Color, contrast, and function: Replicating the flooded forests of South America

We're at a cool phase in our journey into the blackwater/botanical-style aquarium world. We're beyond just creating generic, "proof of concept" aquariums- we're now actively constructing systems which replicate- at least in part, specific natural habitats which encompass an influx of natural terrestrial materials.

It's interesting to study some of the niche environments that exist in nature, which are heavy influenced by terrestrial life. A prime example of this are the South American forests and swamp forests, which are seasonally inundated with freshwater. These forests are perhaps nature's finest example of the interaction between land and water, and how diverse and surprisiingly productive aquatic environments arise in these habitats. The two types of inundated forest areas are blackwater systems known as igapó, and the counterpart "whitewater" systems called várzea.


The igapó is characterized by seasonal inundation caused by a large amount of rainfall, and thus, in some areas, trees can be submerged for up to 6 months of the year. We've touched on the idea of replicating this habitat in "The Tint" some time ago. These forests have sandy, rather acidic soils with a very low nutrient content. The rainwater combines with the humic substances and tannins contained in the soils and the forest floor materials that are found on them. The acidity from the water corresponds to the acidic soils of these forests. They are the more nutrient poor than a comparable várzea forest, carrying less inorganic elements, yet higher concentrations of dissolved organics, like humic and fulvic acids. 

Amazonian várzea forests are flooded by nutrient rich sediments, and thus are very productive environments- some of the most productive in Amazonia. They are flooded by whitewater rivers, which inundate fertile alluvial soils within várzea forests, which helps explain some of the higher nutrient concentrations found in these waters, as opposed to the nutrient-poor blackwater which inundates and characterizes the igapó areas.

Some of the most popular aquarium species, such as Tetras, Apistogramma, and Loricariid catfishes, reside in these systems during the periods of inundation, and studies have revealed a surprisingly high population density within them.

In a comparative study of Amazonian fish diversity and density conducted by Henderson and Crampton in 1994, in nutrient poor blackwater igapó and richer whitewater várzea habitats in Brazil, the whitewater sampling sites were characterized by high turbidity and conductivity, and a pH of 6.6-6.9. By comparison, the blackwater sites had low turbidity, a very low conductivity, and a pH of 5.3-6.0. 

Both whitewater and blackwater sites held high diversity fish communities with many species in common. Whitewater habitats were more diverse, yielding 108 species, compared with 68 from blackwater. However, each habitat has some characteristics which shape the population composition and density, and it bears noting when thinking about stocking our aquariums, doesn't it?

I think it does!

Várzea have a characteristic which makes them very hospitable to fishes: During the flooding season, the more static várzea whitewaters, which develop low oxygen levels, lack predatory species such as Eels and Knifefishes, which are more typically abundant blackwater. There is a lower overall density of other predators, which have migrated or are confined to smaller, dryer areas, and the resident fishes use this time to spawn! The productivity of the varzea generates large amounts of detritus, particularly when the water level falls, which supports the fishes that feed on this material. 

On the contrary, detritivorous fishes are less abundant in the igapó, where far less of this material is found. Fishes in these areas tend to consume greater quantities of insects, wood/leaves (as in the case of some catfishes), and of course, each other!

Another interesting thing about Amazonian streams and flooded forest areas in general is that there is no significant "in situ" (in place) primary production, and that the fish populations that reside in them depend on what is known as "allochthonous input" (material that is imported into an ecosystem from outside of it) from materials like seed pods, fruits, blossoms, leaves, and dead wood from the surrounding forest.

This is why leaf litter beds are so important in blackwater, as they serve as sort of "aggregators" of terrestrial material, and foster decay and biological processes which support what aquatic ecologists call  "food webs." Most of the aquatic life forms which reside in these waters are aggregated in submerged litter.

So, these inundated forests sound like fascinating subjects for replication in our aquaria, huh? Yeah, thought you'd think that. What's pretty cool is that the várzea offers those who like the idea of utilizing botanicals in their aquarium, but are not fans of the" tint" caused by blackwater, to create aquarium displays that are diverse and interesting without the tint, and still know that they are more-or-less "biotopically-on point!"


You would simply run chemical filtration, such as activated carbon, Poly Filter, Purigen, etc., etc. in your filter to negate the tint, something we have discussed and experimented with on our own systems a couple of years back, as a sort of  demonstration that you can do this.

For those of us who love blackwater, just omit the chemical filtration media and you're golden- literally...

For a várzea-themed aquarium, we'd say to omit some of the more heavily "tint-producing" botanicals, and go with stuff like the more "durable" seed pods, like "Lampada Pods", "Savu Pods", "Concha Pods", "Flor Rio Pods", etc. These not only impart less tannins into the water than leaves and such, many of them, such as "Flor Rio" and "Concha" represent the fruits and such that accumulate in these waters.As mentioned above, a good chemical filtration media should counteract any tint imparted into the water by these materials, as well as the nutrients released by them.

For the igapó-themed tank, it's "game on," and you can use just about any botanical items you'd want, particularly, leaves. Since blackwater is your thing, you need not concern yourself about "tint limitation" in your botanical choices. I'd lean towards items like Coco Curls, "Rio Fruta", Catappa and Magnolia leaves as some of your primary "botanical stars" in such a setup.

One thing that I haven't really put much thought into when developing either concept in the past, yet am interested in now, has been the application of proper substrate materials in these aquariums. I'm considering the use of acidic soils in the igapó tank, versus more "alluvial" materials in the várzea aquarium. This is where some of the more specialized aquatic soils that are used in planted aquaria, for example, may come in handy. Some of you intrepid hobbyist with knowledge of planted aquarium substrates need to do some research on this stuff! Interesting, and perhaps just a bit ironic, that our work as natural botanical-style, "hardscape-first" aquarium hobbyists could be aided by aquatic soils created for planted tanks....

Aquascaping in general for these aquariums is fun.

Since you are simulating a flooded forest, the idea of using large, vertically-oriented pieces of wood to simulate submerged trees and shrubs is immediately appealing. Projecting wood out of the water would be perfectly acceptable in such a tank. I'm stealing a pic from our friend Adam Till, who, in addition to being a serious researcher of botanical substrates, just happened to have created an aquarium that demonstrates an interesting "above and below", sort of palludarium-like display a while back, that would really work for this situation!

As you can see, there is so much more to the "concept-aquarium" world besides underwater mountain ranges, beach scenes, and "Middle Earth Fantasy 'Scapes." And you don't necessarily have to go full-on, 100% hardcore "biotope", either. Rather, you can "riff" off of these unique and important habitats and create truly amazing aquariums.

For those aquarists who are not only interested in the aesthetics of these unique habitats, but the execution of incredible functional systems as well, there is so much work to be done.

So many potential discoveries and breakthroughs to be made. Certainly worth devoting some talent, energy, enthusiasm, and tank space to!

Until next time- Stay engaged. Stay creative. Stay curious. Stay original.

And Stay Wet.

Scott Fellman

Tannin Aquatics

Scott Fellman
Scott Fellman


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