JAMP Eutrophication Monitoring Guidelines: Chlorophyll a in Water. OSPAR Agreement 2012-11

Nutrient enrichment of transitional, costal and marine waters can give rise to increased primary production and phytoplankton biomass which is part of eutrophication process. The human driven eutrophication process can cause undesirable disturbance of ecosystems and therefore an integrated monitoring programme to characterise the status and changes in aquatic ecosystems with respect to eutrophication is required.

Chlorophyll a is the most used operational indicator for phytoplankton biomass. However, the chlorophyll a content in relation to organic carbon is different for different taxa and can vary, due to the physiological status, as does the cellular carbon compounds. Chlorophyll includes a variety of different pigments like chlorophyll b, c1, c2, divinyl chlorophyll a and divinyl chlorophyll b.

Extracts of algae pigments contain also variable amounts of the chlorophyll precursor pigments and breakdown products like chlorophyllide a, phaeophorbide a and phaeophytin a. Usually, the phaeopigments have been discriminated from chlorophyll a and chlorophyllide a by measuring the pigment extract before and after the addition of acid: the so called ‘acidification method’. The acidification causes a conversion of chlorophyll to phaeophytin and the chlorophyll a content can be calculated from the difference of both measurements and should be reported as “active chlorophyll a” and “phaeopigments”. But interlaboratory comparisons have shown that the calculated values for phaeopigments are less reliable than the values for chlorophyll a. Therefore the acidification is no longer recommended because it is time consuming and the results are questionable.

The non-acidified methods give an estimate of pigment concentration which is a combination of chlorophyll a, chlorophyllide a, phaeophorbide a and phaeophytin a. Because the standard photometric and fluorometric methods for determining chlorophyll a do not completely separate the different chlorophylls or distinguish between chlorophyll a and chlorophyllide a the term “total chlorophyll a” should therefore be used when reporting results from these methods. For chlorophyll data, analysed by HPLC, which considered the other chlorophyll derivates as well the term “chlorophyll a” should be used.