The measurement of bacterial chlorophyll and algal chlorophyll a in natural waters

Several groups of chlorophyll‐bearing photosynthetic organisms often coexist in anoxic waters. Because of the close similarity between the absorbance spectra of chlorophyll a (in cyanobacteria and algae) and the chlorophylls of green sulfur bacteria (Chl‐GSB) it is difficult to distinguish these two groups of organisms where they co‐occur. We have developed a simple method to measure both chlorophyll a and Chl‐GSB and used it in a meromictic coastal pond. In anoxic bottom waters Chl‐GSB was the major pigment present (averaging 20 mg m −3 ), but chlorophyll a accounted for from 5 to > 50% of the total pigment present throughout the year. Without our method large errors in calculated Chl‐GSB and chlorophyll a concentrations would have resulted.