None detected: What “zero” indicates in direct counts of aquatic microorganisms in treated ballast water

International limits on the concentrations of living organisms in ballast water are now force for commercial ships. Microscopy-based, “direct count” assays estimate organisms. These used performance tests Ballast Water Management Systems (BWMSs), which shipboard technologies designed to reduce below discharge limits. Here, we examine factors that affect method detection limit (MDL) direct count assays. The MDL depends upon volumes sampled, concentrated (or diluted), and analyzed, so will vary as these from analysis-to-analysis. Decreasing detect exceedingly rare individuals (e.g., ~1 individual per m 3 or fewer) is possible, but problematic, given challenges analyzing dynamic communities organisms: increasing sample analysis lower likely accelerate loss individuals, several orders magnitude above situ held extended times. Results may be widely disseminated an indicator BWMSs, when no detected, shown “0 mL” rather than “<MDL”, appropriate. When clearly derived reported, a numerical assay—the lowermost value distinct “zero”—provides context results transparency into sensitivity assay.