So, really, what happens when a leaf falls into a stream?

There is amazing beauty and life to be found in what seems to be disorder and decay in aquatic systems. Yeah, decay and breakdown- processes which we have talked about extensively on these pages. It's a fantastic, dynamic process. Need an example of this dynamic?

Look no further than our friends, the leaves...You know, the plant materials that ecologists call allochthonous input- material imported into the aquatic ecosystem from outside it...Such as...fallen leaves.

The process which happens when a leaf falls into the water is an incredible example of the elegance of Nature and the dependency and interrelationship between terrestrial and aquatic habitats.

So...What makes leaves fall off the trees in the first place?

Well, it's simple- er, rather complex...but I suppose it's simple, too.ūüėÜ

Essentially, the tree "commands" leaves to fall off the tree, by creating specialized cells which appear where the leaf stem of the leaves meet the branches. Known as "abscission" cells. If you're a word enthusiast, you'll love the little fact that they actually have the same Latin root as the word "scissors", which, of course, implies that these cells are designed to make a cut! 

And, in tropical species of trees, the process of leaf drop is vitally important to the surrounding environment.

The nutrients are typically bound up in the leaves, so a regular release of leaves by the trees helps replenish the minerals and nutrients which are typically depleted from eons of leaching into the surrounding forests. And the rapid nutrient depletion, by the way, is why it's not healthy to burn tropical forests- the release of nutrients as a result of fire is so rapid, that the habitat cannot process it, and in essence, the nutrients are lost forever.

Now, interestingly enough, most tropical forest trees are classified as "evergreens", and don't have a specific seasonal leaf drop like the "deciduous" trees than many of us are more familiar with do...Rather, they replace their leaves gradually throughout the year as the leaves age and subsequently fall off the trees.

The implication here?

There is a more-or-less continuous "supply" of leaves falling off into the jungles and waterways in these habitats, which is why you'll see leaves at varying stages of decomposition in tropical streams. It's also why leaf litter banks may be almost "permanent" structures within some of these bodies of water!

So, what is the fate of a leaf that falls into a jungle stream?

Almost immediately after it falls into the water and becomes submerged, the leaf leaches out some sugars and carbohydrates. These compounds tend to be dispersed widely into the water via current or other factors. Interestingly, studies have determined that when leaves enter the streams, their nitrogen content generally increases.

It's a by-product of respiration. This respiration is seen as evidence of microbial colonization on the leaf surfaces, which can be corroborated by the increase of oxygen consumption in the vicinity of the submerged leaf accumulation.

Leaves are surprisingly variable botanical structures, and they can vary compositionally significantly by species, and sometimes, within a species! Studies have shown that different leaves of the same species can contain up to a tenfold difference in difficult-to-digest tannins, with one leaf containing as little as 3 percent tannins and another leaf as much as 33 percent! This impacts the breakdown of the leaves and how quickly and easily they are assimilated into the aquatic environment.

Energy- in the form of organic carbon, nitrogen, and other nutrients- flows from the leaves into the waters, contributing significantly to these habitats. And among the most important roles of these nutrients in the aquatic habitats is to power the process of decomposition. And decomposition, in turn, is the process which powers the development of aquatic food webs.

In the decomposition process, the leaf's constituent elements follow multiple pathways, and as leaf litter is broken down into smaller and smaller particles, there are many opportunities for its elements to reenter the stream food web.

Of course, the physical leaf itself provides "fodder" for the fungi-those gooey "strings" you see whenever you add leaves to your aquarium. The fungi readily colonize the leaf surfaces. Stream-dwelling asexual fungi are called "Ingoldian hyphomycetes" by ecologists, in honor of the mycologist C.T. Ingold, who first described them in detail. These fungi supply much of the energy in stream ecosystems. 

They're really important!

How important?

Well, research by ecologists suggests that not only are fungi needed to start the initial break-down of leaves, but that fungi are the principle organisms involved in converting leaf materials to a more palatable form for other organisms. As the fungi work their biological magic, they facilitate the softening of the leaf, allowing our buddies, the bacteria ,to further break them down physically.

The mesophyll of the fungi produce enzymes that significantly degrade "structural polysaccharides", like cellulose, which results in a softening of the leaf structure which increases their food value for shredders. We call leaves which have been acted upon in this manner by aquatic fungi "conditioned" leaves.

Bacteria arrive on the scene to form part of what we call "biofilms", a complex community of algae, bacteria, fungi, and protozoans living in a matrix of secretions that adhere to the leaf surfaces themselves.

Biofilms form when bacteria adhere to surfaces in some form of watery environment and begin to excrete a slimy, gluelike substance, consisting of sugars and other substances, that can stick to all kinds of materials, such as- well- in our case, leaves. Biofilms continue the work started by fungi to soften the leaves for other organisms along this food chain, typically insects.

Usually, about 4-7 days, aquatic insects (typically "gnats") start to arrive on the scene, joining in on the bounty that the submerged leaf provides. These "grazers" are typically insects like caddisfly larvae, and even aquatic organisms like snails. They specialize in feeding off of the biofilms on the leaf surfaces with specialized mouthparts. Larger organisms, like various insect larvae and worms, shred the leaf with their tearing mouthparts.

Other aquatic insects are collectively known as "collector/gatherers." These insects move slowly along the stream bottom, searching for dead organisms, detritus, or other food particles.

Another group of organisms filter particles of biofilms or tiny bits of the leaves which are dislodged by grazers and shredders as they float downstream in the current. These filter feeding organisms may be either free-swimming or sessile (attaching to the leaf tissue) to feed and grow. They'll consume organic detritus and other materials in the water column as well. Some of these filter-feeding organisms, such as Hydra and amphipods are predators, too, opportunistically feeding on live organism which happen to float on by. 

Finally, some larger aquatic insects are actually predatory upon the other, smaller insects which feed on the bounty of leaf litter. They , in turn, form a part of the diet of the "alpha" species in these bodies of water- fishes.

Of course, fishes will take advantage of the leaves themselves, the fungi and bacterial biofilms which colonize them and soften them, as well as the various invertebrates and insects which break them down, and the resulting detritus which is produced as part of this decomposition process.

The largest proportion of allochthonous organic matter entering most streams comprises leaves, bark, and wood‚ÄĒ notably, twigs.

The carbon and nitrogen contained within leave are assimilated and released at various stages of their decomposition process.

In each step of this aquatic food web, energy is transferred from one organism to another, and from one trophic (energy) level of the aquatic community to the next.

Thus, an entire aquatic food web is built around a community of fungi colonizing fallen leaves!

I come back to this concept again and again, because so many people who are new to the idea of botanical-style aquariums and our practice of adding botanical materials into our tanks simply ask, "Why?"

It makes perfect sense to ask this question. And it also makes perfect sense to be a bit hesitant about the idea of tossing in a bunch of leaves and seed pods and stuff into your "perfectly clean", well-maintained aquarium. We've been told for generations that we need to keep our tanks free from extraneous additions of organic materials. To many hobbyists, this is rightly viewed as adding "bioload" to the aquarium, which it is.

However, rather than just viewing the addition of leaves and such as a means to pollute your tank, I think it's more important to understand that the leaves are actually "fuel"- the power for the ecosystem which is our closed system aquarium.

It's absolutely fundamental to what we do.

Adding leaves to our aquariums is a functional re-creation of a natural process. It's important to note that the primary rationale within our movement for adding leaves to our tanks is not aesthetic.Sure, they do look cool- but that's a collateral benefit of their most beautiful attribute: The function and benefits which they bring.

Leaving these materials in situ to follow the process of softening, recruiting fungal growths, biofilms, and other organisms, and allowing them to completely break down is how we embrace the process of creating a functional closed aquatic ecosystem in our aquariums.

I've said this before, and I'll say it one more time (I can't say it often enough, really!):

A truly "natural" aquarium is not sterile. It encourages the accumulation of organic materials and other nutrients- not in excess, of course. Biofilms, fungi, algae...detritus...all have their place in the aquarium. Not as an excuse for lousy or lazy husbandry- NO- but as part of the 'microbiome", and to serve as supplemental food sources for the other life forms in our tanks.

Some of the decayed material is subsequently recycled as nutrients, like phosphorus (in the form of phosphate, PO4-3) and nitrogen (in the form of ammonium, NH4+) which are readily available for plant growth. Carbon is released largely as carbon dioxide that acts to lower the pH of the aquarium water.

We need to get over the "block" which has espoused a "sanitized" version of Nature. I hit on this theme again and again and again, because I feel like, globally, our community is like 75% "there"- almost entirely "bought in" to the idea of really naturally-appearing and functioning aquarium systems.

Understanding that stuff like the aforementioned decomposition of materials, and the appearance of biofilms- comprise both a natural and functional part of the microcosms we create in our tanks.

This is true in both the wild habitats and the aquarium, of course.

The same processes and function which govern what happens to these materials in the wild occur in our aquariums. And, if we reject our initial instinct to "edit" what Nature does, the aquarium takes on a look and vibrancy that only She can create.

Embrace, don't edit.

Leave the stuff in there until it decomposes.

It's a real mental shift that we as hobbyists have to make. Sure, there will always be a lot of people that don't like the look of brown water, decomposing leaves, biofilms and fungi in their aquariums. It's a radically different look than what we've come to accept an aquarium "should" look like for the better part of the century..

We have to accept Nature's input here. We can't be afraid of it. We need not be.

Nature dictates the speed by which this decomposition process occurs. Nature "recruits" the organisms which power our little ecosystems.

Yes, we "set the stage" for it- but Nature is in full control.

She always was, and always will be.

Our journey starts with a falling leaf. 

Leaves represent a true confluence of terrestrial and aquatic elements, working together to create a unique and inspiring aquatic habitat. By replicating, at least on some level, an "unedited" interpretation of the process which builds these habitats, we open up new possibilities, foster breakthroughs in aquatic husbandry, and further the state of the art of the aquarium hobby.

Yeah, it starts with a falling leaf...

 

Stay inquisitive. Stay studious. Stay observant. Stay persistent. Stay patient. Stay bold...

And Stay Wet.

 

Scott Fellman

Tannin Aquatics 

 


Scott Fellman
Scott Fellman

Author



Leave a comment