If you study up on many of the wild habitats which we love, there is a tremendous amount of thinking by scientists that blackwater systems are relatively devoid of planktonic life. This is a fact borne out by many years of study.
Major rivers like the Rio Negro are often called "impoverished" by scientists, in terms of plankton production. That may give you the impression that these are virtual "wastelands" in the aquatic realm. However, "impoverished" doesn't mean "devoid" of life. There's a lot of it. And these rivers are surprisingly dynamic, in terms of life.
Many blackwater systems do show seasonal fluctuations, such as lakes and watercourses enriched with overflow in spring months. At low water levels, the nutrients and population of these life forms are generally more dense. Creatures like hydracarines (mites), insects, like chironomids (hello, blood worms!), and copepods, like Daphnia, are the dominant fauna that fishes tend to feed on in these waters.
And of course, these creatures are tied intimately to the presence of leaf litter and decomposing botanical materials from the terrestrial environment. This is interesting to contemplate when we consider what to feed our fishes in aquariums, isn't it?
These life forms, both planktonic and insect, tend to feed off of the leaf litter itself, as well as fungi and bacteria present in them as they decompose. The leaf litter bed is a surprisingly dynamic, and one might even say "rich" little benthic biotope, contained within the otherwise "impoverished" waters.
And, as we've discussed before on these pages, it should come as no surprise that a large and surprisingly diverse assemblage of fishes make their homes within and closely adjacent to, these litter beds. These are little "food oasis" in areas otherwise relatively devoid of food. The fishes are not there just to look at the pretty leaves.
It's not really that much different in the aquarium, is it? I mean, as the leaves and botanicals break down, they are acted upon by fungi and bacteria, the degree of which is dependent upon the available food sources. Granted, with fishes in a closer proximity and higher density than in many wild systems, the natural food sources are not sufficient to be the primary source of food for our fishes- but they are one hell of a supplement, right?
That's why, in a botanical-rich, leaf litter dominated aquarium, you see the fishes spending a lot of time foraging in and among the litter...just like in nature.
It's been observed by many aquarists, particularly those who breed loricariids, that the fry have significantly higher survival rates when reared in systems with leaves present. I'm sure some success of this could be attributed to the population of infusoria, etc. present within the system as the leaves break down.
Biofilms, as we've discussed many times before, contain a complex mix of sugars, bacteria, and other materials, all of which are relatively nutritious for animals which feed on them. It therefore would make a lot of sense that a botanical-influenced aquarium with a respectable growth of biofilm would be a great place to rear fry! Maybe not the most attractive place, from an aesthetic standpoint- but a system where the little guys are essentially "knee deep" in supplemental natural food at any given time is a beautiful thing to the busy fish breeder!
And what of the leaves themselves?
Do our aquatic animals feed on them? Well, yes- and no. Some fishes, for example, Loricariids, will feed on some of the materials directly, rasping off surface tissues. Others, like certain characins (notably, Headstanders, Metynis, and similar fishes), will feed off of the algae growth, or aufwuchs, as it's collectively referred to, present on the botanicals and leaves.
Other creatures, such as the beloved ornamental shrimp (Neocaridina, Caridina, etc.) feed on the biofilms, algal material, and directly on the leaves themselves. Not only do they do very well in such systems, in my experience- but they can help keep the biofilms from getting too much of an "upper hand" in such an aquarium.
Leaves and other materials added to our aquariums not only look cool, but they provide an important function (supplemental food production) as well. Something interesting to contemplate, right?
Yeah, we think so! Consider that the next time you toss some leaves in your aquarium!
Stay creative. Stay thoughtful. Stay resourceful...
And Stay Wet.