Background levels of manganese in three representative sea lochs (Ailort, Eil and Duich) on the west coast of Scotland were measured at the surface and at 4 and 8 m depth. Eleven of the 12 samples were <0.01 ppm and the remaining sample was 0.02 ppm, and it was concluded that trials could be confidently undertaken with added concentrations of manganese of 0.25 ppm. The amount of manganese sulphate required for 1 million litres of enclosed volume was calculated as: the molecular weight of MnSO4 is=223, and the atomic weight of manganese is 55, thus each kg MnSO4 contains 55/223 x 1000 g Mn=247 g, therefore, 1 kg of MnSO4 is required to give a manganese concentration of 0.25 ppm in 1 million litres of enclosed volume. The relationship between observed concentration and enclosed volume is given in Table 1.
Differences in the final measured concentrations at different points in the cage would show inadequate mixing, and mixing rates could be monitored by plotting the rate of progress to the final observed maximum concentration.
A net on a 11.4 m square cage was raised to 3 m depth and enclosed with a tarpaulin giving an estimated enclosed volume of 390 m³. Fish were not present in the net as MnSO4 had not been tested for toxic effects. Oxygen was supplied with a 9 m length Dryden coiled diffuser with 0.71 Kgcm-2 pressure positioned on the net bottom 10 minutes before the net was enclosed. The calculated weight of MnSO4 required to give a final concentration of 0.25 ppm in 390 m³ volume was 385 g. This was divided into 4 aliquots, mixed and dissolved in 10 l of 32 ppt seawater at a water temperature of 12°C and dispersed by bucket in a corner of the cage. Water samples were taken with a ISD water sampler and analysed within 24 h using a Merck Aquaquant kit (14406). Samples were taken before treatment at 4 points A, B, C, D, 1 m out and midway along the cage sides at 50 cm and 2 m depth. Water was sampled 5 min after MnSO4 was added and also at 2 m at the cage corners E, F, G, H, and 10, 15, 30 and 60 min after adding Mn SO4 at points A-D at 50 cm depth. Samples were also taken at 50 cm depth 5, 15 and 30 min after the tarpaulin was dropped.