Energy flow: the food web

Once you've had a look at various ways that nitrogen or carbon are transformed in the aquarium, you might consider the ways energy flows into the aquarium and then passes from organism to organism in the food web.
 
The flow of energy is a one-way, non-cyclical process. It is constantly fired by the sun's light and warmth, or your fluorescent substitutes, and constantly dissipated in entropy.  By way of contrast, nutrients are constantly recycled.
 
Trophic levels. The idea that the nutritional needs of organisms bind them all together in a network of interactions is probably the most familiar concept in ecology. In the aquarium's fragmentary captive ecosystem, the fishes occupy the top level of the food web. We've eliminated all their natural enemies, like caiman and river otters, fish-eating eagles and herons. Though your Neon Tetra may get swallowed by a maturing Angelfish, fishes are still the top predators in our captive systems. Down at the base of the food web are bacteria, yeasts and especially the photosynthesizers, upon which all the other organisms ultimately depend. Ecologists think of them as the "primary producers;" in fact autotrophs ("self-feeders"), those organisms that manufacture their own food, are the only true producers. All other organisms are consumers, which depend on them.
 
In the aquarium, you may supplement the contribution of photosynthesizers by introducing Spirulina flakes or spinach leaves, but in a natural freshwater system, the vast mass of green algae and photosynthesizing single-cell protists like Euglæna supports the whole trophic (i.e. "feeding") structure. Some yeasts and non-photosynthetic bacteria are also grazed upon, but on the whole, all the grazers and predators, even strict carnivores, ultimately depend upon the captured sugars and starches that algae, euglenoids and plants have constructed out of sunlight and carbon dioxide. As our whole planetary world of multi-celled organisms depends on this green base, so does the limited ecosystem of a planted aquarium.
 
Energy transfer. Remember that illustration from your High School Biology book that showed the pyramid of trophic levels in an ecological system? Remember how it was built up from a vast mass of green plants at the broad base and came to a point at the pyramid's tip, with a very few carnivores? The reason for the pyramid shape is that not much of the available energy is successfully transferred from one trophic level to the next. Actual number values are hard to fix exactly. The warm-blooded scientists assigning number values to trophic processes have tended to focus on the warm-blooded mammals. It's a natural mammal-centric bias, but we mammals might not be very representative creatures; we waste a lot of potential energy simply keeping cool or getting warm, panting and shivering and chasing a meal just to keep the furnace running. Relying for their numbers perhaps too much on the metabolism of mammals, scientists have encouraged us to rate the effective energy transfer from one trophic level to the next at about 10%. But the efficiency of fishes' metabolism is rated higher, nearly 50%. I've read that an octopus has a food conversion efficiency about 80%. Particularly efficient energy transfers, say among bacteria, might successfully use much more of the available energy. Though the actual number values might be higher, yet the principle holds true: on the whole, only a smallish percentage of the potential energy stored in the tissues of an organism is actualized when it is eaten by the next member of the food web.
 
Only a bit of that potential energy actually results in body growth, active metabolism, maintenance of tissues and so on. The rest is partly excreted, and it is partly dissipated in the process called entropy. How is the energy potential dissipated? Think of a fluorescent light bulb translating electrical energy. Only part of the electrical energy is transformed into visible light; even in an efficient fluorescent light unit, much of the rest is dissipated at a lower energy state, as heat for instance, and as vibrations causing an audible hum.
 
How many trophic levels? Even in the richest, most balanced natural system, it is rare to find a food web that consists of more than five levels, ecologists tell us. Why is this? Only a small amount of available light initially gets trapped and used by a chloroplast at the base of the system, and afterwards the energy that has been stored isn't very efficiently transferred at each succeeding trophic level. If about 90% of the available energy is lost at each transfer, there is usually not sufficient energy left after, say, four transfers to sustain a top-of-the-food-chain population on the remaining one-ten-thousandth of that energy first trapped at the base. A ten-thousandth part simply represents a tenth of a tenth of a tenth of a tenth of the energy potentially available at each trophic level.
 
Omnivores. An ecologist will tell you that "omnivory is rare," that few species feed at more than one level. This is less true in a freshwater system than it is, say, on the East African savannas. Teleost fishes are such successful opportunists, so flexible in their diets, that they often cut right across the food web; your barbs will graze some algae, pick at some copepods, nibble some softening plant leaves, snap up a larval fish or a larval mosquito, or a fish egg— notoriously— and scavenge freshly-dead corpses, such as frozen brine shrimp.
 
Another ecological maxim will tell you that "food webs vary with environmental structure; they are longer in three-dimensional habitats." The aquatic environment is a fully three-dimensional environment, more so than any terrestrial habitats, except perhaps the rainforest canopy.
 
Links. Ron Shimek writes of "Reef Aquaria as ecosystems". Many of the points I'm making here about trophic webs and the recycling of nutrients apply perfectly well to tropical reefs and are reflected in the mini-reef aquaria that capture a little of their diversity. Shimek is a biologist who's highly knowledgable in this role as ecologist. He also writes well.