If you follow our ramblings here in "the Tint" and in social media, you know that we seem to have endless discussions on the merits of leaving in or removing leaves, pods, and other botanical materials as they break down. All things being equal, my current state of thought is to "leave 'em in", and I am fascinated that many of you feel the same!
Now, we've probably beaten this subject up quite a bit over the last year, but it's still our most "asked" topic. And there is still much to learn and discover about it. By looking once again at the wild habitats- truly "nature's aquarium"- you can gain a few insights, and perhaps apply some of the observations to our practices and ideas in the aquarium world.
Now, I realize that an aquarium is a closed system, and unlike nature, has artificial, clearly defined nutrient inputs-outputs, water influx, etc. However, I have always believed that when you really think about it, the practices we engage in with our aquariums are quite a good analogy to the things that happen in nature.
Let's get back to the idea of decomposing leaves.
Interestingly enough, in field studies of rain forest streams in the Amazon, it has been noticed that entire leaves are frequently found buried in the sediments within the streams, leading some scientists to initially postulate that coarse organic matter is degraded really slowly in these streams. However, subsequent observations determined that most leaves are actually shredded by various aquatic insects and creatures; in this case, chironomids (relatives of the adult version of the "bloodworm!") -and that material is selectively removed from the leaves, lending the appearance that it is intact.
So, what this means for those of us who enjoy replicating these unique habitats is that the leaves and botanical materials are initially broken down by smaller insects and such, then acted upon by microbial life forms. Fungi and bacterial biofilms are though to be the dominant forms of life at the base of the food chain in these streams.
Indeed, due to the higher concentrations of structural compounds (e.g., lignin and cellulose) in leaf litter in Amazonian stream systems, microbes (mainly fungi, due to their greater ability to degrade structural compounds) are more important for leaf breakdown in relation to the invertebrate community, because they sort of "soften up" the leaves for the "shredders." Yet, biological decomposition and high fungal biomass were of secondary importance for leaf litter breakdown rates, together with chemical decomposition. An interesting focus was placed on "leaf quality" (chemical and structural components of the leaves), assigning it the single most important factor in leaf breakdown.
And here is another killer tidbit about leaf breakdown from wild observations of Tank, et. al: "In tropical stream systems, leaf litter with high concentrations of labile compounds (e.g., polyphenols and tannins) and nutrients (e.g., nitrogen and phosphorus) generally exhibits a higher leaf breakdown rate than leaf litter with a high percentage of structural compounds (e.g., lignin and cellulose) and high degree of leaf hardness..."
So, where does that leave us, as hobbyists?
Does this observation mean that the more tannin-imparting leaves, like Catappa, for example, tend to break down faster when submerged? Well, yeah, right? Catappa leaves are generally the fastest-decomposing leaves we use. Leaves like Magnolia, Jackfruit and Guava, for example, tend to be "harder" and less prone to fast physical breakdown underwater, usually acquiring biofilms and some fungi and slowly softening over many weeks.
So, the much-feared "pollution" that people who've never worked with leaf litter in aquariums before love to scare us with may not be such a menacing factor, as the breakdown of the leaves either occurs very quickly, as in the case of Catappa, or over a very long term, with the aforementioned "harder" leaves. Most adequately filtered aquariums, managed by hobbyists with solid, consistent husbandry skills, should be just fine, IMHO. It also goes to show you that there are tangible benefits of having a diversity of leaves and other botanicals in your aquarium, right?
This new data from nature tells me that we're doing a lot of things right by letting nature run its course and having these materials beak down in our aquariums. Now, sure, there is "stuff breaking down" slowly, and then their is full-on "pollution", brought about by lack of responsible husbandry (overfeeding, overstocking, etc.). Two totally different things, IMHO. I've had more than one aquarist over the years take a rather confrontational stance and accuse me of advocating sloppy husbandry under the guise of "being natural" when discussing botanical-influenced aquariums.
This, of course, is absolutely NOT what we're advocating, and I think is based on some incomplete or even outmoded thinking. The point is that, with regular water changes, managing feeding and fish stocking levels, it's certainly not an issue leaving these materials in the aquarium until they break down. There are too many variables to make a blanket statement that leaving botanicals in until they break down is an invitation to "pollution."
Granted, we likely don't have "primary" shredders like chironomids and such populating our leaf litter beds (though I'd like to), but we do have populations of beneficial microbial life, fungi, and perhaps even a small population of crustaceans, like Gammarus, Daphnia, etc. However, maybe a few of us keep ornamental shrimp in our tanks, and grazing fishes, like Plecos and such, which help fill this role, right?
And in my experience and the experience of many of our fellow "tinters", the fear of a chemically unstable, "dirty" tank is largely unfounded, IMHO. With leaves breaking down at different rates in the aquarium, I don't think that "mass pollution" becomes an issue. In fact, in aquariums, as in nature, I believe that some of the slow decomposing species are important to fishes, invertebrates and microorganisms as substrates and sources of particulate organic matter.
As previously mentioned, phosphates and nitrates have always been essentially undetectable in my botanical, leaf-litter-dominated aquariums. I simply have not had issues with nitrate and phosphate. Now, your experience may differ, and a lot of other factors could contribute to this. On the other hand, what detrimental effects have you observed in a leaf-litter or botanical-dominated aquarium when detectible (notice I didn't say "higher") levels of nitrate and phosphate are present? Massive algal blooms? Fish death? I'm not being sarcastic, actually...I'm curious, because these observations are important!
I have nothing other than my personal experience with my tanks and some theories to go on, but I can't help but wonder if some denitirifcation occurs in deap leaf litter beds much as it does in sandbeds in a reef tank. Again, taking into account the processes that occur in natural streams, rivers, and lakes, one can only assume that similar ones occur in our aquaria.
All of these things are very interesting, and so much is yet to be learned and experienced by us as hobbyists in relation to leaf litter and botanicals in our aquariums. Yet, one can only hope that many of the positives which occur in natural habitats comprised of leaf litter and botanical cover will occur in our thoughtfully-managed closed system aquaria. The day will come when we have a better understanding of what's really going on in leaf litter systems in our tanks, and that these materials won't be coveted just for their ability to impart tannins and humic substances for lowering pH and tinting the water, but for the true biological "richness", diversity, and utility they provide.
Here's to YOU- working on this stuff every day.
Stay on top of things. Stay observant. Stay open-minded. Stay curious.
And Stay Wet.