Ionic composition of water in recirculating marine ecosystems
Consideration of the water chemistry requirements of a recirculating marine ecosystem, such as may be established and maintained in aquaculture, aquaponics, zoological displays, or research applications, begins with the composition of the seawater, itself, not surprisingly. Following is a brief discussion of the chemical characteristics of seawater as it exists in oceanic waters (defined as water beyond continental margins).
Composition and Salinity.
Seawater is primarily composed of water, inorganic ionic solutes, organic solutes (colloids) and particles (solids), and entrained gases. It is primarily the inorganic ionic solutes and the organic solutes that are of concern in a recirculating marine ecosystem, because they must exist within ranges (specific to each ion, or within what we will refer to as a balanced nutrient budget) suitable for each specific system in order to achieve long-term success as a caretaker of that system. Concentration values and ranges of these substances often change as the system undergoes an evolution in the rate of cohort respiration and waste production and extraction, themselves influenced (directly and/or indirectly) by changes in the population build of the cohort, as well as the efficiency of filtration, illumination characteristics, and flow pattern within the system. As an example, a phosphate concentration of 0.05 mg/L may be tolerated by the system, in general, for a period of months; then, a change in illumination may influence the growth of phototrophs, such as cyanobacteria, requiring that the phosphate concentration be decreased to a value at which the cyanobacteria cease to proliferate and, ultimately, recede to a tolerable presence.
Water will not be discussed in detail within the confines of this section; it will be revisited with the topic of synthetic seawater preparations.
Inorganic ionic solutes are collectively the major, minor, and trace ions, as well as compounds thereof. The sum of these ions, in units of grams, present in 1 kg of seawater is the salinity value. It is expressed as parts per thousand (‰), as would be evident by the g / kg unit, or sometimes simply by the letter ‘S’, denoting ‘salinity’, in this case omitting “‰”. Salinity values within oceanic waters range from 33 to 37. The average salinity value that is most frequently cited in literature is 35; throughout the remainder of this article, the term “standard salinity” refers to this value of 35. Near shore salinity values may be considerably lower due to freshwater input from runoff. Conversely, bodies of water having relatively high rates of evaporation and low rates of freshwater input exhibit salinity values higher than in open water. The Red Sea and Mediterranean Sea are prime examples of such semi-enclosed bodies of water, the former with salinity values up to 42.
Density and Relative Density.
Density of seawater is determined by incorporating salinity, temperature, and pressure values of a sample at the testing site (in other words, in situ). A common unit of density measurement employed by oceanographers is sigma-t (st), which incorporates salinity and temperature of the sample at atmospheric pressure. Density is a physical property of a substance, which differs from specific gravity (SG), a measurement of relative density (in this context, the ratio of seawater density to that of freshwater density at the same temperature) technically having no units of measure (they cancel each other out). Although the density of pure water is often reported as 1.000 g/cm3, this is a temperature- and pressure-specific value. Density of pure water is temperature- and pressure-dependent, and therefore can have a value unequal to 1.0 g/cm3; for example, at 25°C and atmospheric pressure, the density of pure water is 1.00287 g/cm3. It is therefore possible that the specific gravity value of a seawater sample will not mirror sigma-t. So to reiterate in one concise statement: specific gravity is a density ratio, whereas sigma-t is a density value based upon physical and environmental characteristics of a water sample. From a practical, applied perspective for the caretaker of a recirculating marine ecosystem, the slight difference in density of pure water at the temperatures that most recirculating marine ecosystems operate within makes the use of SG as a means of monitoring water density sufficiently accurate for long term success. It is useful to understand the relationship and differentiation between these two units of measure, however. Rarely (if ever) do oceanographers cite characteristics of seawater in units of specific gravity.
As previously mentioned, changes in temperature and pressure influence the density of a fluid. Therefore, a linear conversion between salinity and sigma-t, and consequently between salinity and SG, values occurs only at a standard temperature value. A simple example:
Seawater with a salinity value of 35 equates to a sigma-t value of 1.02336 g/cm3 at a temperature of 25°C.
Seawater with a salinity value of 35 equates to a sigma-t value of 1.02478 g/cm3 at a temperature of 20°C.
These values are calculated utilizing a Sigma-T table, further extrapolated as necessary when specific values are required for water samples with salinity or temperature values existing between the integers normally published.
Inorganic Ionic Solutes.
For the sake of brevity, ‘inorganic ionic solutes’ will be referred to in this section simply as ‘ions’. As previously mentioned, three classifications of ions exist, with respect to their observed concentration, in seawater: Major (≥1 mg/L); Minor (<1 mg/L, ≥1 mg/kl); and Trace (<1 mg/kl). The units “mg/L” and “mg/kl” are used interchangeably with “ppm” and “ppb”, respectively.
The major ions in seawater account for over 98% of the total dissolved substances by unit mass.
Regardless of salinity value, the ratios of major ions in oceanic water remains unchanged. This is a principle known as The Rule of Constancy of Proportions, also referred to as Marcet’s Principle. In the open ocean, this situation is able to exist largely due to the vast resource, or pool, of the major ions from such sources as: sediment; minerals seeping from the sea floor; remineralization of latent material into the components, carried out largely by microbial processes; runoff from terrestrial habitats. Within this article, a point is made to discuss this very important principle of seawater chemistry because caretakers of recirculating marine ecosystems must understand the following:
Ratios of elements remain the same, regardless of salinity value.
Concentrations of elements change in relation to changing salinity value.
It may seem to some caretakers that the latter point is unnecessarily made in this article, however we have come across individuals who did not understand, for example, why the calcium concentration in newly-mixed seawater using an engineered seawater salt blend did not measure 412 mg/L despite the fact that they had adjusted specific gravity to 1.021 (S≈32 at 77°F). At S=32, the calcium concentration would measure ≈377 mg/L.
In oceanic water, concentrations of major ions do not become significantly depleted by biological uptake or incorporation into insoluble material. Major ions are considered to have conservative concentration profiles, a term which reflects the unchanging proportion of the ionic composition of seawater for these ions regardless of salinity. There are exceptions, however, to this principle in natural systems, such as in bodies of water in which interactions (consumption/depletion) occur between major ions and certain aspects of the system exceeding the available pool of those ions. Biological demand may exceed the rate of input of one or more major ions; similarly, chelation with complex organic material, such as that found in sediment, may act as a sink for certain ions (cations, in particular). When the concentration of an ion depletes as a result of these interactions, then it is considered to exhibit non-conservative behavior. The determination is primarily made by testing water samples at various depths within the zone of water to which sunlight can penetrate adequately to support photosynthesis; this is termed the photic zone. If the concentration of an ion over this zone exhibits a common pattern, or curve (a “nutrient profile”), then it is in indicator that the ion is being taken up by organisms or is being incorporated into material within the zone, and is therefore either being removed (in the case of the latter, if that material then sinks beneath the photic zone) from the zone or is becoming part of the latent pool of material, unavailable in an aqueous form. In either case, the ion ceases to exist in the aqueous form, so the concentration undergoes depletion.
A tidal pool is a simple and effective example of such a system. Isolated from the larger body of water when the tide recedes, depletion of certain major ions may occur dependent upon the biological profile and sediment composition of the pool, until the tide rises sufficiently to reestablish the connection with the larger body of water, and the concentrations of all major ions rapidly return to average values (for that body of water). A recirculating marine ecosystem can be considered a permanent tidal pool. Without input from an external source, concentrations of various ions will decrease via the aforementioned mechanisms.
The significance of discussing conservative and non-conservative behavior is quite relevant to the caretaker of a recirculating marine ecosystem, specifically in their assessment of which ions are suitable to adding to systems under their care. This is particularly the case with minor and trace ions. If, in the open ocean, a minor or trace ion has been determined to exhibit conservative behavior (again, the concentration relative to salinity does not change), then one of two scenarios may be assumed to be the cause:
The rate of uptake mirrors the rate of input to the system (this is highly unlikely).
The ion is not, to any significant extent, incorporated into living biomass, organic material, or other substances existing within that system. The concentration of such ions remains unchanged until addition occurs.
The relevance of discussing these points is that minor and trace ions exhibiting conservative behavior routinely appear on the list of ingredients making up synthetic sea salt and supplementary blends. In principle, this practice seems unnecessary given the preceding commentary. To provide one succinct example, if cnidarians living in a natural reef ecosystem ignore the presence of lithium, then they will ignore it in a recirculating marine ecosystem if average values of all critical elements are maintained.
There is slight variation in the observed ionic composition of oceanic water at standard salinity, hence ionic values published by different researchers are often not in agreement.
Various sources of seawater commonly utilized in recirculating marine ecosystems.
Salt from Desalination Processes
The process of removing solutes from seawater is employed in regions (in particular, arid regions) where water suitable for drinking by the populace is in very low abundance. This process constitutes desalination, which is achieved in commercial applications through various means, incorporating one of more of the following: membrane filtration; ion filtration; distillation; freezing; evaporative process (humidification and dehumidification).
Countries with coastline on the Red Sea utilize this process to provide potable water for the citizens, and for industrial and agricultural applications. The primary purpose of the process is to obtain the purified water, with collection of solutes (if they are of interest to the plant operators) used to decrease the cost of operation through sales to interested parties, a point to which we will return shortly. In the event that the solute is unwanted and/or undesirable to the plant operators, then additional seawater is pulled from the Red Sea to rinse the salt residue away, the resulting brine pumped back into the Red Sea, itself. Chemicals employed to decrease buildup of minerals (particularly calcite, dolomite, and gypsum) which accumulate within the collection and treatment vessels are pumped out into the Red Sea along with the brine. Additionally, biocidal chemicals, notably chlorine compounds, are utilized to sterilize the water, the waste being pumped into the Red Sea with the other discharge. As one would expect, the impact on the marine biota varies with respect to their proximity to the discharge site and the tolerance that a species has for the constituent hypersaline water and aforementioned chemicals. All of this taken into consideration, it is not difficult to appreciate the viewpoint put forward by many in the global scientific community that these processes are environmentally destructive, not just in the Red Sea but in any environment which makes use of these commercial desalination measures. In terms of energy consumption, commercial desalination plants, such as those in operation along the Red Sea, are thought to require as much energy as would be consumed in the transportation of freshwater from other locations, though obviously the actual costs associated are locale-specific.
As previously mentioned, the solute, or more simply “salt”, is a byproduct of the desalination process, and may be sold in whole form or further refined to isolate constituent compounds for sale to various specialty industries, should the plant operator be so inclined (as opposed to pumping the salt back out into the sea). Salt from the Red Sea has been sold for many years, under various brand names. Consistency of suitable ionic composition and absence of nutrients and toxins should be the primary criteria by which a salt meant to produce seawater for application in a recirculating marine ecosystem are judged. Ionic content and presence of organic material in salt obtained through desalination varies. In the case of salt destined for use in a recirculating marine ecosystem, the aforementioned precipitation of minerals containing calcium, magnesium, and/or carbonate ions requires the salt to be batch tested for ionic composition, augmented with sufficient minerals to re-establish the appropriate ionic content, and then blended to incorporate these minerals so that a homogenous salt producing consistent ionic values when mixed with water to a standard salinity value results. The companies who market sea salt blends consisting primarily of desalination byproduct, and who readily promote the origin of the salt as having come from a natural body of water (such as the Red Sea), may be contacted and queried about the extent of processing that the salt undergoes in preparation for use in a recirculating marine ecosystem, the user employing their own judgment in determining the suitability of such a product in systems under their supervision. The aforementioned environmental impact that desalination poses on the natural body of water from which the salt is derived may, in and of itself, be a sufficient deterrent for many users who are concerned with the well being of delicate marine ecosystems.
Prepackaged Natural Seawater
Several companies offer for sale filtered natural seawater. Conceptually, the use of natural seawater which has: been passed through a mechanical filter with a pore size not greater than 0.1-micron; undergone a filtration method for the complete elimination of organic material (including bacterioplankton and stable complex chemical compounds (e.g. pharmaceutical residues, hormones, etc.)); undergone a filtration method for the complete elimination of nutrients such as phosphate and silicate, seems plausible. In application, assuming that the aforementioned measures have been taken to yield “clean” seawater, there is nothing principally wrong with using this water in a recirculating marine ecosystem. However, the practice of doing so if the seawater has to be shipped from the point of collection to an end user results in considerable environmental burden. Water accounts for nearly 98% of the total weight of seawater. The carbon footprint associated with shipping seawater from a location such as the Caribbean or the West Coast of the United States to inland destinations is massive. At a salinity value of 35, the weight of 1 gallon (US) of seawater is 8.54 lbs. Of that, 8.33 lbs. is water. If a supplier ships a pallet consisting of 2,500-lbs (as an example) of this pre-packaged seawater, then water accounts for 2,438-lbs. Consider the environmental impact of carbon emissions from the delivery vehicles tasked with moving this pallet, or the individual packages, from an origin along the Florida coast to end users in the Midwestern United States, or worse the PNW.
Water coming from a tap, which has been filtered by the user such that the chemical characteristics meet their requirements, involves no shipping of actual water. It does, of course, require that the user possess reliable filtration equipment, however the cost of such equipment has dropped dramatically within the past decade due to the improved supply from manufacturers and dealers working within the industry of water treatment for potable applications. Similarly, technological advancements continue to improve filtration efficiency of modern systems. A point-of-use system capable of purifying 100 gallons of source water on a daily basis costs less than US$100 at the time of this writing (June 2020), and replacement reverse osmosis membranes are available for US$20.
Of additional concern is the amount of packaging material, specifically the mass of the plastic required, in containing pre-packaged seawater. The production of this plastic requires natural resources which, again, entail a substantial carbon footprint.
Economical and Augmented Synthetic Sea Salt Blends
Two of the potential classifications that a synthetic sea salt may fall under are: Economy (content of specific ions below NSW values at standard salinity); Augmented (content of specific ions exceeding NSW values at standard salinity).
Economy blends initially were formulated for application in re-circulating aquaculture systems, wherein the values of certain ions at a standard salinity were deemed less important than the associated reduced cost of manufacture and procurement. Cohorts composed wholly of crustaceans, such as commercial shrimp farms and lobster holding tanks maintained by restaurants and grocers, have been operated with such economy sea salt blends for decades. Considerable resources have gone into marketing these economy blends on the basis of their low cost relative to many competing blends, however the initial cost of purchase is a poor basis for selection of a synthetic sea salt blend, and in reality the cost of utilizing such a product in a recirculating marine ecosystem may be higher than the utilization of a blend containing the correct ionic ratios and content at standard salinity. This is particularly the case in recirculating marine ecosystems housing cohorts of reef building invertebrates. To understand why this is the case, it is necessary to address the specific ions which are reduced in the economy blend in order to produce a low-cost product, as well as the manner in which the blend is labeled for sale.
Addressing ionic content of seawater, of greatest concern to the caretaker of reef building organisms is the content of magnesium, calcium, potassium, strontium, and the alkalinity value when the seawater is mixed to standard salinity. The least expensive component of a synthetic sea salt blend, in terms of the cost per unit mass, is sodium chloride. Presently, salts of potassium, strontium, magnesium, bicarbonate, carbonate, and calcium (in that exact order) are considerably more expensive than sodium chloride. Reviewing the ionic content of natural seawater, it is evident that magnesium is the second most abundant cation present (only sodium has greater abundance). Providing magnesium in the appropriate amount (and in any anhydrous form), regardless of whether the salt is chloride- or sulphate-based, is the single greatest expense in the component cost of a sea salt blend formulation. Next in line is the cost associated with sodium chloride. Following that is the cost of potassium salts, and then calcium salts. When the magnesium is derived from a hydrated salt, the cost of sodium chloride may exceed the cost of magnesium salts in the blend, however the mass added to the finished blend by way of water incorporated into the hydrated magnesium salt adds to the cost of transportation (and also tends to introduce impurities to the blend, detrimental to the health of the cohort). It follows that a producer wishing to market an inexpensive sea salt blend will reduce the amount of magnesium present, to whatever extent satisfies their cost requirements. If they have paid any attention to potassium content at all, then this would be the next logical ion to reduce relative to NSW content. Finally, the amount of calcium would be reduced to further cut component cost.
Users of such economy sea salt blends are faced with the reality that the concentrations of several critical ionic components of the newly-mixed seawater are below NSW values at standard salinity. To what extent the values are lower depends entirely upon the formulation, however the result is that the recirculating marine ecosystem caretaker must now perform an analysis of the water for each ion of interest and then supplement the water with appropriate compounds such that the ionic content meets the requirements of the cohort. The cohort need not be comprised wholly, or even in part, of reef building organisms. Organizations working with captive breeding of marine fish species, and/or who are raising them from very early life stages through maturity, attest that there is an obvious positive impact on the health of the individuals comprising the cohort when the inorganic water chemistry reflects the ionic ratios of natural seawater. This environment cannot be easily produced by using a sea salt blend which is imbalanced at the onset or requires additional input of critical ions before it is suitable for use. Ironically, the cost of purchase of individual components is always higher than is the cost that the producer would pay to incorporate them into the salt blend at the onset. The costs associated with augmenting such a salt blend can easily negate the initial savings.
As previously alluded to, the labeling of an economy synthetic sea salt blend as treating x-gallons of water can be misleading to the consumer, specifically in that the volume claimed on the package does not indicate to the consumer that, in actuality, the salinity of that x-gallons of water will be short of 35. For example, consider a package of any synthetic sea salt that claims to be sufficient to produce 150 gallons of seawater. When the consumer mixes 150 gallons of water with the entire contents of the package, they might discover that the salinity value is only 31 (1.020 at 25°C / 77°F), for example. If the target salinity value in the recirculating marine ecosystem is 35, then they will need to add more of this synthetic sea salt blend to their mixing vessel (the volume of seawater produced will now exceed 150 gallons by way of displacement due to the additional salt mass added). In this example, ~13% more salt will be required to achieve S=35. Another way of looking at this example is that the package will yield S=35 in ~133 gallons of water.
Taking all of these points into consideration, then, it follows that the use of an economy sea salt blend in any recirculating marine ecosystem entails:
A salinity value <35 for the stated volume of water treated, according to the producer and/or the external packaging;
Caretaker analysis of the mixed seawater solution at the target salinity value in order to determine the course of supplementation required before the water can be introduced into a functioning recirculating marine ecosystem;
The addition of salts (magnesium; calcium; potassium; strontium; bicarbonate; and/or carbonate) in order to produce a seawater solution with the desired ionic values;
The overall allocation of monetary and temporal resources to the adjustment of the seawater solution produced with an economy sea salt blend exceeding the resources associated with purchasing a balanced synthetic sea salt blend from the onset.
Augmented (or “Enhanced”) Sea Salt Blends
In contrast to economy sea salt blends, augmented sea salt blends are formulated to provide elevated concentrations of some ions at standard salinity.
There would seem to be a pragmatic advantage in utilizing an augmented sea salt blend in recirculating marine ecosystems housing reef building organisms, provided that the augmented formulation maintains NSW ratios of cations behaving in a non-conservative manner. The use of a sea salt blend formulated to deliver the actual ionic concentrations present in oceanic water has been observed by some commercial coral aquaculture enterprises to yield equivalent or superior results to some augmented formulations. It is probable that the cohort, made up of organisms which have evolved within and occupied a chemical environment which has been, up until very recently, extremely stable, simply responds better to residing within water with “familiar” chemical characteristics than it does to persistent exposure to elevated content of the aforementioned cations, despite the fact that many of these ions are incorporated into the skeletal material collectively secreted by the cohort.
Additives which do not appear in the inorganic chemistry of natural seawater are incorporated into many sea salt blends, reportedly (according to the accompanying promotional literature) to provide advantages over the use of competing products. A term frequently used by the company selling such a product is “innovative”.
Whenever the term “innovative” is applied to what should be a simple concept, the first thought that arises is “marketing ploy”. The following list, which is not exhaustive but is certainly illustrative, includes additives which have traditionally, and in some cases more recently, been incorporated into synthetic sea salt blends marketed to operators of recirculating marine ecosystems:
EDTA and other organic, chelating compounds, reportedly added to the salt in order to bind toxic multivalent cations (e.g. lead, arsenic), but which also bind cations such as calcium, furthermore introducing an organic component to the salt water which must then be accounted for by the biological filtration;
Organic forms of calcium, reportedly to hasten the uptake of the element by reef building members of a recirculating marine ecosystem cohort, but which (again) contribute to the organic content of the recirculating marine ecosystem and must be dealt with by biological filtration;
Vitamins, reportedly added to benefit the cohort through the presence of these critical compounds, however which are unlikely to survive the harsh chemical environment within a saltwater mixing vessel intact, thereby becoming little more than dissolved organic material to be dealt with by biological filtration in the recirculating marine ecosystem;
Amino Acids, with the same commentary applied to vitamins;
Bacteria, reportedly added to improve biological stability of the recirculating marine ecosystem, which is not already lacking in biological stability.
Bacterial addition is the most recent development in sea salt additives. That bacteria is being promoted in a sea blend salt is not at all surprising owing to the attention that has been paid to the marketing of biological additives (both the microorganisms, themselves, and the various nutrient source options) over the past decade. It is, however, our viewpoint that there is really no need for a sea salt blend to incorporate nitrifying and/or denitrifying bacteria. Any functioning recirculating marine ecosystem will quickly establish a flourishing biological consortium of the appropriate nitrifiers and denitrifiers through the addition of little more than an appropriate microbial suspension, or through the addition of substrate from an established recirculating marine ecosystem, fed with appropriate nutrients. From the perspective of establishing a controlled recirculating marine ecosystem for commercial and/or research purposes, the addition of bacteria to a sea salt blend is no more appropriate than is the intentional incorporation of EDTA, vitamins, amino acids, or other organic compounds. In summary, if a caretaker of a recirculating marine ecosystem wishes to incorporate any of these additives into systems under their supervision, then it should be their prerogative to do so intentionally and in a controlled fashion.
Having reviewed the preceding concepts, this article now arrives at the topic at large, namely seawater interaction within contained marine aquatic ecosystems.
Three decades of recirculating marine ecosystem operational experience, in research and commercial settings, interacting with clients globally, as well as in the establishment and care of systems for research and proof of concept, has firmly established for ourselves and many of our clients the functional requirement of synthetic sea salt: establish appropriate ionic content within water, conforming to the ionic content that marine life commonly maintained in recirculating marine ecosystems has evolved within for at least the last hundred million years, excluding unnecessary components. Inclusion of additives, and/or the skewing of ionic values at a standard salinity value, produces an environment which does not conform to oceanic standard values, and which results in added cost of procurement or application, and (often) inferior outcomes. Similarly, inclusion of minor and trace ions which exhibit conservative behavior in oceanic water is generally unnecessary in recirculating marine ecosystems, unless specific ions are the subject of a study.
As previously mentioned, reported NSW values of major, minor, and trace elements vary by source. Therefore, there is no "exact" ionic template to follow when producing an engineered sea salt formulation. Our approach is to formulate an engineered sea salt according to the client's requirements and/or specifications. Effectively, this practice affords a client the option of having a salt formulated which precisely reflects the ionic values of a natural seawater assay, or having one formulated which provides specific elements in concentrations outside of NSW values. For example, a client growing macro algae may request a salt made with elevated iron, manganese, nickel, vanadium, cobalt, etc. Yet another client may request a salt with elevated potassium, magnesium, calcium, strontium, etc. In any case, the client stipulates the desired ionic content. Our production process ensures that the finished product will be within 1% of formulated ionic values. Clients should note that, unless specified, we incorporate a higher carbonate alkalinity content to counter the tendency of recirculating marine ecosystems to progressively acidify. Unless specified by the client, elements which have been found to negatively impact plants and algae at concentrations above 150% their average NSW values are not included in our general formulation procedure. Because we formulate every blend based upon the client's wishes, we can easily add these elements into the overall formulation process if so directed. All components undergo analysis for nutrient content, with samples testing positive for nutrients (nitrogen- and phosphorus-bearing) being rejected.
Historically, a blend that we have produced to match the ionic values published by Culkin and Cox has undergone ICP/AES analysis by at least four (that we are aware of) independent laboratories, each returning a very favorable report to the respective client in which ionic content did, indeed, conform to the formulated target values. These results have provided a sense of confidence to new clients using the one of our blends in their commercial systems, however of greater value than confidence is the improvement in coloration, growth rate, and overall health of marine organisms observed by commercial aquaculturists occurring within a few weeks of beginning to replace the existing system seawater with that which is created with the blend in question. “Existing system seawater”, in these cases, includes seawater produced with a commercial sea salt blend (various brands), as well as natural seawater collected in offshore marine environments. Such improvements have obvious beneficial implications in the overall operation of a recirculating marine ecosystem.
For additional information regarding the generalities of our engineered sea salt, please visit this page.