July 24, 2017

Falling leaves and rising interest in leaf litter beds!

This comes as no surprise to you, I'll bet, but we have a fair amount of discussion in our botanical-minded blackwater community about the merits of leaving decomposing leaves in your tank versus taking them out. In fact, the last I’ve checked, over the years there have been a bout a dozen-plus discussion threads on our social media outlets, and perhaps 15 or so articles here in the tint which touched on this issue! Tongue in cheek, but this definitely represents the definitive "body of work" on the joys of decaying leaves in the aquarium. I defy you to find a hobby site which has discussed, analyzed, and- well- beaten the shit- out of this topic as much as we have over the past couple of years!

So, in the finest Tannin Aquatics tradition, let’s add one more piece to the discussion on decomposing leaves! Today, however, let’s think about how leaf litter “behaves” in the wild, and how we could emulate it in our aquariums. With perhaps a few new twists and insights that we haven't shared before.

DOWN GO THE LEAVES...

First off, leaf litter comes fro ma variety of trees in the rainforests of the tropical world. There is a near constant leaf drop occurring, which continuously “refreshes” the supply. In monsoonal climates, large quantities of leaves will drop during the dry season, and many will find their way into streams which run through the rain forest. In other habitats, such as the igapo forests of Amazonia,, the leaves fall onto the forest floor and accumulate, and are seasonally-inundated during the rainy season, creating an extremely diverse and compelling environment that we love so much around here!

The leaf litter in an igapo when inundated can be as much as 3 feet (1 meter) or more deep, with a huge amount of surface area available to bacteria (which create biofilms) and are often home to surprisingly large populations of fishes like Apistogramma, which use the shelter and “on-board” food offered by these habitats- to their advantage. And they’re vital to some of the small Elacocharax and other “Darter Tetras” which live almost exclusively in these niche habitats. Oh, and shrimp, too!

One interesting observation I’ve made over the years concerning adding leaves to the aquarium and letting them decay: Dead, dried leaves such as those we favor don’t have nearly the impact on water quality, in terms of nitrate, as fresh leaves would. I’ve routinely seen undetectable nitrate levels in aquariums loaded with botanicals. This is largely because dead, dried leaves have depleted the vast majority of stored sugars and other compounds which lead to the production of nitrogenous substances in the confines of the aquarium. Hence, leaving leaves in to fully decay likely reaches a point when the detritus is essentially inert, consisting of the skeletonize sections of leaf tissue which can decay no further. Dead leaves contain largely inert forms of polysaccharides, and are reach in structures like lignin and cellulose. Oh, and doing regular water changes can’t hurt.

AFTER THE FALL: WHAT HAPPENS NEXT?

To understand this more fully, let’s look at what happens when a leaf dies and falls into the water in the first place. At some point, the leaves of deciduous trees (trees which shed leaves annually) stop photosynthesizing in their structures, and other metabolic processes within the leaves themselves begin to shut down, which triggers a process in which the leaves essentially “pass off” valuable nitrogen and other compounds to storage tissues throughout the tree for utilization. Ultimately, the dying leaves “seal” themselves off from the tree with a layer of spongy tissue at the base of the stalk, and the dry skeleton falls off the tree.

As we know by now, when these leaves fall into the water, or are immersed following the seasonal rains, they form a valuable substrate for fungi to break down the remaining intact leaf structures. And the fungi population helps contribute to the bacterial population which creates the now-famous biofilms, which consist of sugars, vitamins, and various proteins which many fishes in both their juvenile and adult phases utilize for supplemental nutrition. And of course, as the fishes eliminate their waste in metabolic products, this contributes further to the aquatic food chain. And yeah, it all starts with a dried up leaf!

Interesting semi-anecdotal observations from my friends in the know, suggest that the biofilms for decaying leaves form a valuable secondary food for the fry of fishes such as Discus, Uaru, (after they’re done feeding on their parent’s exuded slime coat) and even Loricariid catfishes. And of course, all sorts of other grazing fishes, like some characins and even Cyprinids, can derive some nutrition from the fungi, bacteria, and small crustaceans which live in, on, and among the leaf litter bed. I’ve seen fishes such as Pencilfish (specifically, but not limited to N. marginatus ) spend large amounts of time during the day picking at leaf litter and the surfaces of decomposing botanicals, and maintaining girth during periods when I’ve been traveling or what not, which leads me to believe they are deriving at least part of their nutrition from the leaf litter/botanical bed in the aquarium.

AND, IN THE AQUARIUM?

In the aquarium, much like in the natural habitat, the layer of decomposing leaves and botanical matter, colonized by so many organisms, ranging from bacteria to macro invertebrates and insects, is a prime spot for fishes! The most common fishes associated with leaf litter in the wild are species of characins, catfishes and electric knife fishes, followed by our buddies the Cichlids (particularly Apistogramma, Crenicichla, and Mesonauta species)! Some species of RIvulus killies are also commonly associated with leaf litter zones, even though they are primarily top-dwelling fishes. Leaf litter beds are so important for fishes, as they become a refuge for fish providing shelter and food from associated invertebrates.

So, other applications in the aquarium for a leaf litter bed?

Well, let’s say you love the idea, don’t mind the tint, but simply don’t want any damn decomposing leaves in your hardscape (or whatever) aquarium? How about a leaf litter refugiium! Yup, a dedicated in-line vessel (usually an aquarium or commercially-available unit) which receives rich downstream flow from the display aquarium. You could stock it with leaves to your hearts’s content, and make sure that the returns are sufficiently screened off to keep the decomposing botanical materials from clogging the pump(s) or getting shot back into the display in significant quantities.

In a leaf litter refugium, you’d be able to place small fry, or fishes which require specialized feeding surfaces (like “Darter Tetras”) that only leaf litter can provide. It can form the basis of a “nursery”, breeding tank for Apistos and such, or simply be a supplemental display of some unique fishes which would otherwise be overlooked or out competed by the inhabitants of the main aquairum, while giving the display the benefits of tannins and humic substances. Besides, a refugium is an easy way to study and control this fascinating and dynamic niche environment in the aquarium! Win-win.

As we’ve discussed repeatedly over the past couple of years, there are so many benefits to painting leaf litter in the aquarium in some capacity. Wether it’s for water conditioning, supplemental food, speciality fishes, or simply for a cool-looking display, overcoming our ingrained aesthetic preferences and accepting the decomposing leaves as a natural, transitory, and altogether unique habitat to cherish in the aquarium is a decision that each one of us in “Tint Nation” has to make- but if you look at it from a functional aesthetic perspecitve, it’s pretty easy to appreciate the “beauty”, in my (very biased!) opinion!