Abstract
Abstract. Instrumental equipment unsuitable or unavailable for fieldwork as well as lack of ship space can necessitate the preservation of seawater samples prior to analysis in a shore-based laboratory. Mercuric chloride (HgCl2) is routinely used for such preservation, but its handling and subsequent disposal incur environmental risks and significant expense. There is therefore a strong motivation to find less hazardous alternatives. Benzalkonium chloride (BAC) has been used previously as microbial inhibitor for freshwater samples. Here, we assess the use of BAC for marine samples prior to the measurement of oxygen-to-argon (O2 / Ar) ratios, as used for the determination of biological net community production. BAC at a concentration of 50 mg dm−3 inhibited microbial activity for at least 3 days in samples tested with chlorophyll a (Chl a) concentrations up to 1 mg m−3. BAC concentrations of 100 and 200 mg dm−3 were no more effective than 50 mg dm−3. With fewer risks to human health and the environment, and no requirement for expensive waste disposal, BAC could be a viable alternative to HgCl2 for short-term preservation of seawater samples, but is not a replacement for HgCl2 in the case of oxygen triple isotope analysis, which requires storage over weeks to months. In any event, further tests on a case-by-case basis should be undertaken if use of BAC was considered, since its inhibitory activity may depend on concentration and composition of the microbial community.
Highlights
Marine fieldwork often requires water samples to be collected by ship and returned to a shore-based laboratory for chemical analysis
The chlorophyll a (Chl a) concentration in TS1 samples was 0.4 mg m−3 (Table 1). (O2 / Ar) values in replicate samples to which Benzalkonium chloride (BAC) was added were not significantly different from samples to which HgCl2 was added, and both stayed constant over the seven days of the experiment (Fig. 1)
(O2 / Ar) of untreated samples decreased by (0.4 ± 0.2) % after 2 days and by (1.0 ± 0.2) % after 7 days. This suggests that BAC was as effective as HgCl2 at preserving the O2 concentration in these particular low Chl a concentration–seawater samples for up to 7 days
Summary
Marine fieldwork often requires water samples to be collected by ship and returned to a shore-based laboratory for chemical analysis. The use of HgCl2 has significant disadvantages including its human toxicity, bioaccumulation, long environmental persistence and the expensive disposal of hazardous mercury-containing wastewater. HgCl2 is highly toxic to aquatic organisms, and is efficiently transferred through the food chain, accumulating in top predators such as fish (Morel et al, 1998). Consumption of mercury-contaminated fish can cause gut irritation and kidney damage in humans (Langford and Ferner, 1999). Mercury-containing laboratory waste requires costly disposal to avoid it entering watercourses and wastewater treatment plants. These are strong incentives to find more environmentally benign alternatives to the use of HgCl2, in particular in remote, sensitive and pristine environments such as polar regions
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