Flooded forests, food webs, submerged trees- and their functional benefits for fishes...

Since we've started Tannin Aquatics, we've been going on and on and on about the interesting biological niches which arise when terrestrial environments become inundated with water during the rainy season. We have discussed many aspects of these unique habitats and how we as aquarists can replicate aspects of them to create unique and highly functional aquariums for the fishes that we keep.

Yeah, we've talked a lot about them, and we've seen many of you go on to successfully recreate some of them. And they are as beautiful as they are functional. And the "functional" aspect is, in my opinion, as compelling- if not, even more so- than the aesthetics of these habitats. Just thinking about the dynamics, fish adaptations, and components of these systems can get your creative juices flowing!

And one of the most compelling aspects of these habitats is how they "power" the food webs  which our fishes benefit from.

In the seasonally flooded forests and plains of Brazil, plants grow and take up nutrients from the sediments during the dry "terrestrial phase", store them in their tissues, and release them into the water when they begin to decompose after these habitats flood.

Fishes may directly feed on the organic materials from these plants, such as fruits, flowers, seeds, detritus, and bark. In fact, science has documented over 200 species (primarily larger characins and catfishes) in Amazonia alone which are specifically adapted to feed on fruits, nuts, submerged grasses, and other terrestrial plant materials!

(Well, you're not likely to keep the huge Pacu, Colossoma macropomum, but it's a classic example of a "fruit eater!" Pic by Rufus46, used under CC BY-SA 3.0)

Many other fishes which reside in these flooded forest areas feed mainly on insects, such as beetles, spiders, and ants from the forest canopy. These insects are likely dislodged from wind and rain, and the opportunistic fishes are always ready for a quick meal!


Interestingly, it's been postulated that the reason the Amazon has so many small fishes is that they evolved as a response to the opportunities to feed on insects served up by the flooded forests in which they inhabit! The little guys do a better job at eating insects which fall into the water than the larger, clumsier guys who snap up nuts and fruits with their big mouths!!

Some species of fishes specialize in detritus. As we know by now, decomposing leaves are the basis of the food chain, and the detritus they produce forms an important part of the food chain for many, many species of fishes. Some have even adapted morphologically to feed on detritus produced in these habitats, by developing bristle-like teeth to remove it from branches,tree trunks, plant stems, and leaf litter beds. 

Bacteria, fungi, and algae also act upon the nutrients released into the water by the decomposing organic material from these plants. Aquatic plants, known collectively to science as macrophytes, grow in or near water and are either emergent, submergent, or floating, and play a role in "filtering" these flooded habitats in nature.

Terrestrial trees also play a role in removing, utilizing, and returning nutrients to the aquatic habitat. They remove some nutrient from the submerged soils, and return some in the form of leaf drop. Interestingly, studies show that about 70% of the leaves which fall from the surrounding trees in the igapo habitats occurs when the area is submerged, but the bulk of it is shaded at the end of the inundation period. The falling leaves gradually decompose and become part of the detritus in the food web, which is essential for many species of fishes.

Flood pulses in these habitats easily enable large-scale "transfers" of food items between the terrestrial and aquatic environment, As we've touched on before, aquatic food webs in the Amazon area are very strongly influenced by the input of terrestrial materials, and this is really an important point for those of us interested in creating more natural aquatic displays and microcosms for the fishes we wish to keep.

I think a part of the reason why we're seeing success with utilizing botanicals in our aquariums is that fish are instinctively "programmed" to utilize many of these materials as both feeding substrates and as food items in and of themselves. The addition and replenishment of leaves and other botanical materials definitely mimics, at least to some extent, the processes which occur in these habitats which transfer food and nutrients into the aquatic habitat.

And with the ability to provide live foods such as small insects (I'm thinking wingless fruit flies) and worms (Bloodworms, for sure), there are lots of possibilities for creating really comfortable, natural-appearing (and functioning) biotope aquariums for fishes.

I sometimes fear that the burgeoning interest in biotope aquariums at a contest level will result in the same sort of "diorama effect" we've seen in planted aquarium contests. In other words, just focusing on the "look" (which is cool, don't get me wrong) yet overlooking the reason why the habitat looks the way it does and how fishes have adapted to it...and how we can utilize this for their husbandry, spawning, etc. It goes beyond merely creating the "look" of these systems to win a contest, IMHO. Rather, we can also focus on the structural/functional aspects of these environments to create long-term benefits for the fishes we keep in them.

Leaves, detritus, submerged terrestrial plants- all have their place in an aquarium designed to mimic these unique aquatic habitats. And let's not forget the trees themselves- in their submerged and even fallen state!


The trees present in these forest habitats actually benefit fishes and aquatic life forms in other ways, such as when they fall. Upon the return of the inundating waters, these fallen trees become an important part of the aquatic habitats, providing multiple benefits.  A fallen tree wedged into a stream bottom provides shelter from the currents. The tree trunk changes the flow pattern of the stream to create eddies which may bring in food and wash away fine silt, allowing formation of gravel beds and the accumulation of leaves and fallen botanical materials. Foraging areas are created, in which fishes may find insects, small crustaceans, and fruits and such which come from the terrestrial environment. They provide spawning locations for fishes, and shelter for fry to develop and avoid predators.

Although it's possibly impractical for many aquarists to obtain really large branches and such to simulate these submerged tree trunks, it would be a most interesting aquascape feature if you could source larger, thicker pieces to recreate this fascinating microhabitat in your aquarium! And entire community of fishes could be developed around (literally) one large branch win a modest-sized aquarium. Toss in a bunch of leaf litter and some botanicals- and- bam! Instant biotope! Well, almost...You've got the look down. Bring on the function! Encourage some biofilms, algae, and other epiphytic material to colonize the branch/trunk, and then you're on your way to a functional representation of this unique habitat!

So much to talk about, study, and interpret here. So many unique and compelling aspects of the flooded forests of South America that I know will unlock secrets of many unique and beautiful fishes which we keep in our aquariums. By providing functional biotope aquariums, we're really setting the stage for the ultimate evolution of aquarium keeping. It's a fun and fascinating journey, that will not only yield greater understanding of our fishes, but of the precious and fascinating environments from which they come.

Stay fascinated. Stay curious. Stay resourceful. Stay diligent. Stay obsessive!

And Stay Wet.


Scott Fellman

Tannin Aquatics 




Scott Fellman
Scott Fellman


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