Determining algal assemblages in oligotrophic lakes and streams: comparing information from newly developed pigment/chlorophyll a ratios with direct microscopy

1. Pigment analyses by high performance liquid chromatography (HPLC) are commonly used for determining algal groups in marine and estuarine areas but are underdeveloped in freshwaters. In this study, 15 characteristic pelagic algal species (representing five algal groups) of oligo-/mesotrophic lakes were cultured and pigment/Chl a ratios determined at three light intensities. 2. With the exception of cyanophytes, light treatment had little effect on pigment/Chl a ratios. This justifies the use of the same pigment/Chl a ratios during seasonal studies where light conditions may change. 3. The determined pigment/Chl a ratios were tested on seasonal samples from five oligo-/mesotrophic lakes and three streams using CHEMTAX software. Pigment ratios of both pelagic and benthic algal communities from the lakes and streams were analysed to determine whether the pelagic algae-based ratios can be used for benthic algal communities. 4. HPLC combined with CHEMTAX was useful for identifying freshwater phytoplankton classes and for quantifying the abundance of phytoplankton groups. However, although correlations were significant for six of seven phytoplankton classes studied, they were weak and varied with season. 5. HPLC was valid for quantifying benthic diatom groups in stream samples, whereas for lakes more benthic algal groups were recorded with HPLC than with microscopy and correlations between the two methods were not significant. 6. The use of both HPLC and microscopy is recommended as a cost-efficient method for analysing many samples. It is crucial, however, that the CHEMTAX software is calibrated with the correct information, and the user is aware of the limitations.