The Role of Osmotic Stress (Salinity Shock)

in Protecting the Great Lakes from BallastAssociated Aquatic Invaders

Technical Report

Prepared for the

U.S. Saint Lawrence Seaway Development Corporation

U.S. Department of Transportation

November 30, 2012

David F. Reid, Ph.D.

Consulting Scientist

Ann Arbor, Michigan

dfrAISC@gmail.com
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Table of Contents

Executive Summary................................................................................................................... i

1. Introduction...................................................................................................................... 1

2. Background: Ballast Water Exchange And Saltwater Flushing ..................................... 2

2.1 Ballast Water Exchange........................................................................................2

2.2 Saltwater Flushing – the NOBOB Problem..........................................................7

2.3 The Discovery Rate Record..................................................................................9

3. The Benefits and Limitations of Osmotic (Salinity) Shock........................................... 12

3.1 Osmoregulation...................................................................................................12

3.2 Osmosis, Osmoconformers and Osmoregulators................................................12

3.3 Salinity Tolerance (physiological stress, osmotic shock, salinity shock)...........14

4. Recent Field and Laboratory Studies of Salinity Shock Associated with Ballast Water 

Exchange for the Great Lakes........................................................................................ 18

4.1 Effect of BWE on Diapausing Eggs (Great Lakes) ............................................18

4.2 Salinity Tolerance Experiments – Testing Effects of BWE on Freshwater 

and Estuarine Organisms. ...................................................................................20

5. Conclusions.................................................................................................................... 23

6. References...................................................................................................................... 24

7. Acknowledgements........................................................................................................ 31
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i

EXECUTIVE SUMMARY

This Technical Report examines the history and basis for using ballast water exchange as a 

barrier against aquatic species introductions. It describes the nature of osmotic stress (or salinity 

shock) and its role in the application of BWE and saltwater flushing to reduce the ballast-related 

invasion risk to the Great Lakes ecosystem.

In the late 1980s ballast water exchange (BWE) was implemented as an interim policy aimed at 

protecting the Great Lakes from continued introduction of aquatic nonindigenous species 

attributed to ballast tank transport. Two benefits were ascribed to ballast water exchange: 

nearshore water and associated organisms would be flushed out and replaced by open-ocean 

water and organisms which are less likely to survive in the Great Lakes; freshwater or coastal 

organisms remaining the ballast tanks would be killed by salinity shock (see below). 

In 2004 the International Maritime Organization (IMO) adopted the International Convention for 

the Control and Management of Ships' Ballast Water and Sediments. The Convention will enter 

into force 12 months after ratification, after which use of BWE will be gradually phased out and 

on-board treatment systems will be phased in.

In 2010 Canada submitted a proposal to IMO outlining the potential benefits of continuing to use 

BWE in addition to on-board treatment (BWE+Treatment) of ballast water on ships bound for 

fresh- and brackish-water ecosystems. The proposal is based on concern that treatment systems 

alone would allow discharge of a certain number of live organisms (as determined the IMO D-2 

performance standard) during each ballast discharge event. Live fresh- or brackish-water 

organisms in the discharge, even within the IMO limits, would pose a risk to any fresh- and/or 

brackish-water ecosystem receiving the discharge. Conceptually, if a ship is required to conduct 

BWE and tank flushing, as well as treatment, the resulting treated discharge could contain the 

same number of live organisms, but they likely would not be freshwater species. Salinity 

exposure would have killed most, if not all larval and adult freshwater species. 

In 2011 USEPA issued a draft 2013 Vessel General Permit that includes a requirement for ships 

bound for the Great Lakes to conduct BWE+Treatment if they have ballasted with water <18 ppt 

within a month prior to entry. In 2012 Canada confirmed its plan to require BWE+Treatment for 

vessels using an on-board treatment system and entering Canadian waters with ballast having a 

salinity of 2 psu (~2 ppt) or lower. The plan to require BWE+Treatment has been criticized 

because there has been no conclusive scientific verification of the enhanced protection 

conceptualized in the Canadian IMO proposal, and there hasn’t been an assessment of 

practicability of this procedure. 

Osmoregulation is a mechanism by which organisms maintain internal cellular stability by 

regulating or adapting the salt content of their body fluids to changes in the salt content of their 

external environment. Osmoregulation is well studied for many classes of organisms, and there 

is a large body of literature and a wealth of information available. Most organisms have a 

limited range of environmental salinity they can manage. The range of salinity over which a 

particular species is able to survive is its salinity tolerance, which is closely related to its 

osmoregulatory capability. The basis for using BWE as an invasive species management tool for 

the Great Lakes is that freshwater organisms subjected to sudden salinity increase, such as 

produced by mid-ocean ballast water exchange or tank flushing, experience significant life-
ii

threatening osmotic stress resulting in high mortality. Unfortunately, some organisms are capable 

of surviving exposure to the full range of salinity between freshwater and ocean water, so BWE 

will not necessarily be 100% effective against all freshwater-tolerant organisms. The response of 

aquatic organisms to a sudden change in salinity depends on many factors: the salinity range they 

generally experience in their present habitat, the rate of salinity increase or decrease during their 

exposure to change, the end point (how high or low the endpoint is, compared to their normal 

habitat range), the amount of time they’re exposed, temperature, life stage, and their 

osmoregulatory capability. The effectiveness of BWE and salinity exposure will depend on the 

nature of the organisms inhabiting the ballast tank.

Studies performed with appropriate controls to correct for non-exchange effects in ballast tanks 

suggest that in general, BWE can achieve 80 to >95% removal of coastal planktonic marine 

organisms, mostly through flushing unless the original ballast water is fresh- or low-salinity 

water. In the latter case, salinity shock can cause high mortality (>99%) of freshwater and lowbrackish water organisms not flushed from the tank. BWE appears to be more effective at 

reducing or eliminating coastal planktonic invertebrates in ballast tanks than for reducing or 

eliminating microbial organisms. Experimental evidence suggests that when carried out 

according to regulations and established procedures, BWE can be highly effective at reducing 

invasion risk, especially for freshwater systems. However, the continued discovery of new 

invaders in the Great Lakes after mandatory BWE was established was interpreted by many 

regulators, environmental groups and policy-makers as evidence that BWE doesn’t work. 

After a number of years of applying BWE and still finding new invaders in the Great Lakes, it 

was realized that over 80% of foreign vessels trading in the Lakes were not being required to 

conduct BWE because their tanks were empty, that is, they contained no pumpable ballast water 

(no-ballast-on-board, or “NOBOB”). As documented by research during the early 2000s, such

tanks contain small amounts of residual water and sediment that house live aquatic organisms 

and dormant eggs, so entry of NOBOB vessels into the Great Lakes was a significant gap in the 

protection framework intended when BWE was implemented. This was resolved by regulations 

in 2006 and 2008 that implemented mandatory flushing of empty tanks and strict enforcement of 

a salinity requirement (≥30 ppt) for water carried in ballast tanks, even residual unpumpable 

water. Since then several scientific studies have experimentally documented the biocidal effects 

of salinity, which causes osmotic shock, against a wide range of freshwater and estuarine 

invertebrate taxa, both in laboratory experiments and via shipboard studies.

The primary mechanism by which BWE reduces invasion risk is to flush coastal water and 

organisms out of ballast tanks, with varying degrees of completeness. When the original ballast 

is also low-salinity or fresh water, salinity shock-induced mortality of low-salinity species is an 

additional mechanism that can enhance BWE efficacy. Regardless of the original ballast water, 

the BWE process brings replacement ballast water with live saltwater organisms, which when 

discharged pose less risk to receiving coastal marine ecosystems, and should pose much less risk 

to freshwater ecosystems. 

Since 2006 (through October 2012) no new aquatic invaders have been discovered in the Great 

Lakes. While this is good news, it must be interpreted with caution. It is clear that tank flushing 

closed a major loophole in the protection framework and, in combination with BWE, has 

contributed to a significant reduction in ballast-associated risk. Although the added mortality 

attributed to salinity (osmotic) shock can effectively eliminate many, if not most live freshwater 
iii

taxa, it does not kill viable dormant eggs and cysts. Salinity exposure does prevent them from 

hatching in ballast tanks and tank flushing reduces the amount of residual sediment, where eggs 

and cysts accumulate. Dormant eggs and cysts can be discharged during deballasting, and with 

appropriate environmental cues, they can hatch into live organisms in recipient ecosystems. 

Their presence in ballast tank sediment limits the protection BWE and tank flushing can provide 

for freshwater systems like the Great Lakes. It has been observed that organisms with broad 

salinity tolerance and the ability to produce resting eggs dominated the composition of new 

invaders discovered in the Great Lakes since mandatory BWE was implemented in 1993. These

are also the organisms most likely to be carried in the residual ballast transported by NOBOB 

vessels. Most scientists will not be surprised if new ballast-implicated invaders are discovered in 

the future. 

A deficiency in our understanding of the efficacy of BWE and tank flushing is lack of sufficient 

data related to organisms other than large (>50μ) invertebrates, especially small invertebrates, 

phytoplankton, and microorganisms. So while BWE and tank flushing appear to be highly 

beneficial and protective of the Great Lakes ecosystem, vital information about the introduction 

rates and effects of salinity on other freshwater-tolerant organisms is lacking.

In principle, the use of BWE in combination with on-board treatment (BWE+Treatment) could 

enhance reduction of the risk associated with ballast water discharges to fresh- and brackishwater ecosystems. BWE would potentially eliminate freshwater-tolerant (i.e., high-risk) species 

from the ballast water prior to treatment, leaving less risky marine and high-brackish species in 

the discharge. The efficacy against high-risk species will be dependent on the salinity tolerance 

of the organisms. There are some, mainly estuarine, organisms that have tolerance over the full 

range of salinity, although their ability to survive a rapid change from low salinity to almost fullstrength ocean water is another factor. Using BWE as a precursor to on-board treatment would 

also provide a form of insurance in case of undetected or unexpected treatment system failure, 

especially since these systems are very new and at present we don’t have much actual 

operational experience with them to accurately gauge their reliability and performance. 

There is ample scientific evidence to support the conceptual principles presented in the Canadian 

BWE+Treatment proposal. However, the practicability of the proposed procedure under 

shipboard operating conditions has not yet been evaluated. Also, the effectiveness of 

BWE+Treatment compared to treatment alone needs to be verified by direct experiments. At 

least one series of experiments using simulated exchange+treatment vs. treatment alone has been 

completed at a land-based test facility, but the results have not yet been published, although they 

are expected soon. In addition, a shipboard test program is also underway by Canadian 

scientists.
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1. INTRODUCTION

In 2004 the International Maritime Organization (IMO) adopted the International Convention for 

the Control and Management of Ships' Ballast Water and Sediments (IMO 2004). Once the 

Convention enters into force (12 months after ratification criteria are met), it allows continued 

use of ballast water exchange (BWE) only until discharge standards established by Regulation 

D-2 (Ballast Water Performance Standard) become effective. Regulation D-2 establishes sizebased numeric discharge limits for viable organisms that BWE alone will not meet, and is 

scheduled to be phased in based on vessel ballast capacity and build date. In 2010 Canada 

submitted a proposal to IMO (IMO 2010) for ships bound for fresh- or brackish-water ports to 

use BWE in combination with on-board IMO-approved treatment systems designed to produce 

ballast water discharges meeting the IMO D-2 standard. This concept (BWE+Treatment) is 

based on research suggesting that osmotic stress induced mortality as a result of BWE can 

frequently reduce invasion risk from ballast discharge to fresh- and brackish-water ports to a 

level at least as protective as the D-2 standard. For freshwater recipient ecosystems, it may be 

possible to achieve a discharge of relevant organisms1 even less than the D-2 standard. Use of a 

treatment system alone (and meeting the IMO D-2 standard) would still allow discharge of up to 

9 viable high-risk freshwater organisms per m3 (≥50μ size range) into freshwater ecosystems like 

the Great Lakes. Under the BWE+Treatment scenario, it is possible that most, if not all 

freshwater taxa would be killed prior to treatment, leaving mainly, if not exclusively low-risk 

marine taxa to comprise the (up to) 9 viable organisms allowed in the discharge.

The Canadian proposal is mirrored in the draft 2013 Vessel General Permit (USEPA 2011a, 

Section 2.2.3.7) specifically for ships entering the Great Lakes. Additionally, several Great 

Lakes States, including New York, Michigan, Wisconsin, and Minnesota have included 

BWE+Treatment requirements in their draft and/or final Clean Water Act Section 401(a) 

Certifications of that permit. Croot (2012) argues that there have not been scientific studies that

conclusively demonstrate the benefits of BWE+Treatment. There is also criticism that the 

practicability of the proposed BWE+Treatment combination has not been demonstrated and the 

added cost to ship operations has not been evaluated2

. A recent paper discussing Canadian plans 

for implementing the IMO Convention notes that in addition to minimizing risk to freshwater 

ecosystems, an additional benefit expected under a BWE+Treatment strategy is that BWE would 

provide backup in case of undetected or unexpected treatment system failure (Transport Canada 

2012). Scientific studies to better quantify the effectiveness of BWE+Treatment are planned by 

Transport Canada (C. Wiley, Transport Canada, personal communication), and at least one 

independent field study was started during 2012 led by University of Windsor (H. MacIsaac, 

personal communication).

This Technical Report examines the history and basis for using ballast water exchange as a 

barrier against aquatic species introductions to coastal ecosystems. It describes the nature of 

osmotic stress (or salinity shock) and its role in the application of BWE and saltwater flushing to 

 

1 Organisms that pose high invasion risk to freshwater ecosystems because they have a high probability of survival 

in freshwater habitats.

2 As part of its 2011 economic analysis for the VGP, EPA evaluated the additional operating costs of the ballast 

water exchange requirement for vessels entering the Great Lakes (USEPA 2011b, Table 4-12).
2

reduction of ballast-related invasion risk to the Great Lakes ecosystem. It shows that there is 

ample scientific evidence to support the conceptual principles presented in the Canadian 

BWE+Treatment proposal (IMO 2010). This report does not address practicability or cost 

concerns, or reliability of ballast water treatment systems. 

2. BACKGROUND: BALLAST WATER EXCHANGE AND SALTWATER FLUSHING

2.1 BALLAST WATER EXCHANGE

BWE to reduce the risk of aquatic species invasions to North American waters, especially the 

Great Lakes, is a well-known ballast management practice aimed at reducing the presence of 

aquatic nonindigenous species (ANS) in ballast water (MEPC 1991, USCG 1993). It was 

initially developed in Canada in the early-to-mid 1980s, not for management of ANS, but to 

protect the aquaculture industry of the Magdalen Islands from sewage-contaminated ballast 

water (Wiley, personal communication; SLSDC 2012). By 1988 the International Joint 

Commission and Great Lakes Fishery Commission had become increasingly alarmed by 

continuing discoveries of ANS in the Great Lakes, as illustrated by three new (at that time)

aquatic invaders (ruffe, Gymnocephalus cernua; spiny water flea, Bythotrephes longimanus; and 

zebra mussel, Dreissena polymorpha). They determined the most likely source for many Great 

Lakes aquatic invaders was ballast water discharged from foreign vessels trading within the 

Lakes. In consultation with scientists, shipping experts, and U.S. and Canadian federal scientific 

and regulatory agencies, they proposed implementing ballast water exchange (IJC & GLFC 

1990) as a means of reducing risk of continued ballast-related species introductions. In 1989 

Canada published voluntary ballast water management guidelines for vessels entering the St. 

Lawrence River and the Great Lakes, and in 1993 the U.S. Coast Guard established mandatory 

ballast management requirements, including mandatory use of BWE on ships originating from 

beyond the EEZ and entering the Great Lakes or Hudson River (USCG 1993). Also in 1993, the 

first IMO guidelines concerning ships’ ballast water and harmful aquatic organisms were 

adopted (IMO 1993), and included ballast water exchange and sediment removal at sea as one of 

several possible approaches. 

Two benefits were ascribed to ballast water exchange (Carlton 1990, IJC & GLFC 1990, Locke 

et al. 1991, USCG 1993): nearshore water and associated organisms would be flushed out and 

replaced by open-ocean water and organisms which are less likely to survive in the Great Lakes;

freshwater or coastal organisms remaining the ballast tanks would be killed by the saltwater (i.e., 

the result of salinity shock). Two main BWE processes have been used: 1) empty-refill

exchange, in which ballast water is pumped out until the ballast tank is empty (contains only 

unpumpable ballast water), and then refilled with new water (mid-ocean or from an approved 

coastal exchange zone site), and 2) flow-through exchange, in which new water (mid-ocean or 

from an approved coastal exchange zone site) is pumped into a ballasted tank, generally at the 

bottom, and the tank is allowed to overflow (usually at deck level, see Figure 1) until at least 

three times the total tank volume has overflowed from the tank. 

One of the earliest biological studies of ballast tanks in ships trading in the Great Lakes was 

conducted in 1980 by Bio-Environmental Services (1981), but BWE was not a practice at the 

time and therefore none of the ships had conducted BWE. Salinity of the sampled ballast water 
3

ranged from 0-36 ppt, with 65% of samples classified as marine, 31% estuarine and 4% 

freshwater. Over 150 distinct genera and species of phytoplankton and 56 distinct aquatic 

invertebrate fauna were identified in the samples, including freshwater forms.

Locke et al. (1991, 1993) were the first to evaluate the effectiveness of BWE in eliminating 

freshwater-tolerant organisms on ships specifically entering the Great Lakes. Their results were 

based on the presence/absence of live freshwater-tolerant taxa in ballast water of ships sampled 

while up-bound to the Great Lakes in the St. Lawrence River or at dock in Montreal, Quebec, 

and they did not actually conduct exchange experiments. They calculated BWE effectiveness of 

67-86% based on live freshwater taxa found in 24 vessels originally ballasted in fresh or 

brackish-water ports prior to mid-ocean BWE; only 14 of the 24 had achieved the BWE-required 

≥30 ppt final salinity. These results were instructive, but not definitive, given the lack of 

controls and test protocols. However, their work was an important early attempt to evaluate the 

effects of exchange on ballast water coming into the Great Lakes, and in particular, they 

exhibited considerable foresight when they noted the frequent occurrence of ships entering the 

Great Lakes with unpumpable ballast water. Such ships were not subject to BWE requirements, 

and Locket et al. expressed concern that organisms carried in unpumpable residual water could 

be released during subsequent (ballasting and then) deballasting operations as these ships visited 

multiple ports within the Lakes. They recommended strategies be developed to treat 

‘unpumpable’ residual water, such as flushing of such ballast tanks with saltwater (Locke et al. 

1991, Recommendation (4)).

Numerous studies since the early 1990s attempted to measure the efficacy of ballast water 

exchange for flushing out coastal water and organisms. Ruiz et al. (2007) discussed the 

methodological difficulties of evaluating the effects of BWE and the limitations associated with 

many of the studies published through 2004. Such studies used different experimental 

methodologies and were conducted on ships with different ballast tank configurations; most did 

Figure 1: Water overflowing through a deck hatch on a commercial cargo vessel 

during flow-through exchange. (Photo courtesy of Smithsonian Environmental 

Research Center and the Great Lakes NOBOB Project).
4

not address conditions specific to Great Lakes concerns, i.e., freshwater taxa survival. In 

addition, many studies did not control for effects unrelated to exchange, such as natural mortality 

in the tanks (Ruiz and Reid 2007). 

Wonham et al. (2001) conducted empty-refill exchange experiments on saltwater ballast, with 

control tanks to correct for natural mortality. Natural mortality in the absence of exchange 

reduced live plankton densities by >98%, and diversity3 (number of taxa) was reduced by >50% 

in the unexchanged control tanks over the course of the 16 day voyage. Open ocean exchange 

replaced an estimated 96-100% of coastal water and 80-100% of live coastal organisms, but the 

total population of plankton in exchanged tanks increased significantly, presumably reflecting 

the influx of mid-ocean organisms. Due to the relatively small change in salinity, osmotic 

stress/salinity shock was not a factor, and the effectiveness of exchange against encysted 

phytoplankton, bacteria, and very small plankton was not evaluated. They concluded "Openocean exchange represents an additional selective filter in the ballast invasion pathway that

reduces but does not eliminate coastal taxa." (Wonham et al. 2001, p. 10).

Ruiz et al. (2007) reported results from over two dozen BWE experiments with controls for nonexchange effects. Calculated exchange efficacies, based on dye measurements, ranged from 88-

99% for replacement of coastal water and 80-95% for removal of coastal planktonic organisms, 

depending on the type of ship (Figure 2) and parameters measured. They found no significant 

differences between flow-through and empty-refill or high vs. low starting salinities. 

 

3 Number of taxa

Figure 2: Efficacy of Empty-Refill BWE for removing original 

ballast water on different ship types determined by Ruiz et al.

(2007) using Rhodamine Dye as a tracer. (Figure reproduced from 

Ruiz et al. 2007)
5

Although they concluded that BWE significantly reduces total propagule supply of coastal 

organisms, especially for waterborne life stages, they cautioned that residual ballast and 

microorganisms and cysts posed an unknown and understudied risk. 

Gray et al. (2007) conducted controlled BWE exchange experiments in ballast tanks filled with 

Great Lakes water. They found BWE to be >99% effective at eliminating the freshwater 

zooplankton in the ballast tanks. They concluded that BWE is particularly effective against 

freshwater organisms and can provide strong protection to freshwater ecosystems against

invasions by freshwater taxa.

Klein et al. (2010) focused on BWE effects on diatoms, a highly abundant form of 

phytoplankton in ballast water. Their experiments were conducted using saltwater ballast and 

included unexchanged tanks as controls. They found BWE introduced new diatom species to 

exchanged ballast tanks and several freshwater diatom species survived BWE and were still 

viable at the conclusion of the voyages. While BWE effectiveness was ~87% for reducing total 

abundance of diatoms in the ballast tanks, they concluded that the density of viable diatoms 

present at discharge represented a high risk for establishment at receiving ports. 

Simard et al. (2011) conducted exchange experiments on two cruises between Europe and North 

America, both of which controlled for natural mortality. Their results, which measured the 

flushing efficacy of the exchange process, ranged from 23-54% for zooplankton and 29-40% for 

microplankton when adjusted for natural mortality. Both experiments were conducted on 

saltwater ballast and did not involve effects of salinity shock, as the organisms contained in the 

water were not exposed to a significant change in salinity. 

Briski et al. (2012) found live macroinvertebrates4 in ~10% of ballast tanks sampled on ships 

arriving to the Atlantic coast of Canada, including tanks that had undergone BWE. Most were 

marine species and some are known to be highly invasive. One live crab specimen 

(Rhithropanopeus harrisii) was a gravid5 female. This species has been reported in freshwater 

reservoirs in Texas, suggesting it could pose an invasion risk to the Great Lakes if introduced. 

BWE did not affect macroinvertebrate occurrence, although more than half of those found were 

in ballast tanks of coastal voyages NOT required to use BWE, and the majority of discoveries 

were found in ships after short voyages (<7 days).

Many studies of BWE efficacy have targeted invertebrates, which are common invaders in many 

aquatic ecosystems. Ruiz et al. (2000) pointed out that the biology of many microorganisms 

may facilitate their invasion potential: high capacity for asexual reproduction, ability to form 

dormant resting stages, and broad environmental tolerances, such as salinity and temperature. 

Since 2000 several studies have examined microorganisms (bacterial and/or virus-like-particles) 

in ballast water and/or attempted to evaluate the effects of BWE on microbial communities. 

Ruiz et al. (2000), Drake et al. (2001, 2007) and Sun et al. (2010) measured microorganism 

concentrations in ballast water but did not conduct BWE efficacy tests. Ruiz et al. (2000) was 

one of the first studies of microorganisms (bacteria, virus-like-particles and Vibrio cholerae

 

4 Macroinvertebrates are invertebrates that can be seen with the naked eye.

5 Carrying eggs
6

bacteria O1 and O139) in ballast water and found concentrations were 6-8 orders of magnitude 

greater than population densities of other taxonomic groups reported in ballast water. Drake et 

al. (2001) broadly characterized microorganism communities (bacteria and virus-like-particles) 

in ballast water of ships arriving to Chesapeake Bay. They found considerable variation among 

vessels, but mean abundances of bacteria and viruses were less than typically found in 

Chesapeake Bay water and similar to densities reported for open-ocean surface waters. There 

were some data in common to both Ruiz et al. (2000) and Drake (2001) (same ship and tank 

samples) and neither study differentiated results from exchanged vs. unexchanged tanks. 

Drake et al. (2007) sampled both exchanged and unexchanged ballast tanks in ships arriving to 

Chesapeake Bay over a 7 y period and found no significant differences in mean microorganism 

(bacteria or virus-like-particle) concentrations between exchanged and unexchanged tanks. They 

did, however, subsample different in-tank habitats - water, water and sediment residuals, and 

biofilm - and found concentrations of microorganisms varied over 1000-fold among these 

habitats, with ballast water >> sediment and water residuals >> biofilms6

. 

Similarly, Sun et al. (2010) did not conduct actual BWE experiments, but reported bacterial

abundances from ballast water of ships arriving to Vancouver, British Columbia. Sampled water 

included a mix of mid-ocean exchanged, intracoastal exchanged and intracoastal unexchanged 

ballast. Bacterial abundances were significantly higher in Vancouver port water than in any of 

the ballast water samples, but this should not be a surprise. It is not unusual for water in coastal 

ports, where ballast water is most often loaded, to contain bacterial pollutants from local 

activities, such as sewage and agricultural run-off. Also not surprising, unexchanged coastal 

ballast water had significantly higher bacterial abundance compared to mid-ocean exchanged 

ballast water. No microbial community composition data were obtained.

Drake et al. (2002) conducted microbial experiments using two exchange and two control (no 

exchange) ballast water holds. They observed microbial constituents (bacteria and virus-likeparticles) decreased throughout the voyage, but found no significant differences in microbial 

concentrations between exchanged and unexchanged holds in their final samples, taken 5 days 

after exchange was completed. They measured microorganism cell abundance and biomass, but 

not community composition. Since naturally occurring bacterial communities have distinct 

estuarine, coastal, and oceanic populations (Seiden et al. 2011), the microorganism community 

composition in a ballast tank could change during BWE as coastal water is replaced by midocean water, but this wouldn’t necessarily be reflected in abundance.

Quilez-Badia et al. (2007) also conducted mid-ocean exchange experiments using paired 

exchanged tanks and unexchanged (control) tanks. Total bacterial concentrations generally 

decreased over time in both control tanks and exchanged tanks, but total bacterial abundances in 

the exchanged tanks decreased significantly right after exchange and then became constant 

thereafter. An incubation effect was observed in a few tanks (control and exchange), as revealed 

by bacterial abundances increasing over time. 

 

6 A biofilm is a complex microbial community adhering to surfaces that are regularly in contact with water, 

consisting of colonies of bacteria and usually other microorganisms such as yeasts, fungi, and protozoa that secrete a 

mucilaginous protective coating in which they are encased (www.dictionary.com).
7

Seiden et al. (2011) conducted similar experiments, using two ballast tanks for mid-ocean 

exchange experiments paired with two unexchanged ballast tanks as controls. They found midocean exchange did not significantly reduce bacterial abundances and final abundances in the 

exchanged and unexchanged tanks were not significantly different at the end of voyages. 

Further, ballast water had significantly higher abundances than receiving port waters.

BWE appears to be more effective at reducing or eliminating coastal planktonic invertebrates in 

ballast tanks than for reducing or eliminating microbial organisms, although the effects of BWE 

on the latter have only been characterized by changes in abundance. Changes in microbial 

community composition may occur during BWE due to differences between microbial 

communities typically found in open-ocean surface water vs. coastal and port water. 

While BWE can be effective at removing coastal water and organisms, it is not a perfect filter. 

Results of BWE efficacy studies have been quite varied, and although some of the variance can 

be attributed to differences in methodology and/or differences in tank architecture, efficacy is

also affected by various other factors, such as ship type, adherence to procedural guidelines, and 

the environmental conditions ballast organisms encounter during the voyage. Significant natural 

mortality of ballast tank organisms prior to or without having conducted BWE has been 

observed. 

The primary mechanism by which BWE reduces invasion risk is to flush coastal water and 

organisms out of ballast tanks, with varying degrees of completeness. When the original ballast 

is also low-salinity or fresh water, salinity shock-induced mortality of low-salinity species is an 

additional mechanism that can enhance BWE efficacy. Regardless of the original ballast water, 

the BWE process brings replacement ballast water with live saltwater organisms, which when 

discharged pose less risk to receiving coastal marine ecosystems, and should pose much less risk 

to freshwater ecosystems. 

2.2 SALTWATER FLUSHING – THE NOBOB PROBLEM

Continued discoveries of new ANS in the Great Lakes after mandatory BWE regulations were 

implemented in 1993 (see GLANSIS 2012) raised doubts that BWE is effective in protecting the 

Lakes from aquatic invaders, although the effect of salinity exposure on organisms most likely to 

survive and establish in the Great Lakes was expected to be helpful, if not significant (Carlton 

1990; Locke et al. 1991, 1993). The scientific and regulatory communities soon realized that 

BWE requirements did not apply to vessels entering the Great Lakes with empty (no pumpable 

ballast water) ballast tanks, even though such tanks were known to contain small amounts of 

residual ballast water and sediment. Reeves (1997) revisited the problem of unpumpable ballast 

water raised by Locke et al. (1991, 1993), referring to vessels that reported “no ballast on board” 

as “NOBOBs”. He stated “The problem with the NOBOBs, representing between 75 and 95 

percent of the vessels entering the system, is that some 40 percent of those vessels engage in a 

cross-transfer of ballast inside the Great Lakes.....This is probably the most serious limitation of 

our current regulatory regime.” (Reeves 1997, p. 291-292)

Ricciardi (2006) calculated that from 1960 (the first year the Saint Lawrence Seaway operated 

over a full shipping season) through 2003 the rate of new ANS reported in the Great Lakes was 

1.8 per year, an average of one new species reported every 28 weeks (~7 months), whereas the 

rate from 1840 through 2003 was only 1.1 new species per year (an average of one reported 
8

every 11 months). This was similar to findings by Holeck et al. (2004) that the rate of new ANS

reported in the Great Lakes and attributed to ballast water more than doubled after the 

implementation of ballast water controls (comparing 1959 to 1988 vs. 1989 to 2000), although 

they acknowledged a number of potential drivers not related to ballast water that might explain 

the apparent increase. Ricciardi (2006) also noted that euryhaline benthic7 organisms that can 

produce a resting stage8 dominated the composition of new invaders reported since mandatory 

BWE was implemented in 1993. These species generally have broad salinity tolerance and thus 

may survive exposure to salinity over 30 ppt resulting from BWE. The presence of such species 

limits the protection BWE can provide for freshwater systems like the Great Lakes. These are 

also the organisms most likely to be carried in the residual ballast in NOBOB vessels. Since 

BWE requirements still did not apply to NOBOBs during the period covered by his study, 

Ricciardi (2006) mirrored the concerns expressed by Locke et al. (1991, 1993): “NOBOB ships 

may represent an active vector that plays a role in introducing benthic organisms, especially 

those with resting stages” and “the effectiveness of BWE (note added by author: referring to 

BWE regulations) has been undermined by the increasing proportion of inbound foreign vessels 

that are not subject to regulation” (Ricciardi 2006, p. 426). 

Johengen et al. (2005) investigated the characteristics and biology of residual water and 

sediments in NOBOB ballast tanks. They documented significant numbers of live taxa, as well 

as viable resting stages, including marine and some freshwater species not native to the Great 

Lakes. Numerous NOBOBs trading into the Lakes were found to repeatedly ballast in lowsalinity or fresh water ports in Northern Europe (as well as within the Great Lakes Basin), but 

were not subject to BWE regulations. As a result of this study, and early results from a followon study (Reid et al. 2007), in 2005 the U.S. Coast Guard established a best management 

practice policy stating that vessels operating outside the Great Lakes should conduct saltwater 

flushing of their empty tanks, as well as mandatory BWE on their ballasted tanks, before

entering the Saint Lawrence Seaway (USCG 2005). This was not proposed as a regulation 

because in the U.S. federal policy can be implemented relatively quickly while regulations can 

take years for approval. Canada quickly followed with regulations making ballast water 

management mandatory for all ships entering Canadian waters (Transport Canada 2006). 

Management options provided for in the regulations included mid-ocean BWE, retention of 

ballast water onboard, transfer of unexchanged ballast water to a reception facility, or use of an 

approved ballast water treatment technology. Ships bound for the Great Lakes and choosing to 

use BWE for ballast water management were also required to flush their empty (no pumpable 

water) tanks. In 2008 the Saint Lawrence Seaway Development Corporation harmonized its

regulations with the new Canadian regulations, requiring all ocean-going ships originating from 

beyond the EEZ to undertake saltwater flushing (SLSDC 2008). The U.S. EPA established a 

national regulation for saltwater flushing in their 2008 Vessel General Permit (USEPA 2008).

 

7 These are animals and plants that live on or in the bottom of an ocean or lake (benthic) and are tolerant of a wide 

range of salinity (euryhaline).

8 “Resting stage” or “diapausing stage” refers to a reproductive body (embryo, egg, spore, cyst) in a period of 

dormancy – a strategy to help the organism survive unfavorable or extreme environmental conditions (e.g., 

temperature extreme, desiccation, even digestion); resting stages can become active and produce juveniles of a 

species when conditions become more favorable.
9

A joint Ballast Water Working Group (BWWG) was created in 2006, consisting of 

representatives of the U.S. Coast Guard, Transport Canada-Marine Safety, Saint Lawrence 

Seaway Development Corporation (United States) and the St. Lawrence Seaway Management 

Corporation (Canada). The BWWG coordinates enforcement of ballast water management rules 

for the Great Lakes. It established a Joint Ballast Water Management Exam Program to conduct 

detailed inspections of foreign vessels entering the Great Lakes from outside the EEZ. 

Inspections included detailed review of ballast water reports, logs, records, and ballast water 

management plans. Inspectors interview crew to assess their understanding of the vessel’s 

Ballast Water Management Plan and operational procedures. Ballast tanks are sampled for 

salinity or the presence of mud that would suggest a satisfactory management practice was not 

employed. Initially, detailed inspections were focused on a vessel's first voyage into the Lakes, 

but in 2008 the policy expanded to examine 100% of ballast tanks on all vessels on all transits 

regardless of prior entries. Since 2007, 94-99% (Table 1; BWWG 2008, 2009, 2010, 2011, 

2012) of ballast tanks were found in compliance with salinity requirements. Ships with ballast 

water not in salinity compliance are issued a Letter of Retention which states that the water may 

not be discharged in the Great Lakes Seaway system. When the vessel departs the system, the 

Letter is rescinded after retention of the water is confirmed. Thus, starting in late 2006, the 

discharge of unmanaged low-salinity ballast water was essentially eliminated by strong 

regulations and strict enforcement. 

2.3 THE DISCOVERY RATE RECORD

Bailey et al. (2011) evaluated the annualized discovery rate of new ANS in the Great Lakes 

attributed to shipping (Figure 3, after Bailey et al. 2011 Fig. 4(b).) and found a general increase 

in the mid-1980s, but then a rapid decrease starting in the mid-to-late 1990s, more or less 

coincident with U.S. implementation of mandatory BWE. In addition, since 2006 there have

been no new ballast-associated ANS reported. The last time a gap of more than five years 

between new species discoveries occurred in the Great Lakes was from 1952-1958, just before 

the Seaway opened (GLANSIS 2012).

The observation that no new species have been reported since 2006 is coincident with two 

significant policy changes: 1) inclusion of NOBOBs and most coastal traders in mandatory 

ballast water exchange/flushing and 2) strict enforcement of the salinity standard, with 

Table 1: Results of Ballast Tank 

Enforcement Inspections by Agencies in the 

Great Lakes Seaway Ballast Water 

Working Group (BWWG 2008-2012).

Tanks in Compliance with 

Salinity Requirement

2007 96%

2008 99%

2009 98%

2010 94%

2011 97%
10

monitoring of all tanks for evidence of sediment build-up and to exclude discharge of lowsalinity ballast water. The latter ensured that organisms in ships bound for the Great Lakes will

be exposed to sudden high physiologic stress, whether they’re in pumpable or residual ballast 

water, or accumulated sediments. Exposure of freshwater ballast organisms to high salinity via 

BWE, and exposure of mid-ocean saltwater organisms to sudden low salinity when discharged 

into the freshwater of the Great Lakes induces “salinity” or “osmotic” shock, a potentially deadly 

form of physiologic stress. 

The observed discovery rate record may reflect the protective effects of both the original 

implementation of BWE in 1993 and the NOBOB policy implementations of 2006. 

Interpretation of Figure 3 must take into account that scientific interest in invasion ecology grew 

dramatically starting in the late 1980s (Ricciardi and MacIsaac 2008). Increased scientific 

interest led to increased awareness and more activities to look for aquatic invaders in the Great 

Lakes, and those efforts likely contributed to some of the increase in discovery rate seen between 

1984 and 1992. Also, Sturtevant et al. (2007) calculated that economic changes during the 1980s 

led to a decline in both the number of vessels carrying pumpable ballast water (ships could no

longer afford to carry partial or full loads of ballast water and little or no cargo) and in the annual 

number of foreign ships trading in the Great Lakes during the period 1994 to 2004 (compared to 

1978-1988). These changes translated into a potential reduction of up to 76% in average annual 

ballast water volume carried into the Great Lakes when comparing these same periods. A 

reduction in ballast water volume would produce a lower risk of new species introductions and 

Figure 3: Annualized number of ship-related ANS discovered in the 

Great Lakes from 1959 through 2010, inclusive. Dashed lines mark (1) 

voluntary BWE (1989, Canada), (2) mandatory BWE (1993, U.S.) and 

(3) mandatory BWE and flushing of all tanks (2006, Canada). (After 

Bailey et al. 2011, reprinted with permission. Copyright 2011, American 

Chemical Society).
11

thus could have been a contributing factor in the decline of ship-related ANS discoveries after 

1994 (Figure 3). 

Another complication with interpreting the discovery record of ANS is that once a species 

successfully invades an ecosystem, it may not immediately be discovered, and the length of time 

it takes to be discovered (lag time) can depend on how big it is, how rapidly its population 

increases, how it reproduces, what habitat it occupies, and how much effort is made to look for 

that type of organism. The length of time a newly discovered invader has actually been in an 

ecosystem is often not known with certainty. For example, three nonindigenous testate amoebae 

were found in Great Lakes beach sands in 2002 (Nicholls and MacIsaac 2004). Even though two 

of these species were found at multiple locations in several of the Great Lakes, they had not been 

previously reported anywhere in the Great Lakes basin. While it is highly probable that these 

species were introduced via ballast water, there is no way to determine when they were first 

established. Echinogammarus ischnus was first reported in the Laurentian Great Lakes from

samples collected in 1995 (Witt et al. 1997). Archived samples later revealed E. Ischnus was 

present in western Lake Erie in 1994, and based on size distribution of the 1994 population, it 

was likely present at least as early as 1993 (van Overdijk et al. 2003). Similarly, viral 

hemorrhagic septicemia (VHS) was first identified in Great Lakes fish collected from the Bay of 

Quinte, Lake Ontario, Canada in 2005 (USDA 2006), but subsequent examination of archived 

samples suggest it has been present since at least 1999 (GLANSIS 2012). Some benthic 

invertebrates, such as those noted by Ricciardi (2006) as dominating the composition of new 

Great Lakes invaders discovered since 1993, may be quite small, making immediate discovery 

difficult and resulting in potentially large lag times before they are discovered; some reproduce 

asexually, producing resting eggs that can sit dormant in sediment for decades (Hairston et al. 

1995) until appropriate environmental cues stimulate them to hatch. 

According to the discovery record, new shipping-related invasions dropped to zero immediately 

after implementation of the Canadian tank flushing regulations (Transport Canada 2006), yet it

took several years to achieve full enforcement of these regulations (Table 1). It is not likely that 

the 2006 regulations alone produced such an immediate result; it is more likely that the discovery 

rate reflects cumulative effects over time of the entire ballast water management framework and

the increasingly strict enforcement of regulations. The discovery rate record must be interpreted 

with caution. It may be entirely coincidental that in the Great Lakes, the discovery rate of new 

invaders dropped dramatically around the same time mandatory BWE was originally 

implemented and has been zero since tank flushing was also made mandatory. 

Bailey et al. (2011) assessed the efficacy of ballast water policies enacted for the Great Lakes 

and based on four criteria concluded that “risk of ship-mediated aquatic NIS introductions has 

been markedly reduced” (Bailey et al. 2011, p. 2559) since ballast management regulations and 

strict enforcement were implemented. It is clear that tank flushing closed a major loophole in the 

protection framework and has contributed to a reduction in ballast-associated risk. Even though 

the recent discovery rate has been zero, and all ballast water being discharged from foreign 

sources is saline, the future invasion risk is not zero. It will not be too surprising if new ballastimplicated invaders are discovered in the future.
12

3. THE BENEFITS AND LIMITATIONS OF OSMOTIC (SALINITY) SHOCK

In developing and evaluating the effectiveness of BWE for protecting the Great Lakes from 

species invasions, osmotic or physiologic stress (salinity shock) has been acknowledged as an 

important factor. For example, Carlton (1990) summarized fifteen alternative ballast water 

preventive options. In the case of freshwater ballast the option he outlined was to pump-in 

seawater, for which he stated “This treatment presumes that sufficient addition of saltwater to the 

freshwater would lead to the mortality of the freshwater organisms (via disruption of 

physiological, osmoregulatory processes).” (Carlton 1990, p. 7). Canada’s proposal (IMO 2010) 

stated that ballast water exchange is particularly effective in protecting brackish and freshwater 

habitats because it imposes an environmental salinity barrier: “Any fresh water or brackish 

coastal taxa entrained in ballast tanks that are not purged from tanks by ballast water 

exchange will be subjected to osmotic stress when euhaline ocean water is used to replace 

coastal ballast water.” (IMO 2010, p. 2). In this section concepts of osmoregulation and osmotic 

stress are outlined.

(Note: The following sections (3.1 and part of 3.2) are based on general information and 

discussions about physiology widely available in many references. For this document, four main 

references were used: Wilson 1979, Randall et al. 2001, Marshall and Grosell 2006, and 

Hickman et al. 2007, unless otherwise noted).

3.1 OSMOREGULATION 

French physiologist Clause Bernard (1813-1878) proposed that all living organisms exist in a 

“milieu intérieur” that ensures a stable internal environment for their metabolic functions and 

cellular infrastructure. In the early 20th century Harvard physiologist W.B. Cannon gave a name 

to the tendency of organisms to regulate their internal fluid composition and maintain this 

stability: homeostasis. 

Cells are the fundamental structural units of living organisms. All cells have a cell membrane

(aka plasma membrane), which defines the cell. Some cells also have a cell wall that surrounds 

the cell membrane. Cell walls are stronger than cell membranes and provide structural support to

protect the cell against excessive swelling when water diffuses into the cell. Cell walls are found 

in plants, bacteria, fungi, algae, and some single cell microorganisms. Both the cell membrane 

and cell wall help maintain homeostasis. Two types of processes challenge homeostasis: 1) 

metabolic processes within cells that involve materials essential for survival (e.g., oxygen, salts, 

water, nutrients) and which generate waste products that must be removed; and 2) external 

changes in the environment surrounding the organism. For aquatic organisms, both temperature 

and salinity of the surrounding environment are of critical importance and can pose great 

challenges to organism survival, as well as place limits to their geographic distribution. 

However, the following discussion is limited to the effects of salinity.

3.2 OSMOSIS, OSMOCONFORMERS AND OSMOREGULATORS

Water diffuses across a semipermeable membrane in the direction of lower water concentration 

by a process called osmosis. In aqueous salt solutions, the concentration of water decreases as 
13

the concentration of dissolved salt increases. Two solutions with different salinity will have 

different osmolality9

, and if separated by a semipermeable membrane, osmosis causes water to 

move from the lower salinity (greater concentration of water) to the higher salinity (lower 

concentration of water) solution. Cells are selectively permeable (semipermeable) to water, and 

are thus subject to osmosis, but limit or exclude passage of other molecules. Osmosis across a 

cell membrane and/or cell wall in a living organism is accompanied by a net gain or loss of water 

molecules inside the cell until flow of water molecules in both directions is in equilibrium. At 

osmotic equilibrium no further net gain or loss or water occurs. Essential ions, other compounds, 

and waste products can be transported across cell surfaces by various biochemical/physiological 

transport mechanisms that vary by species. 

The force that moves water in response to different solute concentrations across a semipermeable 

membrane is called osmotic pressure. It is approximately equivalent to the hydrostatic pressure 

needed to achieve osmotic equilibrium between two solutions of different osmolality. J.H. van’t 

Hoff, first Nobel laureate in Chemistry, described osmotic pressure in his 1901 Nobel lecture: 

“What is osmotic pressure? When a solution, e.g. of sugar in water, is separated from the pure 

solvent - in this case water - by a membrane which allows water but not sugar to pass through it, 

then water forces its way through the membrane into the solution. This process naturally results 

in greater pressure on that side of the membrane to which the water is penetrating, i.e. to the 

solution side. This pressure is osmotic pressure.” (van’t Hoff 1901, p. 5). Osmotic pressure is 

hydrostatic pressure linked to difference in the concentration of solutes between solutions 

separated by a semipermeable membrane, such as the inside and outside of a cell. It is solute 

driven. This can be contrasted with turgor pressure, which is pressure exerted on a cell wall

from inside by an excess of water within the cell. Turgor pressure is directly linked to water 

accumulation caused by osmosis. The term “turgor pressure” applies to plants, bacteria, and 

fungi cells and some protists because they have cell walls. Extreme internal osmotic pressure

and uncontrolled accumulation of water in a cell can result in swelling (excessive turgor) of the 

cell and rupture the cell wall and membrane, causing death. Alternately, uncontrolled diffusion

of water out of a cell by osmosis can lead to desiccation of the cell fluids, loss of turgor pressure,

and collapse of the cell membrane, also causing death. 

In aquatic ecosystems, when an organism is faced with changes in the salinity of its environment, 

it either avoids the change, it adapts physiologically, or it dies. Some species have little or no 

physiological ability to regulate their internal osmotic variables (water and solute concentration), 

so osmotic concentration in their body fluids surrounding and/or within their cells changes to 

conform to external salinity. These are referred to as “osmoconformers” and the internal osmotic 

concentration in their body fluids will be approximately equal to (isotonic with) that of the 

surrounding water. Marine animals may be osmoconformers, but not freshwater animals, 

because the molality of freshwater is much too low to sustain cellular processes. 

Osmoconformers have no defense against externally-driven significant osmotic changes. Some 

species are able to employ various mechanisms to regulate their body fluid at constant or nearly 

constant osmotic values different from the surrounding water. These species are called 

“osmoregulators” and the processes and mechanisms by which they do so are collectively called 

osmoregulation. Osmoregulation is a means for these organisms to maintain internal homeo-

 

9 The terms “osmolarity” and “osmolality” are used to describe the relative osmotic strength of a solution, which 

depends on the amount solute (e./g., salt) per liter or kg of solution.
14

stasis, which includes managing intake and output of water as well as maintaining an acceptable 

solute concentration. Some osmoregulators maintain their internal osmotic concentration above 

that of the surrounding water (hyperosmotic regulator) and some maintain it below the 

surrounding water (hypoosmotic regulator). For example, all freshwater animals are 

hyperosmotic regulators. Teleosts (bony, ray-finned fishes, which are the predominate fish in 

ocean and freshwater ecosystems) maintain the salt concentration of their body fluids at about 

1/3 that of seawater (Karnaky 1999), yet are found widely distributed from freshwater to 

saltwater. Freshwater teleosts are hyperosmotic regulators - they maintain the osmolality of their 

body fluids well above the osmotic concentration of freshwater. In these fish, osmosis produces 

a constant influx of water and diffusive loss of important ions, for which their osmoregulatory 

system compensates by eliminating excess water through generation of large volumes of very 

dilute urine and reabsorbing electrolytes from the surrounding water through the gills. In 

contrast, seawater teleosts are hypoosmotic regulators, since the osmotic concentration of their 

body fluid is lower than that in seawater. These fish suffer constant osmotic loss of water across 

their gills, for which they compensate by drinking large volumes of seawater and producing only 

small amounts of urine. Drinking seawater brings an excess of salt ions, which are eliminated 

through the gills and skin.

The detailed cellular-level physiological and biochemical mechanisms by which various types of 

aquatic organisms respond to and control osmotic challenges are quite varied and complex, and 

are the subject of a large body of research and literature well beyond the scope of this discussion. 

3.3 SALINITY TOLERANCE (PHYSIOLOGICAL STRESS, OSMOTIC SHOCK, SALINITY SHOCK)

All aquatic organisms have limitations in their ability to survive changes in salinity. The range 

of salinity over which a particular species is able to survive is its salinity tolerance, which is

closely related to its osmoregulatory capability. Organisms in water outside their salinity 

tolerance range can lose their ability to maintain homeostasis, resulting in physiological stress 

(“osmotic shock” or “salinity shock”), diminished or disrupted metabolic functions (for example, 

ionic regulation, respiration rate, nutrient intake, and oxygen requirements), and potential death.

The outcome of osmotic stress depends on the suddenness and severity of the change, the age or 

life stage, temperature, and possibly other abiotic factors. Aquatic organisms with narrow 

salinity tolerance are called stenohaline and have little ability to maintain homeostasis outside a 

narrow salinity range. Alternately, euryhaline organisms have developed osmoregulatory 

mechanisms that allow them to survive in areas with broad and sometimes rapidly changing 

salinity ranges, such as estuaries. Stenohaline and euryhaline organisms can be osmoconformers 

or osmoregulators and some appear to transition from one to the other depending on salinity. 

The European shore crab (Carcinus maenas) is an osmoregulator in the lower end of its salinity 

tolerance range, but as salinity increases above ~22 ppt the osmotic concentration of its body 

fluid increases and it appears to become an osmoconformer.

A large body of research exists about salinity tolerance, with a wealth of detailed field and 

laboratory results on salinity tolerances for many aquatic species. A survey of relevant literature 

reveals that salinity tolerance within different groups of aquatic organisms (e.g., marine, 

estuarine, and freshwater fishes, invertebrates, phytoplankton, and microorganisms) can be 

highly variable. 
15

Brand (1984) tested the salinity tolerance (based on reproductive/growth rate) of 46 species of 

coastal, estuarine, and oceanic phytoplankton. He found wide salinity tolerance ranges for many 

of the species, although the salinity tolerance of the estuarine and oceanic species was generally 

reflective of the salinity in their source habitats. Most oceanic species were unable to reproduce 

below 25 ppt, although a few could still reproduce at 15 ppt. All were killed by 5 ppt and 0 ppt 

treatments. Estuarine species were all able to reproduce in salinity as low as 5 ppt and as high as 

45ppt, and four were also able to reproduce in 0 ppt water. One of these species, Thalassiosira 

pseudonana, was described as “extremely euryhaline” and is often found in lakes and rivers. 

Coastal species exhibited reduced growth at 45 ppt and were capable of growth in salinity as low 

as 15 ppt; most, but not all, died in 5 ppt water and all were killed by 0 ppt exposure. 

Most stenohaline marine teleosts can live in full-strength seawater and also survive in salinities 

as low as ~12 ppt, which approximates the osmotic concentration of their body fluids. 

Stenohaline freshwater teleosts can survive in fresh- and brackish-water up to salinity ~12 ppt. 

Euryhaline marine and euryhaline freshwater teleosts have broad salinity tolerances and can 

survive the full range of aquatic environments from freshwater to seawater (and also hypersaline 

conditions), although the rate at which salinity changes can be important. Marshall et al. (1999) 

reported freshwater teleosts with euryhaline capability (e.g., tilapia) readily adapt to increasing 

salinity if the change is gradual and occurs over days. They also state “Unlike anadromous 

fishes that change salinities a few times during their life cycle (and generally do not survive

direct transfers from fresh water to full-strength sea water), estuarine-resident teleosts such as 

killifish must readily adapt to high and low salinity extremes and are capable of surviving direct 

transfer from fresh water to full-strength sea water.” (Marshall et al. 1999, p. 1536). Some 

euryhaline organisms are adapted to the low end of the salinity range (low-brackish to 

freshwater), while some are adapted to the upper end (high brackish to full-strength seawater), 

and some are widely euryhaline and can adapt across the full salinity spectrum typical of most 

Figure 4: Salinity tolerance of different populations of 

the European rock shrimp (Palaemon elegans) from 

various locations in the Mediterranean and Black Seas 

(Barnes 1989; reproduced by permission of The Royal 

Society of Edinburgh and R.S.K. Barnes).
16

aquatic ecosystems. BWE, whether empty-refill or flow-through exposes fresh or low-salinity 

species in ballast tanks to a rapid increase in salinity to almost full-strength seawater, which 

should produce the most stress and result in greatest mortality to stenohaline and low-euryhaline 

species. 

Populations of the same species in geographically separated aquatic environments and different 

species of the same genus can exhibit widely different salinity tolerances (Figure 4, Table 2). 

However, there is general confirmation in the literature that salinity tolerance of many aquatic 

organisms reflects to a great degree the salinity range of their existing habitat. 

It would be useful if freshwater, brackish-water and marine ecosystems had clearly defined and 

non-overlapping salinity boundaries to which aquatic organisms are restricted, but salinity in 

natural waters is a continuum from ~0 (e.g., Great Lakes, Lake Baikal) to >100 ppt (e.g., Great 

Salt Lake, Dead Sea). Williams (1980) defined freshwater as <3 ppt dissolved solids, but noted 

this was an arbitrary selection. More germane to consideration of salinity tolerance is the 

classification (i.e., fresh, brackish, marine) of ecosystems based on biology. 

Remane (1934, as discussed in Barnes 1989, see also Locke et al. 1991) compiled data from the 

North Sea and Baltic Sea on relative species diversity of freshwater, brackish water, and marine 

species vs. salinity over the range 0-35 ppt (Figure 5). His analysis showed freshwater species 

not present above 15 ppt, relatively few marine and brackish water species at salinities less than 

5 ppt, and a minimum in species abundances occurs between ~5 and ~8 ppt. Khlebovich (1968) 

examined chemical data from estuaries and suggested that sharp changes occur in the ionic 

composition associated with seawater in the 5-8 ppt range as seawater is diluted with river water. 

He concluded that this zone marks a physico-chemical boundary between freshwater and marine 

fauna. Deaton and Greenberg (1986) reexamined Khlebovich’s data. They found changes in 

ionic ratios are much larger below 2 ppt and disputed the concept that 5-8 ppt is a defining 

boundary between marine and freshwater fauna. They suggested that the species minimum is 

due to few animals having the physiological mechanisms to survive in the highly variable 

estuarine habitat, and those that can have low rates of speciation. Bulger et al. (1993) also 

disputed the validity of Remane’s analysis that salinity is the defining variable based on 

information that Remane’s analysis did not actually include any freshwater forms, and the 

Table 2: Salinity tolerance of five Gammerid 

species. Only G. duebeni is capable of living 

across the full range of salinity from 

freshwater to seawater (data from Morrissey 

and Sumich 2012, Figure 8.8).

Gammarid 

Species

Approximate 

Salinity 

Range

G. pulex <1 - 6

G. zaddachi 4 - 17

G. salinas 8 - 24

G. locusta 22 - 35

G. duebeni <1- ~35
17

salinity gradient across the Baltic Sea parallels a significant latitudinal temperature gradient 

which was not accounted for. They used multivariate analysis of biological data to identify key 

estuarine salinity zones (Table 3) and identified the lowest significant salinity zone as 0-4 ppt 

based primarily on stenohaline freshwater fish. This zone also overlaps with the lower end of an 

“Inner Estuarine Zone” ranging from 2-14 ppt.

Based on Table 3 one might conclude that freshwater could be defined as water with salinity <2 

ppt, but it would be an arbitrary endpoint and ignores that the 0-4 ppt zone was based on 

biological data, not just an arbitrarily selection (as per Williams 1980). Further, the significance 

of this zone would be overestimated, since Bulger et al. (1993) noted in their discussion that 

Table 3: Salinity zones identified through 

Principal Component Analysis of field 

data primarily from Chesapeake and 

Delaware Bays (data from Bulger et al. 

1993).

Salinity 

Zone Name

Salinity 

Range

1: Freshwater to 4 ppt 0-4

2: Inner Estuarine 2-14

3: Mid-Estuarine 11-18

4: Outer Estuarine 16-27

5: 24 ppt to Marine ≥24

Figure 5: Relative species diversity of freshwater, 

brackish and marine species found at various 

salinities from freshwater to saltwater according to 

Remane (1934) as cited by Barnes (1989; 

reproduced by permission of The Royal Society of

Edinburgh and R.S.K. Barnes).
18

these zones are not exclusive, that is, organisms typically associated with, for example, Zone 2, 

are not necessarily excluded from Zone 1. 

Ultimately, the response of aquatic organisms to a sudden change in salinity depends on many 

factors: the salinity range they generally experience in their present habitat, the rate of salinity 

increase or decrease during their exposure to change, the end point (how high or low the 

endpoint is, compared to their normal habitat range), the amount of time they’re exposed, 

temperature, life stage, and their osmoregulatory capability. For examples, bivalves exposed to 

salinity outside their normal range can close their shells and lower their metabolic rate for short 

periods of time, effectively isolating themselves from a source of osmotic stress. Freshwater 

stenohaline organisms subjected to sudden salinity increase, such as produced by mid-ocean 

ballast water exchange or tank flushing, experience significant osmotic stress and likely high 

mortality. Estuarine organisms may be better equipped to survive BWE because of 

osmoregulatory abilities, but most estuarine organisms are stenohaline (Morrissey and Sumich 

2012) and not likely to survive across the full range of salinity. Stenohaline estuarine organisms 

adapted for the lower salinity range would suffer osmotic shock after BWE, whereas estuarine 

organisms adapted for the upper salinity range would suffer osmotic shock after discharge into 

the Great Lakes. Estuarine organisms that are strongly euryhaline, such as Thalassiosira 

pseudonana (Brand 1984) and Eurytemora affinis (USGS 2012) will continue to pose a risk to 

freshwater systems like the Great Lakes even with BWE. 

Salinity shock will not be a 100% reliable barrier to all freshwater-tolerant invaders that could 

threaten the Great Lakes. Locke et al. (1991) reported live freshwater-tolerant species in three 

ships’ ballast tanks, even though salinity was >30 ppt due to BWE. Previous ports for these 

vessels were Ghent, Rotterdam, and Philadelphia. All three ports are located on estuaries with 

tidal salinity zones, although Philadelphia is far enough into the Delaware River estuary that 

salinity is usually less than 4 ppt (Smullen et al. 1984). The live species found in these tanks is 

illustrative of the limitations of salinity shock as a ballast-mediated barrier to ANS, especially for 

euryhaline estuarine species. 

4. RECENT FIELD AND LABORATORY STUDIES OF SALINITY SHOCK 

ASSOCIATED WITH BALLAST WATER EXCHANGE FOR THE GREAT LAKES

There have been many studies of osmotic regulation and salinity shock for various aquatic

organisms, but most have been general physiological studies not linked to BWE. Over the past 

decade some studies have directly examined salinity tolerance relevant to BWE and the Great 

Lakes. These studies fall into two categories: effect of BWE on diapausing invertebrate eggs, 

and salinity tolerance experiments on aquatic invertebrates that mimicked either flow-through 

BWE or empty-refill BWE. In addition, several studies have examined the effect of NaCl (brine) 

on aquatic organisms in ballast tanks, but these are only indirectly relevant to BWE as brine is 

not the same as seawater and it is not utilized as part of BWE.

4.1 EFFECT OF BWE ON DIAPAUSING EGGS (GREAT LAKES)

Bailey et al. (2004) extracted diapausing eggs of three invertebrate species native to the Great 

Lakes from ballast tank sediments in transoceanic ships entering the Great Lakes between 2000 

and 2002 and tested them for hatching under ballast exchange conditions. The eggs were placed 
19

in freshwater and then exposed to stepwise salinity increments simulating partial-to-complete 

BWE. The proportion of eggs successfully hatched declined for all three species as salinity 

increased, and none hatched at full salinity (32 ppt). Some eggs from two of the three species 

hatched when reintroduced to 0 ppt water after 10 days of exposure to 32 ppt salinity, mimicking 

a sequence of BWE followed by discharge into the Great Lakes. They also observed partial, but 

incomplete embryo development in eggs exposed to 8 ppt, suggesting that low-brackish salinity 

may allow hatching cues to initiate embryo development in some species, but is too high for the 

embryos to survive. While BWE significantly reduced successful hatching of test animals, it did 

not provide complete protection against hatching after removal of eggs from the saline 

conditions, as would happen with discharge into the Great Lakes after BWE. 

Gray et al. (2005) conducted similar experiments on diapausing eggs collected from both natural 

freshwater habitats and ballast tank sediments. They compared only two treatments: 0 and 32 

ppt, followed by reintroduction to 0 ppt after 10 days to determine if exposure to salinity as 

experienced during BWE affected the viability (ability to hatch after exposure) and/or taxonomic 

richness after hatching. They did not find a significant reduction in viability (total hatched) or 

species richness after exposure to salt water. 

Bailey et al. (2006) revisited the issue of diapausing eggs, varying the temperature (10o

, 20o, 

30oC) during salinity treatments (0, 8, 35 ppt). They observed some inhibition of hatching during 

salinity exposure at 8 and 32 ppt, but also significant initiation of hatching once the eggs were 

reintroduced to freshwater. In general efficacy of salinity in reducing viability was related to 

temperature, but not in a consistent manner.

Gray et al. (2007) conducted controlled BWE exchange experiments in ballast tanks filled with 

Great Lakes water and then subjected to BWE in the North Atlantic; they also placed in situ

incubation chambers to evaluate 1) BWE effects on two sentinel freshwater benthic invertebrates 

isolated in the chambers, which also contained a deep layer of sediment and 2) BWE effects on 

animals hatched from diapausing eggs during transit. They found BWE to be >99% effective at

eliminating freshwater pelagic organisms in the ballast tanks. Sentinel organism mortality was 

almost 100%, with indications that saltwater does penetrate into the pore spaces of sediment, 

thus exposing buried organisms. They also found BWE significantly reduced survival of 

organisms that hatched during transit. They concluded that BWE is effective at reducing 

discharge of live freshwater species, and is particularly effective against freshwater organisms 

(invertebrates). 

Gray and MacIsaac (2010) conducted in situ experiments in operational ballast tanks to 

determine effects of BWE on diapausing egg viability using incubation chambers mounted on 

the floor of the ballast tanks. They ran parallel laboratory tests for comparison. Their 

conclusions supported previous lab-based findings that saltwater exposure does not significantly 

reduce viability of diapausing eggs in ballast tanks.

Briski et al. (2010) evaluated ballast tank sediment accumulation and conducted hatching 

experiments on ballast sediments from both transoceanic and coastal-trade ships entering the 

Great Lakes between 2007 and 2008, the first two years after implementation of strict BWE and 

flushing regulations by Canada (Transport Canada 2006). Comparison of these post-regulation 

results with similar pre-regulation data (Bailey et al. 2005) revealed post-regulation total 

sediment accumulation was ~2/3 less and mean density and abundance of diapausing eggs per 

ship was 80-90% less. They conducted hatching experiments on diapausing eggs separated from 
20

sediments and also on whole sediment samples. Total abundance of eggs of high risk species 

(defined as nonindigenous species that hatched in freshwater treatments) per ship was 

significantly lower in post-regulation samples. The potential contribution of viable individuals 

to exchanged ballast water by in situ hatching was estimated to be insignificant compared to the 

typical concentration of live organisms already in exchanged ballast water. Ballast management 

regulations enacted in 2006, appear to have markedly reduced the probability of introduction of 

NIS via dormant eggs carried in ballast sediments by both reducing sediment accumulation and 

preventing hatching of diapausing eggs of freshwater invertebrates in ballast tanks.

Briski et al. (2011) evaluated the effect of mid-ocean exchange on invertebrate dormant egg 

density and viability in ballast tanks. They compared data from ships that performed BWE with 

those exempt from BWE and found no significant difference in taxonomic composition or 

abundance of invertebrates or their dormant eggs and also, BWE had no discernible effect on 

dormant egg viability in ballast sediments. Reducing the amount of sediment in ballast tanks

was determined to be a critical factor for reducing abundance and species richness of both 

invertebrates and their dormant eggs in ballast sediment.

In summary, all recent experiments to assess the effectiveness of BWE (and saltwater flushing of 

NOBOB tanks) on diapausing eggs suggest that exposure to seawater does not kill diapausing 

eggs, even those of freshwater organisms, but it does reduce the likelihood of in-tank hatching 

and/or causes mortality of freshwater species hatched in ballast tanks during transit. Further, 

saltwater flushing reduces invasion risk by reducing accumulated sediment and thus potential 

propagule pressure from diapausing eggs. While exposure to saltwater severely reduces or 

eliminates the in situ hatching success of diapausing eggs, it is not clear what the level of 

invasion risk is to an ecosystem receiving repeated small inoculations of a few viable eggs of 

asexually-reproducing invertebrates. This is likely to occur, at least intermittently, because small 

amounts of ballast sediment remain in tanks even after mid-ocean flushing. Such residual 

sediments and any eggs they contain can be resuspended and discharged during deballasting, 

resulting in a potentially small but repetitive inoculation of the recipient ecosystem with viable 

eggs of nonindigenous species.

4.2 SALINITY TOLERANCE EXPERIMENTS – TESTING EFFECTS OF BWE ON FRESHWATER 

AND ESTUARINE ORGANISMS.

Santagata et al. (2008) simulated both empty-refill (E-R) and flow-through (F-T) BWE to test 

the salinity tolerance/response of 54 freshwater and estuarine larval and adult crustaceans. Test 

organisms were collected from freshwater and mesohaline habitats adjacent to ports of the Baltic 

Sea, North Sea, Great Lakes, Chesapeake Bay, and San Francisco Bay. In the E-R experiments 

animals experienced instantaneous salinity increase from their ambient salinity to 34 psu (~34 

ppt), while in F-T tests the organisms were subjected to stepwise increases of salinity (0, 14, 24, 

34 psu) every hour until 34 psu was reached. The effectiveness of both treatment types

decreased as the ambient salinity of the collection sites increased (Figure 6). Organisms 

collected from freshwater habitats (0-2 psu) experienced the most mortality: all individuals in 

82% of the F-T treatments and 88% of the E-R treatments were dead after 48 hours. They found 

both types of BWE were about equally effective at the end of 48 hours, but E-R treatments, in 

which test animals experienced sudden and extreme salinity shock, required less exposure time

in 43% of all cases. Santagata et al. (2008) concluded that salinity shock does not completely 
21

prevent the transfer of all low-salinity biota, but BWE is still a useful management tool to reduce 

species transfers, especially considering the combined effects of removal and mortality. Current 

management practices of BWE and saltwater flushing serve to reduce ship-mediated transfer and 

subsequent risk of introduction of non-indigenous species to the Great Lakes and other lowsalinity recipient systems.

Ellis and MacIsaac (2009) conducted short-term exposures of nine organisms (8 adults, 1 larva)

that had already invaded the Great Lakes; five discovered prior to mandatory BWE regulation 

and three after. As with Santagata et al. (2008), experimental protocols mimicked both F-T 

(stepwise increases: 0, 4, 8, 14, 24, 30 ppt) and E-R (instantaneous increase: 0 to 30 ppt) BWE. 

Salinity exposure by either BWE method reduced survival of all species, and, comparable to 

Santagata et al. (2008), E-R produced mortality more rapidly. Five test organisms exhibited 

100% mortality within 5 hours of initial exposure, regardless of BWE treatment, while adult 

dreissenid mussels (quagga: Dreissena rostriformis bugensis; zebra: Dreissena polymorpha), the 

round goby (Neogobius melanostomus), and the amphipod Echinogammarus ischnus all took 

much longer, although 80-90% mortality was observed after 48 hours. Ellis and MacIsaac 

(2009) concluded that BWE is an effective management tool for preventing the introduction of 

freshwater species into the Great Lakes.

Karsiotis et al. (2012) conducted short- and long-term salinity tolerance experiments on round 

gobies (Neogobius melanostomus) from Lake Erie. Their short-term experiments immersed 

gobies directly from freshwater into water with salinities ranging in 5 ppt steps from 0 to 40 ppt. 

Figure 6: Effectiveness of flow-through (F-T) and emptyrefill (E-R) salinity treatments grouped by the habitat 

salinity observed at the time of species collection. The 

number of experiments (n) is listed above each salinity 

range. Bars represent the proportion of experiments within 

a given salinity category that yielded 100% mortality by 

treatment. (Figure reproduced from Santagata et al. 2008).
22

This simulated E-R ballast water exchange (immediate immersion in water ≥30 ppt salinity) and 

also release into ports having a range of salinities (immediate immersion into water with 5, 10, 

15, 20, or 25 ppt salinity). Test animals exhibited significant mortality at salinities ≥25 ppt and 

none survived in salinities ≥30 ppt. They concluded that this species cannot withstand fully 

completed BWE (i.e., final salinity >30 ppt).

Matheson et al. (2007) examined the efficacy of NaCl brines of various concentrations up to ~70 

ppt to kill a range of freshwater test fish, snails, and macrophytes. Santagata et al. (2009), 

Bradie et al. (2010), and Wang et al. (2012) all tested NaCl brines up to 115 ppt as a potential 

ballast water/tank treatment. NaCl in solution is known to be more toxic to freshwater aquatic 

organisms than seawater at equivalent concentrations (Santagata et al. 2009). Since the present 

paper is concerned with the ability of ocean-water to induce osmotic shock and death in ballast 

tank organisms, the use of NaCl brine is noted but will not be discussed. 

As part of a program to evaluate the effectiveness of the 2006 expanded ballast management 

regulations for the Great Lakes, Bailey et al. (2011) sampled NOBOB residuals and water from 

ballasted tanks coming into the Great Lakes between 2005 and 2008. They compared these

results (“post-regulation”) with data obtained “pre-regulation” and found abundances of all 

invertebrates in NOBOB residuals was significantly lower, and mean and maximum density of 

Figure 7: Mean (+/-S.E.) abundance of invertebrates 

in NOBOB residual ballast (upper panels) and water 

from ballasted (lower panels) ships, before (black 

bars) and after (white bars) the introduction of 

saltwater flushing and ballast water exchange, 

respectively. Thin horizontal lines on bars are median 

values. (Bailey et al. 2011, reprinted with permission. 

Copyright 2011, American Chemical Society).
23

invertebrates in ballasted tanks were also lower (Figure 7). When only high-risk taxa (tolerant of 

salinity up to 18 ppt) were considered, NOBOB residual ballast water had a median density of 0 

individuals/m3 (note: this is not the same as 0 individuals in every sample) and the median 

density in ballast water was 1 individual/m3. They concluded that strict enforcement of BWE 

and flushing has significantly reduced the probability for rare, high propagule ballast water 

discharges and nearly eliminated high-risk taxa from incoming ballast. Although BWE and tank 

flushing will not provide complete protection against all aquatic invasions, they suggest that the 

Great Lakes ballast management regime may frequently reduce the effective invasion risk for 

freshwater ecosystems receiving ballast tank discharges to approximately the same level 

expected under the IMO D-2 standard. 

Strictly speaking, data to support Bailey et al.’s supposition are only available for zooplankton in 

the >50μ size category. Exchanged ballast water typically carries thousands to millions of other 

viable organisms (microbes, viruses, protists, small zooplankton, phytoplankton, perhaps the 

occasional fish, etc) per m3. Even so, the general concept they highlighted is that freshwater 

tolerant species, regardless of size range, pose the greatest risk to the Great Lakes and salinity 

shock brought on by BWE could reduce the abundance of such organisms to at or below the 

IMO D-2 target level, depending on the salinity tolerance of the particular organisms in a ballast 

tank.

5. CONCLUSIONS

Salinity shock is caused by osmotic stress, a well documented physiologic response in organisms 

unable to adequately control their internal osmotic condition in the presence of external changes 

in salinity. Osmotic stress affects the ability of organisms to carry out required metabolic 

functions, often leading to death, although some organisms can adapt if the change in salinity is 

gradual and exposure is short-lived. Few larval and adult freshwater (and low-salinity brackish 

water) organisms can survive a sudden large change in salinity, such as happens during BWE in 

ballast tanks containing low-salinity water, or when saltwater organisms are discharged into 

freshwater. 

Flushing out coastal water and organisms is the primary mechanism by which BWE reduces 

invasion risk when the original ballast is saltwater or brackish water in the upper-salinity range. 

However, it is not a perfect filter for removing potentially invasive species from ballast water 

discharges because there is little if any change in salinity. 

For protection of fresh- and lower-salinity brackish-water ecosystems, BWE includes the added 

advantage of salinity shock for enhanced risk reduction. The combination of physical flushing of 

ballast tanks (water and sediment) and salinity shock can be very effective for reducing risk from 

organisms adapted to fresh- and lower-salinity brackish habitats.

A deficiency in our understanding of the efficacy of BWE and tank flushing is lack of sufficient 

data related to organisms other than large (>50μ) invertebrates, especially small invertebrates, 

phytoplankton, and microorganisms. So while BWE and tank flushing are highly protective of 

the Great Lakes ecosystem, vital information about the introduction rates and effects of salinity 

on other freshwater-tolerant organisms is lacking. 
24

Some organisms can reproduce asexually and produce resting eggs (e.g., benthic invertebrates) 

or cysts (e.g., dinoflagellates, bacteria, protists) which are able to survive harsh conditions until 

appropriate environmental cues initiate continuation of development. Studies have shown that 

BWE and tank flushing can reduce the invasion risk associated with resting eggs and cysts by 

resuspending and flushing out ballast tank sediment where they accumulate, but salinity has little 

or no effect on long-term survival of these dormant stages.

In principle, the use of BWE in combination with on-board treatment (BWE+Treatment) could 

enhance reduction of the risk associated with ballast water discharges to fresh- and brackishwater ecosystems. BWE would potentially eliminate freshwater-tolerant (i.e., high-risk) species 

from the ballast water prior to treatment, leaving less risky marine and high-brackish species in 

the discharge. The efficacy against high-risk species will be dependent on the salinity tolerance 

of the organisms. There are some, mainly estuarine, organisms that have tolerance over the full 

range of salinity, although their ability to survive a rapid change from low salinity to almost fullstrength ocean water is another factor. Using BWE as a precursor to on-board treatment would 

also provide a form of insurance in case of undetected or unexpected treatment system failure, 

especially since these systems are very new and at present we don’t have much actual 

operational experience with them to accurately gauge their reliability and performance. 

There is ample scientific evidence to support the conceptual principles presented in the Canadian 

BWE+Treatment proposal. However, the practicability of the proposed procedure under 

shipboard operating conditions has not yet been evaluated. Also, the effectiveness of 

BWE+Treatment compared to treatment alone needs to be verified by direct experiments. At 

least one series of experiments using simulated exchange+treatment vs. treatment alone has been 

completed at a land-based test facility, but the results have not yet been published, although they 

are expected soon. In addition, a shipboard test program is also underway by Canadian 

scientists.

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7. ACKNOWLEDGEMENTS

Preparation of this Report was supported by the Saint Lawrence Seaway Development 

Corporation. The author extends his appreciation and sincere thanks to the following individuals 

for taking the time to provide comments and suggestions on an early draft: Ryan Albert, Sarah 

Bailey, Marvourneen Dolor, Richard Everett, Anthony Ricciardi, and Chris Wiley

Any errors or misinterpretations are the author’s alone.