Summary
Species invasions depend on the abundance and rate at which organisms are introduced to new localities, known as propagule pressure. Due to the challenges of measuring propagule pressure, ...proxy variables are often used; however, untested proxy variables may obscure the role of propagule pressure vs. ecological factors that facilitate invasion, leading to uncertainty about the invasion process and confounding management response.
To generate absolute estimates of propagule pressure and facilitate meaningful comparison among pathways of species introduction, we extend proxy variables by combining ecological and global trade data with hierarchical statistical models. We derive absolute propagule pressure with probability distribution functions (propagule size: the probability of introducing n propagules per event; and overall propagule pressure: the probability of introducing n propagules per year) and, based on these projections, evaluate the performance of ballast water volume (m3) as a common proxy variable.
Hindcast accuracy of ballast water volume was low but strongly scale‐dependent, exhibiting poor accuracy (R2 = 0·058) at small scales and only marginal accuracy at large scales (overall R2 = 0·169). As a result, conclusions about propagule pressure are likely to be biased based on the ballast volume proxy, as may be conclusions about species invasion when ballast water volume has been used. Irrespective of geographic pathway, estimated propagule sizes demonstrated extreme leptokurtosis and long right tails, with maximum values between 131‐fold and 2966‐fold greater than median values. These characteristics signify a subset of introduction events involving extreme propagule abundance that may provide opportunity to overcome Allee thresholds or marginal environmental conditions.
Developing meaningful propagule supply functions is critical to resolve the role of propagule pressure within the invasion process and test hypotheses about species colonization at landscape scales. By joining absolute propagule pressure with existing theory about species extinction, the expected ecological outcome of environmental policy to manage propagule pressure can be estimated within a statistical framework.
Global coastal aquatic ecosystems are negatively impacted by the introduction of harmful aquatic species through the discharge of ships’ ballast water. To reduce discharges of harmful aquatic ...organisms and pathogens, such as toxic phytoplankton species, ships are now transitioning to the use of ballast water management systems (BWMS) instead of ballast water exchange (BWE). This study examines the abundance and diversity of phytoplankton in ballast water managed by BWMS (or a combination of both BWE + BWMS) in comparison to those in ballast water managed by BWE alone (collected from ships arriving to Canada’s Pacific coast in 2017–2018 and 2008, respectively). The abundance and diversity of phytoplankton species were also examined in relation to key variables such as ballast water salinity and ballast water age. Total abundance of phytoplankton was significantly lower in preserved samples managed by either a BWMS or BWE + BWMS compared to BWE alone. Abundances in preserved samples were higher than observed in fresh (unpreserved) samples at the time of collection, with all samples managed by a BWMS meeting international limits for the number of viable organisms ≥10 and <50 μm in minimum dimension (based on six 1-mL live counts). While there was no apparent influence of factors such as treatment type e.g., ultraviolet (UV) or chlorine, presence of filtration, ballast water salinity, ballast water age, nor location of last ballast water uptake on phytoplankton abundances in preserved samples, power to detect differences may be limited by sample size. Ballast water managed by BWMS also tended to have lower abundances of harmful phytoplankton species, although the difference was not statistically significant – additional research into the community composition of live cells in fresh samples could be valuable to discriminate the risk associated with phytoplankton surviving ballast water treatment.
We examined the risk of introduction associated with potentially toxic or otherwise harmful algae (HA) or nonindigenous species (NIS) of dinoflagellates in ballast water from 63 commercial ships ...visiting ports of eastern Canada in 2007–2009. Ship categories included transoceanics undergoing ballast water exchange (BWE) and coastal ships with or without BWE. Of 159 species of dinoflagellates observed in Lugol-preserved samples, 15 were potential HA (six Dinophysis spp.) and 46 were NIS (including three HA). We found at least one species of HA in 81% of all ships examined, and maximum cell concentrations reached nearly 4000 cells·L
–1
. Coastal nonexchanged tankers carried the greatest cell concentrations of HA. NIS dinoflagellates were found in 56% of ships, significantly more in ships with BWE. There was no evidence that ships with BWE contained significantly fewer taxa or lower concentrations of HA dinoflagellates, indicating that BWE is not efficient in controlling the introduction of these organisms. In fact, BWE promoted the transport of NIS dinoflagellates, possibly because of the wide distribution of several of these species. Coastal ship traffic is a significant introduction pathway for HA (ships with and without BWE) and NIS (ships with BWE) dinoflagellates in eastern Canada.
Understanding the implications of different management strategies is necessary to identify best conservation trajectories for ecosystems exposed to anthropogenic stressors. For example, science-based ...risk assessments at large scales are needed to understand efficacy of different vector management approaches aimed at preventing biological invasions associated with commercial shipping. We conducted a landscape-scale analysis to examine the relative invasion risk of ballast water discharges among different shipping pathways (e.g., Transoceanic, Coastal or Domestic), ecosystems (e.g., freshwater, brackish and marine), and timescales (annual and per discharge event) under current and future management regimes. The arrival and survival potential of nonindigenous species (NIS) was estimated based on directional shipping networks and their associated propagule pressure, environmental similarity between donor-recipient ecosystems (based on salinity and temperature), and effects of current and future management strategies (i.e., ballast water exchange and treatment to meet proposed international biological discharge standards). Our findings show that current requirements for ballast water exchange effectively reduce invasion risk to freshwater ecosystems but are less protective of marine ecosystems because of greater environmental mismatch between source (oceanic) and recipient (freshwater) ecoregions. Future requirements for ballast water treatment are expected to reduce risk of zooplankton NIS introductions across ecosystem types but are expected to be less effective in reducing risk of phytoplankton NIS. This large-scale risk assessment across heterogeneous ecosystems represents a major step towards understanding the likelihood of invasion in relation to shipping networks, the relative efficacy of different invasion management regimes and seizing opportunities to reduce the ecological and economic implications of biological invasions.
Ships’ ballast water and sediments are vectors that contribute to the unintentional spread of aquatic non-native species globally. Ballast water management, as well as commissioning testing of ...ballast water management systems and compliance monitoring under the regulations of the International Maritime Organization (IMO) aim at minimizing the unwanted spread of organisms. This study compiles data for treated ballast water samples collected and analyzed from 228 ships during 2017–2023. The samples were collected from the ballast discharge line or directly from the ballast tank for enumeration of living organism concentrations in the categories of ≥50µm and <50 to ≥10µm -sized organisms, as well as indicator microbes in comparison to the ballast water performance standard of the IMO (Regulation D-2). In addition, several ship-specific factors were examined to infer potential factors affecting compliance rates. Nearly all ships were compliant with the ballast water performance standard for indicator microbes and <50 to ≥10µm -sized organisms, whereas almost half of all samples exceeded the limit of ten viable organisms m -3 for the ≥50µm -sized organisms. Compliance testing results did not differ significantly between sampling years, indicating that compliance rate did not change through time. The rate of compliance was higher for commissioning testing than compliance testing. Clear ship- or system-specific factors that lead to compliance or non-compliance were not detected, even though type of ballast water management system, filter mesh size associated with the system and source of ballast water affected compliance significantly either for the samples taken from the discharge line, or ballast tank. As compliance did not improve significantly over time, compliance testing of ships’ ballast water should be undertaken to ensure that the systems remain operational after commissioning and ships meet requirements of the D-2 standard. Furthermore, the study outcomes promote further research on the efficiency of filter mesh sizes and different filtration units associated with ballast water management systems, to improve mechanical removal of larger organisms. Finally, as several ships exceeded the compliance limit by hundreds or thousands of living organisms, technological advancements and operational measures may be needed to improve the overall reliability of ballast water management.
A recent model demonstrated that the adenosine triphosphate (ATP) content of spherical aquatic organisms with a 10 to 50 μm diameter is between 0.16 and 19.9 pg cell−1. Here, the model is validated ...by comparing microscopy-based counts with ATP concentrations from a commercial ATP kit.
The measured ATP content of both freshwater and marine organisms 10 to 50 μm size range falls in the 0.16 to 19.9 pg cell−1 model range. On average, freshwater organisms contain 0.33 pg ATP cell−1, have a spherical equivalent diameter (SED) of 13 μm, while marine organisms have 0.89 pg ATP cell−1 and a SED of 18 μm. In addition, their 13 to 18 μm size is within the 10 to 50 μm ballast water size range and in agreement with the 15 μm mean SED of a coastal plankton size-distribution model.
This study concludes that the ATP-model is reliable, emphasizing the need for caution when converting three-dimensional biomass proxies into linear cell concentrations.
•A recent model demonstrated that the ATP content of spherical aquatic organisms with a 10 to 50 μm diameter is between 0.16 to 19.9 pg cell-1. The model was validated by comparing microscopy-based counts with ATP concentrations from a commercial ATP kit.•On average, freshwater organisms contain 0.33 pg ATP cell-1, have a spherical equivalent diameter (SED) of 13 μm, while marine organisms have 0.89 pg ATP cell-1 and a sed of 18 μm.•The ATP content of both marine and freshwater organisms falls in the 0.16 to 19.9 pg cell-1 model range. Their 13 to 18 μm SED is within the 10 to 50 μm ballast water size range and in agreement with the average of 15 μm SED of a coastal plankton size-distribution model.•This study concludes that the ATP-model is reliable, emphasizing the need for caution when converting three-dimensional biomass proxies into linear cell concentrations.
With the entry into force of the International Maritime Organization's International Ballast Water Management Convention in September 2017, ships have begun to install and operate onboard ballast ...water management systems (BWMS) to reduce the number of live organisms in ballast water. Scientific methods were developed to assess the effectiveness of BWMS at reducing the number of live organisms in ballast water. However, detecting low organism concentrations in treated ballast water is challenging when considering the small sample volume (6 mL) analyzed for organisms in the 10–50 μm size class. The volume analyzed can be increased by concentrating the sample prior to analysis, but it is important to assess the effects of the sample concentration method due to potential cell loss experienced during the concentration step. Therefore, laboratory experiments were conducted to assess the effects of a gravity filtration method to concentrate samples to a factor of 40:1. Experiments were conducted for both low and high organism abundances. For unpreserved samples at low organism abundances (∼10 cells mL−1), concentrated samples had on average 31% fewer live cells mL−1 than unconcentrated samples for four out of five experiments. At high organism abundances (≥ 120 cells mL−1), unpreserved concentrated samples had on average 55% fewer live cells than unconcentrated samples. Alternatively, with preserved samples at low organism abundances, concentrated samples had on average 4.5× more cells than unconcentrated samples. At high organism abundances, concentrated samples had on average 6.4× more cells than unconcentrated samples. Differences were also observed between preserved and unpreserved samples. These findings can help to improve ballast water monitoring procedures and BWMS assessments, addressing a critical challenge to maritime environmental protection.
•Concentration by gravity filtration led to cell loss in live phytoplankton samples.•Alternate concentration methods should be explored to mitigate cell loss.•Concentrating had different effects on live and preserved samples.•Concentrating samples is important to improve precision at low abundances.•Counting six subsamples improved precision in low abundance samples.
To minimize the global transfer of harmful aquatic organisms and pathogens, the International Maritime Organization (IMO) has introduced the standard in Regulation D-2 to limit the number of viable ...organisms in ballast water discharged by ships. To meet the standard, many ships are installing ballast water management systems. Concurrently, regulators are looking for indicative analysis devices able to assess compliance with Regulation D-2, producing rapid, accurate and reliable results while being easy to operate. The purpose of this research is to compare four indicative analysis devices against detailed microscopy for measuring the size class of organisms ≥10 to <50 μm in minimum dimension (e.g., phytoplankton, including autotrophs, heterotrophs or mixotrophs), using field and laboratory tests. Comparisons were conducted on (treated) ballast water discharge samples collected across Canada during three consecutive years (2017–2019). During seven tests in 2019, paired ballast water uptake samples were also obtained, facilitating measurements before and after treatment was applied. Indicative analysis devices also were challenged with natural environmental samples containing different organism abundance levels, ranging from low (nominally <10 cells mL−1) to high (nominally >150 cells mL−1) during laboratory tests. While the indicative analysis devices examined during this research produced numeric estimates having weak correlations with the standard reference method, categorical outcomes (above/below the D-2 standard) had high agreement (89% or better) when assessing ballast water samples, but lower agreement (67% or poorer) during laboratory tests. There was a relatively high rate of false negative results measured by all devices during laboratory tests. Results provided by indicative analysis devices had higher uncertainty when organism abundances in ballast samples are below and close to the D-2 standards.
•Indicative devices had weak correlation with microscopy based on numeric estimates.•Categorical outcomes had high agreement for ballast water samples.•Uncertainty was higher for abundances below and close to the D-2 standard.
The movement of ballast water by commercial shipping is a prominent pathway for aquatic invasions. Ships' ballast water management is now transitioning from open ocean exchange to a ballast water ...performance standard that will effectively require use of onboard treatment systems. Neither strategy is perfect, therefore, combined use of ballast water exchange plus treatment has been suggested to provide greatest protection of aquatic ecosystems. This study compared the performance of exchange plus treatment against treatment alone by modeling establishment rates of nonindigenous zooplankton introduced by ballast water across different habitat types (fresh, brackish, and marine) in Canada. Treatment was modeled under two efficacy scenarios (100% and 50% of ship trips) to consider the possibility that treatment may not always be successful. The model results indicate that exchange plus treatment will be more effective than treatment alone at reducing establishments when recipient ports are freshwater (58 140 vs 11 338 trips until ≥1 establishment occurs, respectively). Exchange plus treatment also serves as an important backup strategy if treatment systems are partially effective (50% of trips), primarily for freshwater recipient ecosystems (1442 versus 585 trips until ≥1 establishment occurs, respectively).
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