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.😆

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.

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