Ballast water samples which have been filtered through a 50µm nylon mesh usually contain phytoplankton as well as zooplankton organisms. For the detection of viable organisms from both of these two physiological groups different analytical methods may be applied.
To distinguish viable zooplankton organisms from dead zooplankton organisms it is indispensable to clearly define the term “viable”.
A zooplankton organism has to be recorded as viable when the organism is able to perform the species specific movements across the visual field of the microscope for more than 10 seconds.
Although many microalgae are equipped with flagella and are thus able to perform considerably quick and arduous movements the above definition cannot be applied for the analysis of phytoplankton.
Optical counts of the viable zooplankton organisms of this size class are easy to perform and ideally plankton chambers are used to simplify the counting procedure. Optical counts of the phytoplankton organisms is easy to perform, too, due to the size of the phytoplankton organisms. However, the differentiation between live and viable can only be executed in combination to the PAMF values of the samples and is of estimative character. Another aspect of the optical count method is crucial. A considerable number of micro-algae species are able to form cell aggregates, in which the single individuals are glued to each other by mucus. Allthough the single individual of the species is smaller than 50µm these aggregates are retained in the 50µm filtration step of a ballast water sample and therefore have to be recorded. The same applies for bilateral-flat shaped plankton species. These individuals are longer than 50µm but their diameter is less than 50µm. Thus they may pass the filter mesh of 50µm only in the longitudinal direction. Species with this habit, too, have to be recorded.
Selective (live-dead) staining methods may be applied to distinguish live cells from dead cells.
The Pulse Amplitude Modulation Fluorometry (PAMF) would describe the physiological status of the phytoplankton organisms and thus indicates, if the phytoplankton organisms in the sample are dead, will probably recover or are viable. However, PAMF does not generate any numerical data.
Flow cytometric analysis will generate numerical data solely for the phytoplankton organisms.
The further development of the Fluorescein-diacetate fluorometry and the Adenosin-triphosphate fluorometry may allow for the transfer of concentration data into numerical data for all viable plankton organisms in the sample.