Stratification of Living Organisms in Ballast Tanks: How Do Organism Concentrations Vary as Ballast Water Is Discharged?

Abstract

Vertical migrations of living organisms and settling of particle-attached organisms lead to uneven distributions of biota at different depths in the water column. In ballast tanks, heterogeneity could lead to different population estimates depending on the portion of the discharge sampled. For example, concentrations of organisms exceeding a discharge standard may not be detected if sampling occurs during periods of the discharge when concentrations are low. To determine the degree of stratification, water from ballast tanks was sampled at two experimental facilities as the tanks were drained after water was held for 1 or 5 days. Living organisms ≥50 μm were counted in discrete segments of the drain (e.g., the first 20 min of the drain operation, the second 20 min interval, etc.), thus representing different strata in the tank. In 1 and 5 day trials at both facilities, concentrations of organisms varied among drain segments, and the patterns of stratification varied among replicate trials. From numerical simulations, the optimal sampling strategy for stratified tanks is to collect multiple time-integrated samples spaced relatively evenly throughout the discharge event.

Hybrid system increases efficiency of ballast water treatment

“…Our experiments were conducted under realistic scenarios on board an operating vessel that was outfitted to allow collection of samples from major sections of ballast tanks, thereby incorporating vertical variation in distributions of biota (Murphy, Ritz & Hewitt ; First et al . ). Reductions in abundance of bacteria, microplankton and macroplankton in untreated (control) ballast water in relation to voyage length are consistent with previous studies (Drake et al .…”

“…Three additional aliquots of unfiltered port water were collected at different times during the ballasting process, though we avoided the initial and final 20 min in order to collect representative samples (First et al . ), and then integrated the samples into a single 20‐L sample. Sample volume was monitored using a Hydrobios flowmeter.…”

Implications of heterogeneous distributions of organisms on ballast water sampling

“…Unfortunately, there are not many studies designed to identify the form of the curves representing the variation in the concentration along the discharged volume; a study paving the way in that direction is presented in First et al (2013). We consider two examples with simulated data using the functions represented in Fig.…”

Optimizing methods to estimate zooplankton concentration based on generalized patterns of patchiness inside ballast tanks and ballast water discharges

“…There has been extensive research to develop equipment for collection of in‐line samples (e.g., Richard, Grant, & Lemieux, 2008; Wier et al., 2015) and to establish minimum required sample volumes (e.g., Frazier, Miller, Lee, & Reusser, 2013; Hernandez, Johansson, Xiao, Lewis, & MacIsaac, 2017; Miller et al., 2011), as well as multiple studies examining the distribution of zooplankton within sequential samples collected by in‐line sampling (e.g., Carney et al., 2013; First et al., 2013; Gollasch & David, 2013). In general, these studies report that plankton concentration can vary widely depending on the timing (sequence) of sample collection, leading to recommendations for collecting multiple 1‐m 3 volume samples.…”