Effects of using synthetic sea salts when measuring and modeling copper toxicity in saltwater toxicity tests

Abstract

Synthetic sea salts are often used to adjust the salinity of effluent, ambient, and laboratory water samples to perform toxicity tests with marine and estuarine species. The U.S. Environmental Protection Agency (U.S. EPA) provides guidance on salinity adjustment in its saltwater test guidelines. The U.S. EPA suggests using commercial sea salt brands, such as Forty Fathoms (now named Crystal Sea Marinemix, Bioassay Grade), HW Marinemix, or equivalent salts to adjust sample salinity. Toxicity testing laboratories in Canada and the United States were surveyed to determine synthetic sea salt brand preference. The laboratories (n = 27) reported using four brands: Crystal Sea Marinemix (56%), HW Marinemix (22%), Instant Ocean (11%), and Tropic Marin (11%). Saline solutions (30 g/L) of seven synthetic sea salts were analyzed for dissolved copper and dissolved organic carbon (DOC) content. Brands included those listed above plus modified general-purpose salt (modified GP2), Kent Marine, and Red Sea Salt. The synthetic sea salts added from < 0.1 to 1.2 microg Cu/L to the solution. Solutions of Crystal Sea Marinemix had significantly elevated concentrations of DOC (range = 5.4-6.4 mg C/L, analysis of variance, Tukey, alpha = 0.05, p < 0.001) while other brands generally contained < 1.0 mg C/L. The elevated DOC in Crystal Sea Marinemix was expected to reduce copper toxicity. However, the measured dissolved copper effective concentration 50% (EC50) for Crystal Sea Marinemix was 9.7 microg Cu/L, similar to other tested sea salts. Analysis indicates that the organic matter in Crystal Sea Marinemix differs considerably from that of natural organic matter. On the basis of consistently adding little DOC and little dissolved copper, GP2 and Kent Marine are the best salts to use.

A comparison of the copper sensitivity of six invertebrate species in ambient salt water of varying dissolved organic matter concentrations

“…The salinity of the GCML water sample was lowered to the prescribed test salinity using laboratory reverse osmosis-deionized water. The salinity of the SSFB water sample was increased to prescribed test salinity using a modified synthetic sea water medium (GP2) made with reagent grade chemicals as per U.S. EPA [20]; the GP2 salt has been shown to contribute minimal DOC to the test sample [23]. The test water samples at four DOC concentrations were prepared from the GCML and SSFB salinity-adjusted ambient waters as follows: low DOC ¼ 100% water from GCML, intermediate low DOC ¼ 67% water from GCML þ 33% water from SSFB, intermediate high DOC ¼ 67% water from SSFB þ 33% water from GCML, High DOC ¼ 100% water from SSFB.…”

Salinity tolerance of Great Lakes invaders

“…Salt water for ballast water treatments was produced by mixing filtered (0.7 μm pores size) site water with Instant Ocean® (Spectrum Brands Inc., Atlanta, GA, U.S.A.), a synthetically manufactured aquarium salt. Instant Ocean® is a widely used aquarium salt having ionic and trace element concentrations similar to natural seawater (Arnold et al. , 2007).…”

Temperature-dependent formation of NaCl dihydrate in levitated NaCl and sea salt aerosol particles

“…The content of these organics is on the order of a few µmol/kg of sea salt water, whereas major ions are in the range of tens or hundreds of mmol/kg sea salt water. In the work of Arnold et al, 32 a mean dissolved organic carbon (DOC) value of less than 0.2 mg/l was reported. Recent experiments have shown that organic material (i.e., carboxylic acids) in concentrations of a few ppm can inhibit the nucleation of calcium sulfate dihydrate and lead to the initial formation of calcium sulfate hemihydrate.…”