Storage duration and water type determine the need for mercury chloride preservation of dissolved inorganic carbon

Abstract To quantify aquatic carbon budgets, it is important to accurately measure dissolved inorganic carbon (DIC). The standard approach requires the use of mercury chloride (HgCl 2 ) poison to preserve samples, but the real need and overall constraints of using such a toxic compound remain elusive. Here, we evaluate the need for filtration and mercury poisoning to maintain DIC stability in six contrasting water types across the freshwater–seawater continuum. DIC concentrations were analyzed at nine time points between the sampling day and 6 months later. Filtered and unfiltered samples did not differ significantly, suggesting that filtration alone is insufficient to maintain DIC stability during long‐term storage. Unpoisoned samples started to exceed stability limits after periods ranging from 24 h (Baltic seawater) to 30 d (oligotrophic lake). In contrast, poisoning with HgCl 2 preserved most samples for up to half a year. When deviations of up to 2% are acceptable, HgCl 2 preservation may be unnecessary for storage periods of up to 4 d in the investigated surface water types. The timing of HgCl 2 addition therefore represents a practical compromise between minimizing analytical bias and reducing handling risks. Overall, filtration may be beneficial for particulate‐rich samples, whereas HgCl 2 preservation is recommended when DIC cannot be analyzed within a few days after collection.