24 JUNE 2014 • WORLD AQUACULTURE • WWW.WAS.ORG TABLE 1. Performance of microalgae and shellfish grown in media based on groundwater sources in the Delta Area of the southwest Netherlands (locations are in Fig. 1). Legends: (++) growth rate comparable to that in (enriched) seawater media; (+) growth rate comparable to that in seawater media, but with additional effects such as discoloration of shellfish or growth reduction only when shellfish are fed on feed grown in the same groundwater medium; (+/-) no growth or growth inhibition, but no mortality; (-) 50% mortality; (--) total mortality. Location Seawater SEA Lab Grovisco VAM Zeeland Seafarm La Solitude Vlissingen Stavenisse Borsele Aquacultuur Kamperland St. Philipsland Yerseke Depth (m) 28.8 300 15 15 70 160 Salinity (g/l) 35 29.7 16.2 29 18 30 17 EXPERIENCE Algae ++ ++ + +/- + + ++ Shellfish ++ ++ + - - +/- ++ - - ppt salinity, mortality in cultured algae and shellfish has occurred in the Delta Area of the southwest Netherlands. Table 1 provides an overview of the performance of algae and shellfish in groundwater based on short-term growth experiments (bioassays). To unravel the adverse effects caused by groundwater composition, aquaculture-related studies were consulted from other parts of the world, including Australia (Ingram et al. 2002, Flowers and Hutchinson 2004, Partridge et al. 2008), Central America (Mace 2008), North America (Boyd 2012) and the Middle East (Mourad et al. 2012). In many of these, water salinity may be marginal or sub-optimal or their ionic composition (chloride, sulphate, bicarbonate, sodium, magnesium, calcium, potassium) deviates greatly from that of normal seawater. Effects of differences in ionic balances on micro-algal cultures are not known. This calls for a thorough chemical analysis of problematic groundwater sources. A graphic presentation of the ionic composition of a groundwater type may give some information with respect to its suitability for aquaculture. With the use of pie-charts presenting the concentrations of major anions: bicarbonate, chloride, sulphate (HCO3 -, Cl-, SO4 2-) and cations: sodium, potassium, magnesium and calcium (Na+, K+, Mg2+, Ca2+), unsuitable groundwater and normal sea water were compared (Fig. 2). The groundwater of VAM caused a complete mortality of shellfish and was not suitable for microalgae culture (Table 1). At the time of the study, the water was ‘bicarbonate rich’ and ‘sulphate poor’, but this is not necessarily the reason why it was unsuitable for aquaculture. Certain ion ratios may be related to disturbances of osmotic and metabolic processes and ultimately to growth inhibition. Decreased SO4 2- :Cl- ratios indicate anaerobic microbial activity in soil (Magaritz and Luzier 1985, Stuijfzand 1989). Low ratios were found in organic soil layers where sulphate-reducing microbes formed sulphides. For aquatic organisms, a good balance between potassium and sodium ions is required. In many cases ‘low potassium’ was identified as the cause of growth disturbance. In the Australian studies, low potassium concentrations may have negative effects on cultivated organisms at K:Na below 0.01 (Partridge 2008). In the Delta Area of southwest Netherlands, no serious potassium deficit is present in saline groundwater. Quite the opposite, VAM water had a relative K+ ‘excess’ over Na+, as discussed below. Water hardness may influence the uptake chemistry of ions by organisms. In most groundwater types examined, Ca:Mg ratios deviated from those in seas and oceans (normally ~0.2); in groundwater it increased to ~0.5. We propose a water hardness index (Ca+Mg):(Na+K). The hardness index of ‘normal seawater’ is around 0.13, whereas in problematic groundwater types it is usually between 0.25 and 0.30. To diagnose any physiological effect on organisms, possibly due to ion imbalances, a signature plot of relevant ratios: SO4:Cl, K:Na, Ca:Mg, (Ca+Mg):(Na+K), as shown in Figure 3 for different water types was designed. The VAM water source has a high K:Na (relative Na+ shortage), a high Ca:Mg (relative Mg2+ shortage) as well FIGURE 2. Ionic composition (concentration in mM) of North Sea water (left), groundwater suitable for aquaculture (center) and groundwater unsuitable for aquaculture from the Delta Area of the southwest Netherlands (right). The sum of the concentration of cations in mM is not equal to the sum of the concentration of anions in mM.
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