Charted waters
Sometimes we hobbyists can’t see the forest from the trees when we discuss and worry about the fine details about reef aquariums. It might be useful to remember why we spend so much time trying to find that optimal protein skimmer or argue about the merits of different additives: the ultimate goal of almost all our actions is to create and maintain water with dissolved salts and organics that resembles as much as possible natural sea water (NSW).
Sometimes it seems to me that even experts who publish articles about marine aquariums get confused about the actual composition of seawater. For example, there is a lot talk about iron (Fe) even though it is one of the least concentrated elements – no one seems to care that there is 320 times more barium in seawater. Barium concentration follows so called “nutrient profile” meaning it’s concentration is typically controlled by biological processes so it should be of interest for aquarists. The natural concentration of iron for surface water would be around 0,0000006 grams per liter. For a 400 liter (100 gallon) tank it means that you can bring the water from zero to NSW levels with just 2,2 µgrams (0,000022 grams) of iron!
Some important notes:
- I use the actual amounts, not the weights! Aquarists use almost exclusively weight as a measure of element concentrations which is not very logical. Actually, it’s just plain wrong! For example, there is about 5x more magnesium than calcium in seawater (52.83 vs. 10.28 mM) and yet I’m sure many people tend to think it’s more like 3x (1200 mg vs 400 mg). The reason for this is that magnesium is much lighter than calcium. For salts in solution the weight of atoms of some elements is really a kind of backwards way to express their amounts. For some less abundant elements this distinction can make a large difference in relative portions of elements
- However, to make it more easy to understand the relative amounts of different groups I have calculated the sum of weights of each group which is displayed on top of the graph. The unit is micrograms (0.001 mg or 0.000001 g) per one kilogram of seawater
- Every successive graph in each section contains elements that combined represent so small amount of total that they don’t even register on the previous graph
- Data does not contain gases
- For elements that have “surface depleted” or “nutrient” distribution profile in seawater I have chosen the low point of concentration range. Typically tabulated data about seawater composition use mean values for elements but since almost all seawater is very deep and very cold the mean value might not be the best representative for waters over corals reefs
- I have excluded Na and Cl (which combined is table salt) because they would dominate all groups they belong to. I assume you know that seawater is almost completely made of water and what little is left is almost completely made of table salt
- If you want the data I’ve compiled, please leave a comment and I will send it to you.
- I wouldn’t be surprised if there are some errors
Everything
Seawater must contain all elements that are found on Earth although not all of them have been detected there yet. Typically seawater is split into two parts, the major and minor (including trace metals) constituents. A common lower limit chosen for major elements is 1 ppm (1 mg/kg). Substances at this and larger concentration can have detectable influence on the density and are usually found in nearly constant proportions to each other and to the salinity. For this reason they are called conservative elements. Most elements present in concentrations of less than 1 ppm are not conservative.
It is quite difficult to present the constituents of the seawater visually because the range of concentrations span several orders of magnitude and yet some very minor element can be as important (or even more important) as the major components, at least from the aquarist’s point of view. This is the reason why I have chosen the percentage scale and have split the elements so that each pillar shows elements whose combined concentrations would not even register in a previous one. It would be possible to combine all of them in one graph but it would have to be very tall pillar…
An interesting factoid: it is not possible to accurately measure sodium in seawater and thus it’s concentration is determined by summing up equivalent concentrations of all the anions and then subtracting the equivalent concentrations of all the cations except sodium. In theory this means that if it is found that the accepted concentration of some major element should be changed it would also change the reported concentration of sodium. It highly unlikely that there will be a large change to any of the major elements’ concentrations, however.
Note that all chemical elements in the first two pillars are conservative.
Conservatives
This is a group of elements that really define what seawater is. All elements in the piecharts above are always found in those proportions, no matter where in world you are. Their total amount is defined only by salinity and local processes do not alter their concentrations in a significant amounts in “normal” conditions. This is the reason they are called “conservative elements”. Note: I have listed carbon as a conservative element which it technically isn’t because CO2 concentration can be highly variable.
This also means that by measuring only one parameter you’ll get a very good estimate what the concentration of any other conservative element is. And since the salinity is the only variable affecting the absolute concentrations, you really only need to measure salinity to get, for example, potassium concentration.
The relative proportions of conservative elements is also the ultimate guideline for artificial seawater (ASW) and should in my opinion be the standard that ASW mixes are compared to. Unfortunately, there are only a few mixes that come close. If you are creating a salt mix that is supposed to mimic seawater, you better get at least the elements from the first three piecharts balanced correctly. I just can’t understand how someone can formulate a mix that results in 120% magnesium and 140% calcium concentration and still call it “artificial seawater”. It is saltwater but not even close to seawater.
Follow the nutrients
This is a group of elements that should be most interesting to marine aquarists. All these elements have a distribution model called “nutrient-like distribution” in nature which does not mean they are necessarily considered to be essential nutrients. Instead, “nutrient-like” means that their concentrations follow either one or more of the elements classically labeled as nutrients, namely nitrogen, phosphorous and silicon.
The uptake and release of these elements by biota is large and fast enough to have measurable effect and the effect is so large that the water exchange is not fast enough to either replace or dilute local concentration. These elements are all likely to be incorporated into body materials and transported downwards and then released back when the organic matter is metabolically destroyed.
It is likely that some of these elements will become depleted in a well working reef aquarium with an efficient organic matter removal equipment.
Poor surface
This class of elements are known to be depleted at surface. There can be many reasons for this and for some elements nobody really knows why they are depleted. Some are well known to have important role in biology, like iron and vanadium for example. They do not follow the previous nutrient-like distribution model however because they are not necessarily released back to water column when an organism is broken down but are precipitated and buried into sediments, for example.
One common process that causes the surface depletion is some elements’ tendency to stick to particles and thus be swept out of water column in all depths.
In a reef aquarium these elements are exported together with organisms or particles. If you use efficient mechanical filtration or your skimmer is able to remove particulate matter, it is possible that at least some of these become depleted.