It must be the water... more blackwater musings...




We've talked a lot about what exactly makes wild blackwater habitats so acidic (As you know, natural blackwater in the Amazon region can average as low as 3.75 or so...). Recent studies found that most of the acidity in black waters can be attributed to dissolved organic substances, and not to dissolved carbonic acid. In other words, organic acids from compounds found in soil and decomposing plant material, as opposed to inorganic sources. Blackwaters are almost always characterized by high percentages of organic acids.

Interestingly, however, these waters are surprisingly low in dissolved organic compounds (DOC). In fact, Rio Negro black waters are theorized to have low DOC concentrations because of thdiluting effect of significant amounts of rainfall, and because they are diluted by clear waters from nearby systems low in dissolved organic compounds. Sort of  self-regulating, to an extent, right?

In the podzol soil where black water originates, most of the of the extractable substances in the surface litter layer are humic acids, typically coming from decaying plant material. Scientists have concluded that greater input of plant litter leads to greater input of humic substances into ground water. In other words, those leaves that accumulate on the substrate are putting out significant amounts of humic acids, as we've talked about previously! And although humic substances, like fulvic acid, are found in both blackwater and clear water habitats, the organic detritus (you know, from leaves and such) in blackwater contains more extractable fulvic acid than in clearwater  habitats, as one might suspect!

The Rio Negro, for example, contains mostly humic acids, indicating that suspended sediment selectively adsorbs humic acids from black water.  The low concentration of suspended sediments in rivers like the Rio Negro is one of the main reasons why high concentrations of humic acids are maintained. With little to no suspended sediment, there is no "adsorbent surface" (other than the substrate of the river, upon which these acids can be taken hold of (adsorb).

When you think about it, all of this this kind of contributes to why blackwater has the color that it does, too. Blackwater in the Amazon basin is colored reddish-brown. Why? Well, it has  those organic compounds dissolved in it, of course. And most light absorbtion is in the blue region of the spectrum, and the water is almost transparent to red light, which explains the red coloration of the water!

And many of those organic compounds come from the surrounding land, as touched on above...

The relationship between terrestrial habitats and the aquatic environment is becoming increasingly apparent in these areas. And, the lack of suspended sediments, which create a "nutrient poor" condition in these habitats, doesn't do much to facilitate "in situ" production of aquatic food sources; rather, it places the emphasis on external factors.  Many blackwater systems are simply too poor in nutrients to offer alternative food sources to fishes.The importance of the relationship between the fishes and their surrounding terrestrial habitat (i.e.; the forests which are inundated seasonally) is therefore obvious.

Fishes depend upon the fruits, seeds, insects, and other materials which come from the forest for food ("allochthonous input", remember?). When these areas become seasonally inundated, more food sources are available to the fishes which reside in these habitats. So, it kind of goes without saying that preservation of the forests themselves is really important for the fishes! If you take away the forest, you take away the fishes, too! 

And, as we've hinted on previously- the availability of food at different times of the year in these waters also contribute to the composition of the fish community, which various from season to season based on the relative abundance of these resources.

Like so many things in nature, the complexity of blackwater habitats is more than what meets the eye. Chemically, biologically, and ecologically, blackwater habitats are a weave of interdependencies- with soil, water, and surrounding forest all functioning together to influence the lives of the fishes which reside within them. No single factor could provide all of the necessary components for fish populations to thrive. To damage or destroy any one of them could spell disaster for the fishes- and the ecosystem which supports them. It is therefore incumbent upon us to understand, protect, and cherish these precious habitats, for the benefit of future generations. 

It's not only vital for us to understand how these habitats work in nature- it's important for us to be able to replicate some of its functions if we want to be able to keep and breed the fishes that we keep which hail from these habitats.

Keep researching. Keep learning. Keep sharing.

Stay intrigued. Stay diligent. Stay devoted.

And Stay Wet.


Scott Fellman

Tannin Aquatics 




Scott Fellman
Scott Fellman


2 Responses

Scott Fellman
Scott Fellman

September 20, 2019


Really interesting hypothesis…I think you’re correct…It’s interesting, however, that many- not all- blackwater habitats have limited macrophyte growth…wonder if the epiphytes perform a similar function? I think you’re really on to something here! Now you’ve got me thinking, lol

Thanks for sharing!



September 17, 2019

Im thinking about the reaction between Humic acids with ammonia and nitrate waste.

My hypothesis is that this reaction precipitates salts with water as a by-products resulting in low DOC. This also converts the nitrates into nitric salts which are readily absorbed by plants, thus nitrifying bacteria is not required. However if plants do not consume these minerals from a tank, then ultimately there will still be accumulation of waste.

Scholarly articles seems to support this theory although the studies done are focused on using humic acids to reduce the volatile loss of ammonia based fertilisers but is resting on the same fundamental idea.

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