Storage of dissolved inorganic nutrients in seawater: poisoning with mercuric chloride

Seawater samples were poisoned with mercuric chloride to assess its suitability for the long-term storage measurement of inorganic nutrients. Two series of seawater samples from the open ocean were taken, and nitrate, silicate, phosphate, nitrite and ammonium measured immediately on board. The same samples were poisoned by adding 105 μg of mercuric chloride per milliliter of sample (total sample volume 50 ml) and measured again after about 1 and 2 years storage. The results show that the poisoning with mercuric chloride is a suitable method for the storage of inorganic nutrients. For nitrate, silicate and phosphate a good accordance was found between the direct measurements and those after 1 and 2 years storage. The results of the determinations coincided with, or were very close to the optimal 1:1 relationship. The variations for nitrite were higher, but due to the generally low concentrations in the oceans these differences will cause no serious problems for the nitrate determination. In contrast, ammonium data, obtained after long storage periods, should be taken with care.