestrial, then our vertebrate community is aquatic-poor.  This suggests that the riparian zone and stream are not very productive as far as the aquatic community.

Hydrochemistry and What it Shows

A small handful of chemical properties can communicate crucial data regarding a stream's health.  The properties my students test in the Dakota Drain are pH, dissolved oxygen, phosphates, and nitrates.  They also measure the temperature of the water, and the general stream crew measures discharge in cubic meters per second.

A low pH is a bad thing, because it means the stream is acidic.  Acidic water is caused by pollution and high carbonic acid levels, which impoverish the biological community.

Obviously a high dissolved oxygen (DO) level in the water is positive, because that means the stream can support macroinvertebrate assemblages with a high oxygen demand, such as stoneflies and waterpennies.  DO levels are reduced by decomposing matter such as dead algae, because the decomposing bacteria absorb oxygen during respiration.  This tends to acidify the water and cause a low pH.  Low DO levels are also bad for fish because it makes it more difficult for them to breathe and survive.  Strongly related to DO is temperature.  Water dissolves more gas if the water is cold, just like how a cold soda stays fizzy longer than a warm one. (Try warming up a glass of Coke and see how long it stays fizzy.)  So we want a cold temperature.

The physical attributes of the stream also affect DO.  A rapid velocity, high-discharge stream has a lot of whitewater, splashing and gurgling (cavitation).  These mixing zones dissolve oxygen into the water purely by physical mixing.  A sluggish stream has fewer opportunities to mix oxygen into the water.  This is the main reason why we measure the physical dimensions of our stream.

Lastly, high nutrient levels of nitrates and phosphates tend to have negative impacts on stream ecology.  Nitrates and phosphates feed algae, which bloom under high-nutrient conditions.  The algae then die and sink to the benthos, where they are decomposed by bacteria.  This leads to a lot of stink, muck, and low DO levels, as mentioned above.  Nitrates and phosphates are found in fertilizers.  So if the subdivision upstream of our drain is using a lot of fertilizers on their lawns (they are), then those nutrients eventually find their way into our stream and cause it to become "eutrophic," or having too many nutrients.  This upsets the pH and DO balance and wreaks havoc on our biological community.