WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2014 25 (CONTINUED ON PAGE 26) as a SO4 2- shortage. With signature plots as shown in Figure 3, a quick impression of deviations in ionic composition can be obtained that might indicate the cause of growth problems in marine organisms, particularly algae and shellfish. Yet questions remain: Is this the cause of growth problems? If so, will ion correction help? Good Ionic Balance, No Osmotic Problems Growth problems in groundwater-based aquaculture may have quite a different cause than misbalance in ionic composition. Causes of growth inhibition may be related to inorganic or organic substances. As to inorganic substances, certain trace elements may be toxic. Chemical analyses usually presents total concentration of a single element (e.g. copper) that is not representative of its toxic effects (Rijstenbil and Gerringa 2002). Complexation with organic substances (humic acids) reduces the toxicity of metals such as copper (Ma et al. 2003, Kungolos et al. 2006). Therefore, too little organic matter in groundwater may increase metal toxicity; too much organic substances immobilize essential metals and, therefore, inhibits growth. The question is how to find and selectively remove a substance that causes an inhibition. Most of the chemical tools that can be used to upgrade groundwater to seawater standards are not selective; they bind favorable and unfavorable metals equally. The chelator ethylenediaminetetraacetic acid (EDTA) forms soluble metal complexes, so metals remain present in water. Stripping of metals is a more useful tool in reducing the toxic effect; it eliminates the metals by sorption to resins, such as Chelex® 100 (Bio-Rad) (Florence and Batley 1976). Arsenic may be removed by adsorption onto iron oxides, hydrotalcite or chitosan (Ardau et al. 2007, Mohan and Pittman 2007). Dissolved organic substances in groundwater may have negative effects on growth or evoke avoidance reactions in shellfish. Activated-carbon filtration, ozonation or heat treatment may be remedies. However, this eliminates beneficial substances, such as vitamins, which are favorable to most microalgae. Case Studies of Microalgae Grown on Saline Groundwater Land-based shellfish farming needs microalgae as a food source. Therefore, saline groundwater should be of suitable quality for algal and shellfish cultivation. Groundwater sources, such as La Solitude, with a salinity around 17 ppt, caused shellfish mortality, but the algae Skeletonema sp. and Tetraselmis sp. grew well (Dubbeldam and Van Nieuwenhuijzen 2009). Other groundwater types had a lower yield of microalgal biomass because of a suboptimal N:P ratio or low nutrient levels (Grovisco) but remained sufficient for production, although at a lower growth rate than would be achieved in enriched seawater2. LEFT, FIGURE 3. Signatures of ionic ratios (sulphate reduction, potassium index, calcium excess, hardness) representing (from top to bottom) sea water, soft (potassium-deficient) brackishwater, hard brackishwater, hard low-sulphur water, and typical potassium-deficient groundwater in Australia causing osmotic problems in aquatic animals (Partridge 2008). Note the logarithmic scale. TOP RIGHT, FIGURE 4. Responses of ‘non-diatom’ microalgae (Nannochloris sp., Rhodomonas sp. and Porphyridium cruentum) in hard, saline medium of VAM. BOTTM RIGHT, FIGURE 5. Scheme of pre-treatments and secondary treatments for an experimental setup (bioassays) using saline groundwater. Algae should be tested in manipulated groundwater with or without nutrient enrichment (Andersen 2005). Enrichment can be omitted for marine aquaculture animals.
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