A most intimate relationship

The relationship between land and water is something we talk about quite a bit here. And I think it deserves mentioning more often- and occasionally, examining in greater detail. 

With blackwater systems, there is a unique set of environmental relationships which create and  maintain these habitats and their chemical/physical/ecological makeup. And it starts, of course, with water! 

We've often mused in the hobby about what pH is “appropriate” for blackwater aquariums, and it’s really hard to pin down one number.  The reality is that many natural blackwater systems are far more acidic than we could ever hope to achieve in our tanks. Where does this acidity come from?

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 surrounding terrestrial habitat for food. 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.



Another example of these unique interdependencies between land and water are when trees fall.

It’s not uncommon for a tree to fall in the rain forest, with punishing rain and saturated ground conspiring to easily knock over anything that's not firmly rooted. When these trees fall over, they often fall into small streams, or in the case of the varzea or igapo environments in The Amazon that I'm totally obsessed with, they fall and are submerged in the inundated forest floor when the waters return.

And of course, they immediately impact their (now) aquatic environment, fulfilling several functions: Providing a physical barrier or separation from currents, offering territories for fishes to spawn in,  providing a substrate for algae and biofilms to multiply on, and providing places for fishes forage among, and hide in. An entire community of aquatic life forms uses the fallen tree for many purposes. And the tree trunks and parts will last for many years, fulfilling this important role in the aquatic ecosystems they now reside in each time the waters return.

Shortly after falling into the water, fungi and other microorganisms act to colonize the surfaces, and biofilms populate the bark and exposed surfaces of the tree.  Over time, the tree will impart many chemical substances, (humic acids, tannins, sugars, etc.) into the water.

The  fallen tree literally brings new life to the waters.

The materials that comprise the tree are known in ecology as "allochthonous material"-  something imported into an ecosystem from outside of it.  (extra points if you can pronounce the word on the first try...) And of course, in the case of fallen trees, this includes includes leaves, fruits and seed pods that fall or are washed into the water along with the branches and trunks that topple into the stream.  

These materials are known as “coarse particulate organic matter”, and in the waters of these inundated forest floors there is a lot of CPOM, and the community of aquatic organisms (typically the aforementioned aquatic insects and crustaceans) has a high proportion of “shredders”, which feed on the CPOM and break it up into tinier bits called (wait for it) "fine particulate organic matter" (FPOM).

And of course, some fishes, like larger characins, catfishes, etc., consume fallen fruits and seeds as part of their diet as well, aiding in the "refinement" of the CPOM. Other organisms make use of the fine particulate matter by filtering it from the water or accessing it in the sediments that result. These allochthonous materials support a diverse food chain that's almost entirely based on our old friend, detritus!

Yeah, that detritus.

The stuff of nightmares for many dyed-in-the-wool hobbyists...the stuff of dreams for many fishes who consume it and the associates fauna within it.

And, although the forest floor receives substantially less sunlight than open rivers, the nutrients and available light are utilized by algae, which may colonize the surfaces facing up into the sun. 

We see similar results in our aquariums, right?

And of course, the tree, like almost anything that is submerged,  will gradually decompose over long periods of time. This process is actively exploited by aquatic life forms at all levels. Hollowed-out sections will be inhabited by fishes and exploited for the shelter they offer, and of course, the aforementioned crustaceans and insects will utilize the tree in various ways.

And interestingly, when you think about it, fish movement and species richness and population is directly affected by the physical and biological influences of... fallen trees! And the deep beds of leaves that may be "corralled" by the fallen trees- a sort of natural "dam"- will definitely limit some fish species, which cannot tolerate the lower oxygen concentrations found in these areas.

Other fishes take advantage of the "physical barrier" that a fallen tree presents to shelter from predatory species. Many adaptations have taken place over eons to allow fishes to exploit these changes to their environment caused by fallen trees!

It's pretty hardcore stuff. And pretty fascinating.

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.

It truly IS a most intimate relationship.

Stay studious. Stay diligent. Stay observant. Stay curious...And Stay Wet.


Scott Fellman

Tannin Aquatics 

Scott Fellman
Scott Fellman


Leave a comment