# [Wet Thumb Forum]-Carbon in the aquarium



## Roger Miller (Jun 19, 2004)

Carbon is the most common element in living matter. Tracing where carbon comes from, where it goes to and how it gets there and quantifying the amounts of carbon in the flux are important goals in ecology. In ecology carbon and energy are expected to move together through a system. The intimate relationship between carbon and energy is also likely to hold true in aquaria, though the prevalent mechanisms may differ from natural conditiions. Carbon must be understood in order to understand the ecology of an aquarium.

The following sketch shows how carbon gets into and out of an aquarium, how it is found in the aquarium and how it changes form in the aquarium.








Those of you who were reading APD a few years ago might remember the multipart presentation I gave about the phosphorus balance in aquariums. That was done in hopes that people who understood phosphorus a little better would be less superstitious about it. Hopefully I can be much more brief in my discussion of carbon.

Carbon occurs in aquariums in many forms. The forms can be grouped into:

Particulate Organic Carbon (POC) -- mostly mulm, detritus and decorative wood.

Dissolved Organic Carbon (DOC) -- a large class of mostly biologically-derived chemicals that are soluble in water. Includes e.g. coloring agents, some sugars and organic acids, and plant exudates.

Particulate Inorganic Carbon (PIC) -- carbonate minerals. This is mostly in stones and substrate materials, snail or clam shells and marl.

Dissolved Inorganic Carbon (DIC) -- CO2, bicarbonate and carbonate.

Carbon in living organisms.

The large rectangle in the drawing represents the aquarium boundaries, inclusive of any filter. Carbon comes and goes through the boundaries in several ways.

POC is added as fish food and removed during aquarium cleaning.

DOC is added in medicines, additives and fertilizers and removed (sometimes added) through water changes.

PIC is not usually added, but it is often removed during cleaning.

DIC is the most mobile of all the forms, and much of that mobility is as CO2 gas. DIC is exchanged (added or removed) with the atmosphere, added through CO2 fertilization and added or removed during water changes.

Living organisms are added and removed from aquaria on a regular basis. Quantitatively, the removal of live plant matter is probably a very large part of the carbon balance in a planted aquarium.

In the aquarium most of the interchange between different forms of carbon happens through biological activity. The principle exception is the precipitation and dissolution of carbonate minerals. There are also some non-biological exchanges between dissolved and particulate organic carbon.

Heterotrophic organisms (including fish, snails and other higher organisms as well as protists and bacteria) consume POC and DOC as food and respire DIC. Upon their death their organic remains become POC and DOC and their mineral tests become PIC.

Autotrophic organisms (including plants and algae) consume DIC and also produce DIC through respiration. DOC is produced as a variety of exhudates and upon the death of all or part of the organism the organic remains become POC and DOC.

There are minor pathways that I have omitted from this chart. Can anyone think of any major pathways that I may have left out?

Roger Miller


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## Roger Miller (Jun 19, 2004)

Carbon is the most common element in living matter. Tracing where carbon comes from, where it goes to and how it gets there and quantifying the amounts of carbon in the flux are important goals in ecology. In ecology carbon and energy are expected to move together through a system. The intimate relationship between carbon and energy is also likely to hold true in aquaria, though the prevalent mechanisms may differ from natural conditiions. Carbon must be understood in order to understand the ecology of an aquarium.

The following sketch shows how carbon gets into and out of an aquarium, how it is found in the aquarium and how it changes form in the aquarium.








Those of you who were reading APD a few years ago might remember the multipart presentation I gave about the phosphorus balance in aquariums. That was done in hopes that people who understood phosphorus a little better would be less superstitious about it. Hopefully I can be much more brief in my discussion of carbon.

Carbon occurs in aquariums in many forms. The forms can be grouped into:

Particulate Organic Carbon (POC) -- mostly mulm, detritus and decorative wood.

Dissolved Organic Carbon (DOC) -- a large class of mostly biologically-derived chemicals that are soluble in water. Includes e.g. coloring agents, some sugars and organic acids, and plant exudates.

Particulate Inorganic Carbon (PIC) -- carbonate minerals. This is mostly in stones and substrate materials, snail or clam shells and marl.

Dissolved Inorganic Carbon (DIC) -- CO2, bicarbonate and carbonate.

Carbon in living organisms.

The large rectangle in the drawing represents the aquarium boundaries, inclusive of any filter. Carbon comes and goes through the boundaries in several ways.

POC is added as fish food and removed during aquarium cleaning.

DOC is added in medicines, additives and fertilizers and removed (sometimes added) through water changes.

PIC is not usually added, but it is often removed during cleaning.

DIC is the most mobile of all the forms, and much of that mobility is as CO2 gas. DIC is exchanged (added or removed) with the atmosphere, added through CO2 fertilization and added or removed during water changes.

Living organisms are added and removed from aquaria on a regular basis. Quantitatively, the removal of live plant matter is probably a very large part of the carbon balance in a planted aquarium.

In the aquarium most of the interchange between different forms of carbon happens through biological activity. The principle exception is the precipitation and dissolution of carbonate minerals. There are also some non-biological exchanges between dissolved and particulate organic carbon.

Heterotrophic organisms (including fish, snails and other higher organisms as well as protists and bacteria) consume POC and DOC as food and respire DIC. Upon their death their organic remains become POC and DOC and their mineral tests become PIC.

Autotrophic organisms (including plants and algae) consume DIC and also produce DIC through respiration. DOC is produced as a variety of exhudates and upon the death of all or part of the organism the organic remains become POC and DOC.

There are minor pathways that I have omitted from this chart. Can anyone think of any major pathways that I may have left out?

Roger Miller


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## wetmanNY (Feb 1, 2003)

Truly excellent for its clarity. Thank you, Roger Miller.

Doubtless I tend to overestimate the microcosmic characteristics of a planted aquarium. But in your statement *" The intimate relationship between carbon and energy is also likely to hold true in aquaria, though the prevalent mechanisms may differ from natural conditions"* I'd have removed "likely" --as a hedge that you don't need, because I can't identify any mechanism-- a pathway-- that obtains in the aquarium that does differ from an undisturbed lentic natural system.

What made you hold back? Not the widespread practice of diffusing additional CO2, for the mechanisms/pathways remain the same even if the source is artificial.

What I mean is that your rectangle could identify a pond or a lake, and your diagram would still apply.


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## Roger Miller (Jun 19, 2004)

> quote:
> 
> Originally posted by wetmanNY:
> What made you hold back?


To the best of my knowledge noone has ever worked out the energy budget for an aquarium and established that it can be correlated to carbon. Would you like to give that a shot?

In natural systems the vast majority of the energy budget originates from sunlight and that energy enters the food web through photosynthesis where it becomes tied to carbon. In aquaria the heaters and pumps are a large part of the total energy budget and that energy isn't closely related to carbon.

Consider an experiment; set up two tanks, one with light as the sole energy source (no heater, no circulation) and one with the same amount of light but with heating and circulation. Supply both with carbon and nutrients in excess of demands. Would you see a difference in the 
carbon budget? I think you would, but the difference would not be in proportion to the difference in the amount of energy in the budget; the non-light sources of energy would be significant, but different from the natural condition.



> quote:
> 
> What I mean is that your rectangle could identify a pond or a lake, and your diagram would still apply.


At a very broad level that is true, though you would need to generalize many of the terms to include things (e.g. waterfowl poop) that we don't deal with in aquaria, and the relative importance of some terms will change. If you start quantifying terms then a lot of the broad similarities start to disappear.

Roger Miller


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## Wheeler (Feb 8, 2004)

So, Roger, can we discount energy sources like gravity (circulation) and geothermic processes (heater & circulation) in natural systems with regards to comparison with aquaria? It seems obvious that the sun is the major player, but can we make comparisons using other events as well?

My 100-level geological hazzards class may be failing me now(!), but I seem to remember a list of sources for geological energy. Are they insignificant in the carbon game and the comparison of aquaria to natural systems?

I'm sure that this isn't where you wanted this topic to go, but it's interesting...

BTW, very good topic and graph. Some of those acronyms are tossed around and I never know what they mean. Thanks.

Best wishes,
John Wheeler


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## Roger Miller (Jun 19, 2004)

> quote:
> 
> Originally posted by Wheeler:
> So, Roger, can we discount energy sources like gravity (circulation) and geothermic processes (heater & circulation) in natural systems with regards to comparison with aquaria? It seems obvious that the sun is the major player, but can we make comparisons using other events as well?


Ecological studies typically ignore energy sources other than light. The main exceptions are cases where there is no light and where some other energy source -- typically geothermal heat -- is unusually prominent.

Nature does provide other energy sources. Gravity causes currents in streams and rivers and density-driven turnover in temperate lakes and ponds. Wind is actually solar-powered, but in any given locale wind provides a source of energy that isn't derived from local solar input

The link between the carbon flux and non-light sources of energy are not very clear. Further, they tend to be constant over a given study area and not very amenable to study. It seems reasonable that ecologists should relegate their effects to background noise.

Unlike natural ecosystems, aquariums are very well-defined systems. We shouldn't need to depend on a correlation between energy flux and carbon flux.

I *think* that most of the energy that gets into the biological system can probably be related to the carbon flux either through photosynthesis or through the fact that chemical sources of energy (foods and additives like Seachem's Excel) also contain carbon.

On the other hand, I'm pretty sure that experiment would show that energy added through heating or circulation does appear in the carbon flux. Is it significant? I'm not so sure about that, but I think it's best to assume that other sources of energy are signficant until shown otherwise.

Digressing somewhat, the aquarium-to-nature analogy doesn't allow a direct comparison between the heater and geothermal heat or between circulation and gravity.

In aquariums as in nature, heat is provided mostly by light. Aquarium heaters are required to make up for heat lost through the container walls. Heaters aren't needed for an aquarium in a warm room and are little-used in large, well-lit aquariums. The heater is providing energy that in a natural environment would be provided by sunlight.

The circulation in an aquarium could "stand in" for natural circulation driven by a moderate wind. Our artificial circulation doesn't compare well to gravity-driven currents. The circulation we provide might be analogous to the current in a stream as long as we're talking about a very sluggish stream. It generally is not comparable to the currents in a river.

Roger Miller


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