I have an obsession for small little tropical streams.
They are remarkable little habitats found worldwide, with literally thousands of different fishes found residing in them.
The definition of a "stream" is: "...a body of water flowing in a channel or watercourse, as a river, rivulet, or brook..."
And of course, these little bodies of water flow through jungled areas, where they're bound to pick up some leaves, twigs, and other plant parts as they wind along their path. Leaves, the "jumping off point" of our botanical obsession, form a very important part of these stream habitats.
It is known by science that the leaf litter and the community of aquatic animals that it hosts is, according to one study, "... of great importance in assimilating energy from forest primary production into the blackwater aquatic system."
There is something that calls to me- beckons me to explore, to take not of its intricate details- and to replicate some of its features in an aquarium- sometimes literally, or sometimes,. just taking components that I find compelling and utilizing them.
Streams also function as a means to preserve the nutrients that would be lost to the forests which would inevitably occur if all the material which fell into the streams was simply washed downstream. The fishes, crustaceans, and insects that live in the leaf litter and feed on the fungi, detritus, and decomposing leaves themselves are very important to the overall habitat.
Have you ever heard of an igarape before? You must have, right? We've talked about them incessantly, right?
For good reason.
The igarape habitat is a fascinating, botanical-influenced biome which is an example of the remarkable interdependency between the terrestrial and the aquatic realms.
It translates roughly as "canoe road" in the Hengatu dialect of the Tupi language (spoken all over the Western Amazon region), and that's exactly what it is: A little jungle stream that is used by indigenous people in Brazil to navigate through the rain forests in their small canoes.
These little bodies of water flow between forest trees, and often diminish to even smaller shallow creeks during the dry season. As such, the influence of the soil, plants and grasses of the forest floor is significant. These components are ever-present, providing food, shelter, and spawning areas for a remarkable diversity of fishes.
The lighting is rather dim, as the sunlight is often partially obscured by the low, overhanging trees and vegetation. As one might guess, these little streams often have botanical materials, such as leaves, seed pods, and branches falling into them throughout the year.
As you might guess, an important part of these little streams is the abundant presence of fallen leaves from the overhanging trees. These fallen leaves contribute not only tannins and humic acids to the water, they provide much of the food sources for the aforementioned fungi, bacteria and algae present in the habitats.
Leaf accumulations also function as a means to preserve the nutrients that would be lost to the forests which would inevitably occur if all the material which fell into the streams was simply washed downstream. That's a hugely important ecological role, isn't it?
There is actually some evidence that some fishes may actually consume some of the leaf litter materials as part of their diet (Walker, 1990), in addition to detritus, insects and other small crustaceans, which we've discussed before right here in this blog!
And I think we need to look beyond just the cool looks of the natural habitats from where our fishes hail, and focus on the attributes which comprise their function. We need to understand why fishes are attracted to certain habitats, and apply these lessons into our aquariums. As we all know, a successful aquarium is not just a pretty look. It's a "complete package" sort of thing.
Obviously, we can take some "artistic liberties", being in this for fun and not try to be 100% biotope-centric, down to the last rock and twig. However, there are some general physical characteristics of these small streams that we can use to help us plan our little representation of them!
Water movement: Generally, a very modest current, without significant surface movement. This makes sense, because you do find a fairly large number of surface-active Hatchetfishes, like Carnegiella myersi and Carnegiella strigata in these streams, along with the aforementioned Rivulus, which tend to accumulate at or near the surface.
In this world of decomposing leaves, submerged logs, twigs, and seed pods, there is a surprising diversity of life forms which call this milieu home. And each one of these organisms has managed to eke out an existence and thrive.
A lot of hobbyists not familiar with our aesthetic tastes will ask what the fascination is with throwing palm fronds and seed pods into our tanks, and I tell them that it's a direct inspiration from nature! Sure, the look is quite different than what has been proffered as "natural" in recent years- but I'd guarantee that, if you donned a snorkel and waded into one of these habitats, you'd understand exactly what we are trying to represent in our aquariums in seconds!
Of course, water chemistry of flooded forests and streams is influenced by the many terrestrial components of the habitat. The trees in the ecosystem enrich the habitat and the resulting organic detritus from decomposing bark and organic exudates from the trees themselves help concentrate nutrients within the ecosystem itself.
Learning more about the dynamics of stream habitats and the ecology of the surrounding terrestrial environments is just one fascinating and compelling area of study that we as aquarists can really get into. Yes, it requires some study. It requires trying some new and seemingly wacky ideas (encouraging the accumulation of detritus, decomposing leaves, and epiphytic biofilm growth, for one thing!), and embracing some different aesthetics.
And of course, the very composition of bottom materials and the depth of the channel are always changing in response to the flow in a given stream, affecting the composition and ecology in many ways.
Some leaf litter beds form in what stream ecologists call "meanders", which are stream structures that form when moving water in a stream erodes the outer banks and widens its "valley", and the inner part of the river has less energy and deposits silt- or in our instance, leaves.
There is a whole, fascinating science to river and stream structure, and with so many implications for understanding how these structures and mechanisms affect fish population, occurrence, behavior, and ecology, it's well worth studying for aquarium interpretation! Did you get that part where I mentioned that the lower-energy parts of the water courses tend to accumulate leaves and sediments and stuff?
And stream structures offer very distinct types of "microhabitats", which perform different functions of sequestering materials, nutrients, and creating habitats for the fishes which reside in them. These structures are fascinating subjects for replication in our aquariums!
Amazonian streams typically feature two interesting biotopes that we haven't really discussed in much detail here, and both of which are quite profoundly impacted by the seasonal rains: Pools, with slower current and a substrate covered mainly by deposits of leaf litter, detritus and driftwood; and "rifﬂes" (defined as shallow sections of a stream with rapid current and a surface broken by gravel, rubble or boulders), with a moderately-fast-ﬂowing current and mostly sandy bottom with tree roots, driftwood pieces, and small rocks and pebbles.
And interestingly, in South America, you'll find an unexpected abundance of some species familiar to us as hobbyists in these "riffles." Species like Pyrrhulina brevis, Hyphessobrycon melazonatus, and Hemigrammus of various forms, and even some Nanostimus, and the killie Rivulus compressus!
I find this intriguing, because we tend to associate a lot of these little fishes with sluggish water and more static environments, not areas exposed to greater current and movement.
There are numerous examples of much-loved aquarium species residing in various parts of streams. The idea of recreating the specific characteristics of the parts of the streams where your fave fishes reside in the aquarium is incredibly compelling and a lot of fun!
I can go on and on, sharing with you numerous examples of stream features and functions, and how they affect the lives of the fishes that live in them. However, at some point, I think it's more fun for us to "back engineer" our fishes habitats through our own research. There are as many examples of how to create these habitats as there are fishes which reside in them.
It's time to get to them!
Stay inspired. Stay intrigued. Stay excited. Stay curious. Stay diligent...
And Stay Wet.