February 11, 2021

How long do leaves last underwater? Cues from Nature. Question for aquarium applications.

One of the many enduring questions we hear about use of botanicals and leaves in our aquariums concerns how long the materials that we add to our tanks last once we prepare them and add them to our systems.

It's a good one, too!

As the leaves are affected by the aquatic environment, they are acted upon by water conditions, the presence of bacteria, fungi, and fish/invertebrate populations.

We're often asked which leaves last longer than others, and what the reasons might be for this. Is it strictly the durability of the leaves? Is it that they're somehow distasteful to the organisms which are expected to consume them? Is it "something in the water?"

What gives?

Let's look at how natural aquatic systems process leaves for some clues.

Decomposition is an amazing process by which Nature processes materials for use by the greater ecosystem. It's the first part of the recycling of nutrients that were used by the plant from which the botanical material came from. When botanical material decays, it is broken down and converted into more simple organic forms, which become food for all kinds of organisms at the base of the ecosystem.

In aquatic ecosystems, much of the initial breakdown of botanical materials is conducted by detritivores- specifically, fishes, aquatic insects and invertebrates, which serve to begin the process by feeding upon the tissues of the seed pod or leaf, while other species utilize the "waste products" which are produced during this process for their nutrition.

In these habitats, such as streams and flooded forests, a variety of species work in tandem with each other, with various organisms carrying out different stages of the decomposition process.

Interestingly, in some wild aquatic habitats, such as the famous Peat swamps of Southeast Asia, the decomposition of leaves which fall into these waters is remarkably slow. In fact, ecologists have observed that the leaves typically do not break down.

Why?

It's commonly believed that these low nutrient waters, which are high in tannins, and highly acidic, seem to impede microbial activity. This is seemingly at odds with the understanding that passive leaching of dissolved organic compounds (DOC) from leaf litter has been found to be a major source of energy in tropical stream habitats, fueling the microbial food chains which we are so fascinated by.

No doubt the water parameters have something to do with this. These are unique habitats. Here are a few stats from the peat swamps in which some studies on leaf decomposition were conducted:

Water temperature: 25C/77F-32C/89F

pH: 2.6-3.8

TDS: 89-134mg/l

Nitrate: <0.-0.2mg/l

Dissolved oxygen: 1.8-16mg/l

In the studies, leaves of native species found along the swamps submerged in the waters of the swamps lost very little biomass, which other leaves from trees did break down more substantially. This tends to rule out the generally-held theory that ecologists have which postulates that the slow decomposition rate in the peat swamps is due to the extreme conditions. Rather, as mentioned above, it's believed that the resistance to decomposition is due to the physical and chemical properties of the leaves which are found right along the swamps.

(image by Marcel Silvius)

The reason? Well, think about it.

Leaf litter in tropical peat swamp forests builds up into peat many feet deep over thousands of years, and thus impedes nutrient cycling. And when you think about it, inputs of nutrients into most peat swamps come solely from rainfall, because rivers and streams in the region don't always flow into the swamps. In such nutrient poor, highly acidic conditions, it is more beneficial for plants to protect their leaves, rather than to replace them when subjected to elements like wind, and herbivore damage (mostly by insects) with new growth.

And interestingly, bacteria and fungi are known to be responsible for leaf breakdown in the peat swamps, because ecologists typically don't encounter aquatic invertebrates in the peat swamp which are known to ingest leaf material!

Our friends, the fungi!

Yeah, those guys again.

Fungi are regarded by biologists to be the dominant organisms associated with decaying leaves in streams, so this gives you some idea as to why we see them in our aquariums, right?

Here's a fascinating conclusion from a study by researchers Catherine M. Yule and Lalita N. Gomez on leaching of dissolved organic carbon (DOC) in the early stages of the leaf litter decomposition in these peat swamps:

"Most of the DOC appears to be leached within a few weeks of leaves falling into the swamp and thus it appears likely that the cycling of DOC is rapid, and occurs before the leaves become part of the peat deposits. This would further explain the presence of the thick, superficial root mat layer (also a response to waterlogging) that is a key feature of tropical peat forests, since the processes of nutrient cycling would occur in the upper leaf litter layer, rather than the deeper, waterlogged peat."

Okay, neat stuff. It kind of reminds me of those "bog mummies" from Europe, in which the ancient remains s of humans are very well preserved because of the acidic, oxygen-poor conditions of these bogs where the bodies are found.

During the wet season, the peat swamps are inundated with water, which slows down the aerobic decomposition which occurs in the substrate- conditions which facilitate the formation of peat. The breakdown of leaves in the wild is fascinating, as are the implications for the process in our aquariums.

This is a dynamic, fascinating process- part of why we find the idea of a natural, botanical-style system so compelling. Many of the organisms- from microbes to micro crustaceans to fungi- are almost never seen except by the most observant and keen-eyed hobbyist...but they're there- doing what they've done for eons. They work slowly and methodically over weeks and months, converting the botanical material into forms that are more readily assimilated by themselves and other aquatic organisms.

The real cycle of life!

And another reason why the surrounding tropical forests are so vital to life. The allochthonous leaf material from the riparian zone (ie; from the trees!) as a source of energy for stream invertebrates, insects and fishes can't be understated! When we preserve the rain forests and their surrounding terrestrial habitats, we're also preserving the aquatic life forms which are found there when the waters return.

In our aquariums, we're just beginning to appreciate the real benefits of using leaves and botanicals. Not just for cool aesthetics or to "tint" the water- but to create truly natural, ecologically stable aquatic systems for the health and well-being of the fishes we love so much!

So, the reason why some leaves last longer than others in our aquariums is open for debate, although I tend to favor the argument that some leaves that we play with (like Magnolia, Live Oak, Loquat, and Mangrove), and are physically more durable than leaves like Catappa, Jackfruit, and Guava, and thus more resistant to being physically broken down by the fauna and water conditions typically found in our aquaria.

Magnolia leaves, for example, have a waxy coating which renders them more resistant to damage from salt and pollution. According to botanists, the purpose of this covering is to help the plant retain water and repel pollutants. It's analogous to the protection mechanisms which the tropical leaves mentioned above possess.

Sure, very soft, acidic water could have a definite impact on both the density of the microbial community and the higher organisms which "work" the leaves in our aquariums, but I think that our typical pH leaves are no where near as low as you see in natural habitats like peat swamps and flooded forests.

It's a frustrating, difficult-to-answer-with-complete-certainty question!

The reality is that you could probably count on virtually every leaf to reach a point where they will eventually be broken down via one force or another, However, for the purposes of working with botanical-style aquariums, we should assume that all leaves are "consumables", ultimately needing replacement. Sure, leaves like Catappa tend to be the most "ephemeral" in our systems, but every leaf is ultimately subject to decomposition when submerged.

One concept about botanical-style aquariums that I can't seem to bring up enough is the idea that many of the habitats we like to represent in our tanks- and the materials which we utilize to 'scape them, are ephemeral. In other words, they are not permanent features, breaking down and decomposing following long-term submersion.

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 rich in structures like lignin and cellulose. Oh, and doing regular water changes can’t hurt...😆

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! We've covered this idea so many times here it's probably yawn-inducing for some of you, no doubt!

How often do you need to replace your leaves? Well, another great question for which there is no "rule" involved! The reality is that you can simply add new leaves on a regular basis, so you'll always be making up for the ones that have decomposed. Some hobbyists like to remove the decomposed leaves, preferring a more "pristine" look. It boils down to aesthetics, really.

It's important to understand that,when we add leaves to aquariums, we're replicating on many levels the processes which occur in Nature.

So, how long do different leaves last?

Damn, I wish I had an answer. The reality is that, as with so many things we play with in the botanical-style aquarium game- there is no way to know for sure. No guarantees and certainties.

As we’ve discussed repeatedly over the past couple of years, there are so many benefits to playing with leaves in the aquarium in some capacity. Whether 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 has to make- but if you look at it from a functional aesthetic perspective, it’s pretty easy to appreciate the “beauty”, in my (very biased!) opinion!