## 1 Introduction  
Ships' ballast tanks may hold in excess of 100,000 tons of ballast water, transporting as many as 7000 species every day [1, 2]. Invasive aquatic species can have damaging effects on fisheries and aquaculture through significant and irreversible effects on biodiversity. Human health may also be affected through the introduction of bacterial pathogens. Invasions have also resulted in huge economic losses and costs. These range from medical costs to adverse effects on fisheries and recreation. They may also be responsible for fouling hard surfaces including industrial structures and water inlet pipes. Harmful algal blooms may result from the release of nonindigenous species into coastal waters, and releases of bacterial invasive species have been responsible for outbreaks of diseases such as cholera. In the U.S. the annual costs of bioinvasions has been estimated at over $100 billion [3] although perhaps one tenth of this figure relates to aquatic organisms. In the U.K. the annual cost of invasive species to the national exchequer has been estimated to be \(\varepsilon\)1.7 billion [4; 5]. To combat this threat the International Maritime Organization adopted the Convention for Ballast Water and Sediments (BWMC) in February 2004 [6]. This legislation sought to mitigate the risk of new invasions of non-native species through the implementation of standards governing the release of potentially harmful organisms during ballast water discharge. The interim D1 standard applied to mandatory ballast water exchange in ocean water, but following full ratification of the BWMC in 2017, this has been superseded by the D2 standard which stipulates post-treatment discharge limits for different classes of organisms, as follows [6]:  
* Size class greater than 50 \(\upmu\)M in minimum dimension (normally assumed to be zooplankton) Less than 10 viable organisms per cubic metre. * Size class between 10 and 50 \(\upmu\)M in minimum dimension. Less than 10 viable organisms per mL. * Indicator bacteria: \[ \begin{array}{ll}\odot&\textit{Vibrio cholera};\text{Less than 1 colony forming unit per 100 mL}.\\ \odot&\textit{Escherichia coli};\text{Less than 250 colony forming units per 100 mL}.\\ \odot&\text{Intestinal Enterococci};\text{Less than 100 colony forming unit per 100 mL}.\end{array} \]  
In addition to discharge densities for different classes of organism, shipboard certification tests also stipulate requirements for acceptable uptake (challenge) water densities, which are not always met. In a recent series of test of ballast water treatment aboard Great Lakes freighters [7] phytoplankton numbers in challenge water did not always reach the U.S. Federal (ETV 46, CFR 162.060-28) shipboard testing requirements for uptake densities. Levels of both indicator bacteria (_E. coli_ and Enterococci) were also much lower than the prescribed uptake water levels for land-based testing. Additionally, the great majority of the phytoplankton identified in these trials appeared in the \(<\)10 \(\upmu\)m size class, and as such were not subject to U.S. Federal or IMO ballast water regulation as standards are currently written [6]. A review of several other studies shows similar results. For example, [PERSON] and [PERSON] [8] point out that many phytoplankton, including those forming harmful algal blooms such as _Phaeocystis_ and _Pfeisteria_ are smaller than 10 \(\upmu\)M. Using size classification derived from flow cytometry, analysis of samples taken from the Wadden Sea showed that cells in the \(>\)10 \(\upmu\)M size category represented only 3.6% of the total cell number and contributed only 28.7% of the total chlorophyll-a concentration. Other important dinoflagellates fall into the \(>\)50 \(\upmu\)M, size class, normally associated with zooplankton, which are subject to a discharge standard 10\({}^{6}\) times lower than the 10-50 \(\upmu\)M size class. In waters around the British Isles [PERSON] et al.