Differences in major ions composition of artificial seawater from aquarium tanks at the New Jersey State Aquarium GORDAN GRGURIC1 , JAMES A. KOMAS1 and LISA A. GAINOR2 1

Marine Science Program, The Richard Stockton College of New Jersey, Pomona, NJ 08240, USA (Tel.: +1 609-652-4492; Fax: +1 609-748-5515; E-mail: [email protected]); 2 New Jersey State Aquarium, 1 Riverside Drive, Camden, NJ 08103, USA Accepted 21 April 1999 Key words: artificial seawater, salinity, major seawater ions, calcium carbonate, magnesium carbonate ABSTRACT Concentrations of major seawater ions were monitored in six aquarium tanks at the New Jersey State Aquarium over a three-year period. The ratios of these ions to chlorinity were compared to those in freshly prepared artificial seawater. The largest aquarium tank (Ocean Tank) exhibits statistically significant (p < 0.01) relative enrichment of potassium, calcium and strontium, and relative depletion of magnesium and sulphate. The likely source of excess potassium is from potassium iodide added to prevent goiter in sharks. Based on the excess potassium, a total amount of 650 µmol/kg iodide added over the years was calculated for the Ocean Tank. The excess calcium observed in several tanks correlates with the presence of concrete or carbonaceous shells in these tanks. In Ocean Tank, a calcium leaching rate of 6.7 kg/month was calculated. Continuous formation of white carbonaceous precipitates serves as a sink for magnesium in Ocean Tank, and a magnesium removal rate of 5.1 kg/month was calculated. These and other results show that concentrations of major ions in artificial seawater aquaria are affected by leaching, precipitation and addition of food supplements, and these factors should be taken into account when preparing artificial seawater for aquarium tanks with long water residence time. INTRODUCTION

Closed seawater aquaria employ artificial seawater when the use of natural seawater is impractical or uneconomical. Specific reasons for the use of artificial seawater may include the presence of contaminants in the local ocean waters or the expense of transporting large volumes of seawater to a facility located far from the ocean (Lawson, 1995). The two largest fish aquaria on the East Coast of the United States, the Living Seas in Orlando (Florida) and the New Jersey State Aquarium in Camden (New Jersey) both use artificial seawater in their tanks. The number of experimentally prepared artificial seawater formulations in the literature is quite large (e.g. Cavanaugh, 1964; Kester et al., 1967; Kinne, 1976; Bidwell and Spotte, 1985; Adams and Bubucis, 1998). Differences with respect to nutrients and trace elements in these formulations are usually due to the types of organisms to be cultured or maintained. Artificial seawater for culturing algae and some invertebrates must contain all the macro and trace nutrient elements (McLachlan, 1973), while fish populations are normally maintained in artificial seawater of a simpler formulation, such as that provided by Segedi and Kelly Aquarium Sciences and Conservation 2: 145–159, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.

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(1964) or King and Spotte (1974). Marine mammals (dolphins and seals) may be raised in media as simple as NaCl solutions (Faulk, 1990). Even with respect to major seawater ions, artificial seawater will often vary from natural seawater. Aquaria that use ozone for disinfection generally do not add bromide to their salt mix (Grguric et al., 1994), in order to prevent its oxidation to bromate (Steslow, 1991). Artificial seawater recipes may also vary in the types and amounts of salts containing the six most abundant seawater ions (Cl− , Na+ , SO2− 4 , Mg2+ , Ca2+ and K+ ) because different combinations of salts can produce the same concentration of individual ions in solution (Grguric, 1990; Adams and Bubucis, 1998). In open ocean, major seawater ions behave conservatively and their ratio to each other is very nearly constant. This is due to the fast rate of ocean mixing relative to the rates of geochemical processes that control seawater composition (Libes, 1992). Major ion concentrations in a closed, recirculating seawater system may be affected by leaching, precipitation, and even corrosion, such as that associated with the use of sacrificial magnesium anodes (Rossi et al., 1996). In this study, we monitored concentrations of major ions in six aquarium tanks at the New Jersey State Aquarium. The concentrations observed over time in these tanks were compared to those in freshly prepared artificial seawater, used to supply all the tanks. We found statistically significant differences in the relative concentrations of major ions in several aquarium tanks, especially in the largest tank in the facility. We attempt to explain these differences and, where possible, to quantify the effect of underlying processes on the fluxes of chemical elements in the aquarium tanks.

DESCRIPTION OF THE AQUARIUM TANKS

The New Jersey State Aquarium in Camden, NJ is a closed-system marine facility that started its operation in 1992 and whose centerpiece is a 2.87 million L Ocean Tank. At least 50 other freshwater and saltwater tanks house smaller exhibitions in distinct environments that extend from the local New Jersey pine barrens to the Caribbean reefs. The tanks monitored in this study are described below. Ocean Tank (OT) is by far the largest tank in the facility and houses approximately 40 different fish species, ranging from blueback herring and mullet to sand tiger sharks and rough-tail stingrays. The environment on display mimics the continental shelf adjacent to the New Jersey coast, including the Hudson River Canyon. The tank contains artificial seawater of average salinity 28 g/kg, although salinity variations from 25 to 31 g/kg have been observed during this study. The average seawater temperature in the tank is 20◦ C. Flow dynamics through Ocean Tank involve surface skimming to produce a total flow rate of 16,000 L/min that is delivered to twelve parallel sand filters and treated afterward in a biofilter. A small fraction (less than 10%) of the total flow is ozonated in a separate line. The filtered and disinfected water is reintroduced to the tank through a silica quartz gravel bed on the aquarium floor. The volume of the entire system recirculates in about 3 h.

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The tank exhibits a loss of salinity averaging 0.5 g/kg per month, due mainly to backwashing of sand filters with freshwater. There are also losses of seawater such as that taken out in divers’ suits. To compensate for these losses, artificial seawater in the tank is replenished at a rate of 57,000 L/month, resulting in a water residence time of 50 months. Coral Reef (CR) tank has a volume of 15,000 L and houses an exhibit of Caribbean reef fish such as french angels and smallmouth grunts. The habitat is artificial coral made of fiberglass. Seawater in the tank has an average temperature of 23◦ C and salinity of 34 g/kg. It is replaced at an alternating rate of 50% or 90% a week. The average water residence time is thus 1.4 weeks. Octopus (OCT) tank houses an exhibit displaying a giant octopus. The tank has a volume of 2600 L and mimics the Pacific Northwest coastal environment. It contains seawater with an average salinity of 33 g/kg and an average temperature of only 13◦ C. The tank is dimly lit to simulate the low light intensity in this environment, and the habitat consists of artificial rocks made of fiberglass. Seawater is replaced at a rate of 20% per week, resulting in a water residence time of 5 weeks. Nautilus (NAUT) tank has a volume of 1300 L and houses an exhibit of chambered nautilus and pinecone fish. It has an artificial (fiberglass) rock habitat and is also dimly lit to simulate the environment where the specimens originate. The average seawater temperature in this tank is 18◦ C and average salinity is 32 g/kg. The water replacement regime is the same as in OCT. Rainbow Seas (RS) tank contains an exhibit of live soft corals, similar to those found in Indo-Pacific reefs. The tank has a volume of 2600 L and its seawater has an average salinity of 34 g/kg and a relatively high average temperature of 24◦ C. Solutions of Ca(OH)2 , SrCl2 , KI and several trace elements are added regularly to this tank as chemical supplements for the corals. The water change regime is 15% a week, yielding a water residence time of 6.7 weeks. Delaware Bay (DB) tank is a 2800 L tank housing an exhibition of local fish species such as black seabass, striped bass and croaker. Juvenile diamondback terrapins also inhabit this exhibit. The average temperature of this tank is 23◦ C and the average salinity is 21 g/kg. The brackish seawater in this tank is made by mixing artificial seawater with dechlorinated city water. It is replaced at a rate of 10% a week and 50% once a month. The water residence time is thus variable, with an average of 5 weeks. Saltwater Tanks (ST) are three identical 38,000 L tanks where artificial seawater is prepared by mixing a blend of preweighed salts with filtered and dechlorinated city water. They are considered as one tank for the purposes of this paper. Artificial seawater formulation. The chemicals and their amounts needed to prepare one 38,000 L batch of artificial seawater are listed in Table 1. The recipe is a modified General Purpose 2 Medium from Bidwell and Spotte (1985). Each batch of artificial seawater must meet the following criteria: salinity must be in the range 33–35 g/kg, total alkalinity must be greater than 2.5 meq/L and pH must be between 8.1 and 8.3.

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G . G R G U R I C ET AL. Table 1. Artificial seawater formulation used at the New Jersey State Aquarium∗ Major elements Sodium chloride Magnesium sulphate (anhydrous) Magnesium chloride (hexahydrate) Calcium chloride (dihydrate) Potassium chloride Minor elements Sodium bicarbonate Sodium tetraborate Strontium chloride Lithium chloride Ethylenediamine dihydroiodide Water

1100 kg 140 kg 212 kg 76 kg 32 kg 7.94 kg 1.61 kg 1.00 kg 0.040 kg 0.003 kg 38,000 L

∗ The formulation is a modified General Purpose 2 Medium from Bidwell and Spotte (1985).

MATERIALS AND METHODS

The aquarium tanks described above have been sampled every 3 months in the period from August 1995 to February 1998. To check for spatial variability of major ions in Ocean Tank, two locations were sampled: surface water at the top of the tank and mid-depth water in the acclimation area. Samples were collected in plastic bottles and analyzed in the laboratory for chlorinity, sulphate, total alkalinity and the following cations: Na+ , mg2+ , Ca2+ , K+ and Sr2+ . Chlorinity was determined by titration with a standardized solution of AgNO3 (Strickland and Parsons, 1972). Sulphate was determined by a turbidimetric BaCl2 method (Rand, 1975). Total alkalinity was determined by titration with standard 0.01 N HCl (Strickland and Parsons, 1972). Cation concentrations were determined by flame atomic absorption spectrometry (Greenberg et al., 1992), using a PerkinElmer 3100 instrument. All methods except total alkalinity were standardized with IAPSO Standard Seawater. The coefficient of variation was 3% for the sulphate method, 2% for sodium and total alkalinity and 1% or less for all other methods. Overall quality of data was verified by performing charge balance calculations and determining the % difference in measured vs. charge balanced sodium concentration. When this difference was greater than 5%, either the analyses on the sample were repeated or calculated sodium concentrations were used. Samples of solid precipitates were taken from artificial rocks in OT. The precipitates were thoroughly rinsed with distilled water and placed in a dry, constanttemperature incubator at 37◦ C for 3 h. After cooling, a known weight of precipitates was dissolved in a 50 : 50 (V/V) HCl/HNO3 solution. The digest was analyzed for Na+ , Mg2+ , Ca2+ , K+ and Sr2+ using flame atomic absorption.

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RESULTS

The six aquarium tanks examined in this study exhibited variations in salinity over the 3 year period. The greatest range of salinities was observed in DB (18–28 g/kg), while RS had the least variations in salinity (33–36 g/kg). In order to normalize major ion concentrations relative to overall salinities, weight-to-weight ratios of each ion to chlorinity were calculated for every tank. These ratios and their standard deviations are shown in Figures 1–6. To enable comparison of relative variations in different figures, the vertical axis scale in each figure covers one-third of the average ratio. The only exception is Figure 5 (Sr/Cl ratios) where the vertical scale covers two-thirds of the average ratio, in order to include the range of variations between the tanks. Statistically significant differences between each aquarium tank and ST were determined by the Student’s t-test at 95% and 99% confidence levels. Figure 1 shows Na/Cl ratios in the aquarium tanks. No tank exhibits a statistically significant difference relative to ST (p > 0.05). Relative standard deviations of the Na/Cl ratios range from 1.6% in OCT to 4.1% in RS and are the lowest of all major ions. Potassium to chlorinity ratios (Figure 2) are quite uniform in all tanks except in OT, which shows a statistically significant increase of 7.5% relative to ST (p < 0.01). Relative standard deviation in K/Cl ratios ranges from 2.5% in RS to 5.9% in CR.

0.63

Na/Cl Ratio

0.60 0.57 0.54 0.51 0.48 0.45 OT

CR

OCT NAUT RS Aquarium Tanks

DB

ST

Figure 1. Sodium to chlorinity ratios in aquarium tanks at the New Jersey State Aquarium. Histograms represent the mean values, together with standard deviations. OT – Ocean Tank, CR – Coral Reef Tank, OCT – Octopus Tank, NAUT – Nautilus Tank, RS – Rainbow Seas Tank, DB – Delaware Bay Tank, ST – Saltwater Tanks.

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Figure 2. Potassium to chlorinity ratios in aquarium tanks at the New Jersey State Aquarium. Histograms represent the mean values, together with standard deviations. OT – Ocean Tank, CR – Coral Reef Tank, OCT – Octopus Tank, NAUT – Nautilus Tank, RS – Rainbow Seas Tank, DB – Delaware Bay Tank, ST – Saltwater Tanks.

Calcium to chlorinity ratios in the aquarium tanks (Figure 3) show large variabilities, with three tanks exhibiting statistically significant differences relative to ST (p < 0.01). These include OT (16.6% increase), DB (12.1% increase) and OCT (2.8% increase). Relative standard deviations in Ca/Cl ratios range from 5.4% in RS to 8.0% in OT. The average Ca/Cl ratio in ST (Figure 3) is 36% greater than that in ambient surface seawater (Wilson, 1975), showing the enrichment of calcium in the artificial seawater formulation. One reason for this enrichment is to provide for more successful maintenance of invertebrates and elasmobranchs (Steslow, 1998). Except for strontium, the ratios of all other major ions to chlorinity in ST are within 6% of their ambient seawater ratios. Figure 4 shows mg/Cl ratios in the aquarium tanks. Relative to ST, statistically significant differences (p < 0.01) are found in OT (8.7% decrease) and in DB (5.0% decrease). Relative standard deviations in the tanks vary from 5.7% in OCT to 8.0% in NAUT. Figure 5 shows Sr/Cl ratios in the tanks. Strontium concentration in seawater is an order of magnitude lower than that of the other ions discussed here (Wilson, 1975) and as a result, the relative differences between the tanks are the largest of all ions. The average OT, OCT and DB Sr/Cl ratios are 32.8%, 8.0% and 10.2%, respectively, higher than the ratio in ST. All of these differences are statistically significant (p < 0.01). Relative standard deviations of Sr/Cl ratios are the largest of all ions, ranging from 7.3% in ST to 21.4% in OCT.

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Figure 3. Calcium to chlorinity ratios in aquarium tanks at the New Jersey State Aquarium. Histograms represent the mean values, together with standard deviations. OT – Ocean Tank, CR – Coral Reef Tank, OCT – Octopus Tank, NAUT – Nautilus Tank, RS – Rainbow Seas Tank, DB – Delaware Bay Tank, ST – Saltwater Tanks.

Sulphate to chlorinity ratios for the aquarium tanks are shown in Figure 6. Statistically significant differences (p < 0.01) from ST are observed in OT (4.1% decrease), NAUT (4.1% decrease) and DB (11.5% decrease). Relative standard deviations are quite variable, ranging from 4.4% in NAUT to 11.6% in DB. It is interesting to note that, relative to ST, the average ratios of ions to chlorinity in the aquarium tanks show either an increase (K/Cl, Ca/Cl and Sr/Cl) or a decrease (Na/Cl, Mg/Cl and SO4 /Cl). This could mean that similar processes are responsible for the observed variations. However, each tank is maintained in a different way and those differences have to be taken into account when attempting to explain the observed patterns. DISCUSSION

Ocean Tank Ocean Tank is the only tank that shows a significantly different (higher) K/Cl ratio. As an alkali metal, potassium is not likely to participate in precipitation/dissolution reactions in an aquarium environment. An examination of possible external sources revealed that potassium iodide tablets have been added occasionally, either as a food supplement or in solution, in order to prevent goiter in sharks (Stoskopf, 1993). While the artificial seawater formulation contains iodine in the form of

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Figure 4. Magnesium to chlorinity ratios in aquarium tanks at the New Jersey State Aquarium. Histograms represent the mean values, together with standard deviations. OT – Ocean Tank, CR – Coral Reef Tank, OCT – Octopus Tank, NAUT – Nautilus Tank, RS – Rainbow Seas Tank, DB – Delaware Bay Tank, ST – Saltwater Tanks.

ethylenediamine dihydroiodide (Table 1), it is reasonable to assume that ozone continuously oxidizes this iodide to iodate (Hoigne et al., 1985). The amount of iodide added to OT through potassium iodide tablets can be calculated from the relative excess of potassium in the tank. When the difference in K/Cl ratio between OT (0.0228) and ST (0.0212) is multiplied by the average chlorinity of OT (15.71 g/kg), an excess potassium concentration of 0.0252 g/kg or 650 µmol/kg is obtained. This concentration of iodide is orders of magnitude greater than the average seawater concentration of 0.1 µmol/kg (Tsunogai, 1971; Rebello et al., 1990), but most of it has probably been consumed by the sharks or oxidized to iodate. A plausible explanation for the high Ca/Cl ratio in OT is leaching of calcium from artificial rocks, which are made of cinder blocks and concrete. The calculated excess calcium concentration in OT relative to ST is 0.0754 g/kg. From this excess calcium concentration, the volume of OT and its water residence time, an average calcium leaching rate of 4.3 kg/month was calculated. Magnesium to chlorinity ratio in OT is significantly lower than that in ST, suggesting that there are processes removing magnesium from OT seawater. The calculated magnesium deficit in OT is 0.0895 g/kg. We hypothesized that white precipitates which continuously form on artificial rocks contain magnesium and possibly other alkaline earth elements. Samples of the precipitates were collected from five different locations in OT, at two different times. Digestion and analysis

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Figure 5. Strontium to chlorinity ratios in aquarium tanks at the New Jersey State Aquarium. Histograms represent the mean values, together with standard deviations. OT – Ocean Tank, CR – Coral Reef Tank, OCT – Octopus Tank, NAUT – Nautilus Tank, RS – Rainbow Seas Tank, DB – Delaware Bay Tank, and ST – Saltwater Tanks.

revealed that the cationic composition of the precipitates is principally due to Mg and Ca, with traces of Sr and K present as well (Table 2). The average Mg/Ca ratio in the precipitates is 21% higher than that in OT seawater, indicating that magnesium is preferentially removed by the precipitates relative to calcium. The ratio is within 6% of that in ST, but the long water residence time in OT makes it unlikely that the composition of seawater additions controls the relative removal rates of magnesium and calcium. The ratios of Mg/Sr and Ca/Sr in the precipitates are much more variable (Table 2). However, in all samples except one, they are higher in the precipitates than in OT seawater. Thus, both magnesium and calcium are preferentially removed by the precipitates relative to strontium. The widespread occurrence of the precipitates and the abundance of magnesium in them indicate that the precipitates are probably the most important sink for the ‘missing’ magnesium in seawater. From the magnesium deficit in OT seawater (0.0895 g/kg), a removal rate of 5.1 kg Mg/month was calculated. Based on the average Mg/Ca ratio in the precipitates, 2.4 kg Ca/month is removed from OT seawater by the precipitate formation. When this calcium flux is taken into account, the total amount of calcium leaching from artificial rocks in OT is recalculated as 6.7 kg Ca/month. Of that amount, 4.3 kg Ca/month remains in OT seawater, while the rest is incorporated into the precipitates. Figure 7 shows these magnesium and calcium fluxes within OT.

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Figure 6. Sulphate to chlorinity ratios in aquarium tanks at the New Jersey State Aquarium. Histograms represent the mean values, together with standard deviations. OT – Ocean Tank, CR – Coral Reef Tank, OCT – Octopus Tank, NAUT – Nautilus Tank, RS – Rainbow Seas Tank, DB – Delaware Bay Tank, and ST – Saltwater Tanks. Table 2. Composition and ratios of the principal cations in Ocean Tank precipitates Sample∗ no.

Mg2+ (mg/L)

Ca2+ (mg/L)

1 485 251 2 584 329 3 475 268 4 442 209 5 630 280 6 577 289 7 808 341 8 1342 563 9 1499 660 10 189 83 Mean 703 327 Ratios in Saltwater Tanks

Sr2+ (mg/L)

K+ (mg/L)

Mg/Ca ratio

Mg/Sr ratio

Ca/Sr ratio

4.41 5.39 3.87 4.13 4.19 3.81 3.32 4.63 4.51 1.70 3.99

13 B.D.L.∗∗ B.D.L. 11 12 19 12 22 B.D.L. 17 10.6

1.93 1.77 1.77 2.12 2.25 2.00 2.37 2.38 2.27 2.28 2.15 2.27

110 108 123 107 151 151 243 290 333 111 176 146

57 61 69 51 67 76 103 122 147 49 82 64

∗ Samples were taken from five locations in Ocean Tank, at two different times; the concentrations given are in 1 L of the final digest. ∗∗ Below detection limit for potassium, which is 0.010 g/L.

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Figure 7. Fluxes of calcium and magnesium in Ocean Tank. Solid arrow: leaching of calcium from artificial rocks. Open arrows: removal of calcium and magnesium by precipitate formation. Table 3. Mass of Ocean Tank precipitates as carbonate Sample∗ no.

Dry weight (mg)

1 2 3 4 5 6 7 8 9 10

2533 2533 2357 2069 2963 2721 3235 5988 6997 874

∗

Weight of Mg as carbonate (mg) 1702 2049 1667 1551 2211 2025 2835 4709 5260 663

Weight of Ca as carbonate (mg) 628 823 670 523 700 723 853 1408 1650 208

Total weight Mg + Ca as carbonate (mg) 2330 2872 2337 2074 2911 2748 3688 6117 6910 871

Total Mg + Ca carbonate wt./dry wt. (%) 92 112 99 100 98 101 104 102 99 100

Samples were taken from five locations in Ocean Tank, at two different times.

Vigorous bubbling of the precipitates during digestion was a strong indication of their carbonaceous composition, and the weight of magnesium and calcium as carbonate was calculated (Table 3). In all but 3 samples, the difference between the dry weight of the precipitate and the calculated weight as carbonate was less than 3%. This is compelling evidence that the precipitates are indeed some form of a Ca–Mg–CO3 product. Precipitation of magnesian calcite from artificial seawater has been studied under controlled laboratory conditions (Rushdi et al., 1992; Sabbides and Koutsoukos, 1993; Hartley and Mucci, 1996), in order to explain compositional variability in the marine calcite sediments. Our results

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provide additional evidence for this process in an environment that more closely approximates ambient ocean. To date, we have not been able to test or quantify possible explanations for the relatively high Sr/Cl ratio and relatively low SO4 /Cl ratio in OT. Due to its low concentration, strontium can be affected by relatively small fluxes, possibly leaching from concrete. A more puzzling question is the sink for sulphate in OT. Based on the difference in SO4 /Cl ratios between OT and ST (Figure 6), a sulphate flux of 5.4 kg/month was calculated. Sulphate was not found in any of our precipitate samples, but there may be crevices in artificial rocks where ion exchange is taking place and sulphate substitutes for carbonate. This process has been observed in natural calcites and dolomites (Pingitore et al., 1995). Small Aquarium Tanks Among the small aquarium tanks (CR, OCT, NAUT, RS and DB), DB shows the most difference relative to ST, with significantly higher Ca/Cl and Sr/Cl ratios, and significantly lower Mg/Cl and SO4 /Cl ratios. One source of calcium in DB is freshwater, which is used to make the brackish environment in this tank. However, the calcium concentration of freshwater (average 40 mg/L) can only account for one-third of the observed calcium enrichment in DB. The additional source of calcium is likely to be leaching from a single piece of concrete in this tank. In addition to OT and DB, another tank where seawater has exposure to nonliving carbonaceous material is OCT, which contains shells from crabs and other shellfish that have been used as a feed. All three of these tanks exhibit relatively high Ca/Cl and Sr/Cl ratios and relatively low Mg/Cl ratio, although the Mg/Cl difference in OCT is not statistically significant. In RS, the weekly addition of potassium through potassium iodide supplement for live corals is three orders of magnitude less than the potassium inventory in the tank. In contrast to OT, the relatively short water residence time in RS does not allow the K/Cl ratio to change appreciably from that in ST (Figure 2). The weekly additions of strontium and calcium for RS corals are equivalent to, respectively, 1% and 2% of their seawater inventories. The average Sr/Cl ratio in RS is very close to that in ST (Figure 5) indicating that most of the added strontium is absorbed by the corals. The average Ca/Cl ratio is somewhat higher than that in ST (Figure 3), but the excess calcium in seawater is less than 20% of that added over the span of water residence time. The remaining two tanks in this study, NAUT and CR, have major ion ratios close to those in ST. These two tanks have quite different sizes – while NAUT is the smallest tank in this study, CR is by far the largest of the small tanks, with a volume five times greater than the next largest, DB. Our findings indicate that the closeness in the major ions composition of these two tanks relative to freshly prepared artificial seawater is due to factors such as the absence of exposed concrete or other carbonaceous material and the short water residence time, particularly in the case of CR.

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CONCLUSIONS

Results from this study show that in closed, recirculating artificial seawater aquaria, ratios of major ions to chlorinity are measurably affected by processes such as leaching, precipitation and ion exchange. In addition, occasional or regular introduction of food supplements containing major seawater ions also may have an effect on their ratios to chlorinity. In the largest tank at the New Jersey State Aquarium, we found statistically significant relative enrichment of potassium, calcium and strontium, and relative depletion of magnesium and sulphate, compared to freshly prepared artificial seawater. The likely source of excess potassium in this tank is from potassium iodide, added to prevent iodine deficiency in sharks. In a situation where the past addition records are incomplete, this excess potassium can be used to track the total amount of iodide added to the tank over the years. The excess calcium in OT can be attributed to the presence of exposed concrete in artificial rocks in the tank. Some of this calcium, as well as an even larger amount of magnesium, is removed from OT seawater by the formation of precipitates. Our calculations show significant fluxes of these two elements between solid phase and OT seawater. Two of the small aquarium tanks, DB and OCT, show similar trends in calcium, magnesium and strontium as OT. All three of these tanks have exposure to nonliving calcium carbonate either through concrete or through carbonaceous shells. In contrast, live corals in RS utilize most of the calcium and strontium added as food supplements. The observed variations in artificial seawater composition among the tanks in this study show that factors such as addition of food supplements, the presence of carbonaceous material and formation of precipitates have to be taken into account when artificial seawater recipes are used to prepare aquarium seawater. These recipes should be adjusted to the empirically measured concentrations of major ions, especially in aquarium tanks with long water residence time. ACKNOWLEDGMENTS

We wish to thank Frank Steslow for his support when starting this project. We greatly appreciate discussions and assistance from Brian DuVall, Robert Fournier and other members of the New Jersey State Aquarium. We are grateful to John Bellace, Renee McLaughlin and William Schmalz of Richard Stockton College for their help in the laboratory.

REFERENCES Adams, G. and Bubucis, P.M. (1998) Calculating an artificial seawater formulation using spreadsheet matrices. Aquarium Sciences and Conservation 2, 35–41.

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