Effects of water color on underwater irradiance and consequences for photosynthetic pigments of primary producers in boreal lakes

Photosynthetic pigment contents of phytoplankton and macrophytes ( Nuphar lutea) were studied along a water color gradient of lakes. Chlorophyll a concentration in the water was affected by species composition of phytoplankton and the highest values were observed at intermediate color. Chlorophyll b concentration and chlorophyll b:biomass ratio of phytoplankton increased together with water color. The highest chlorophyll a: b ratios occurred at intermediate water color, because phytoplankton species with chlorophyll b were scarce. In N. lutea, chlorophyll a and b in the petioles were not affected by water color or light attenuation. For chlorophyll a: b of N. lutea the effect of water color was stronger than the effect of any light wavelength band. This was probably because the floating leaves have a strong effect on the light spectrum underneath the leaves. Chlorophyll a: b decreased moderately with increasing water color. Hence, the chlorophyll a: b ratio of N. lutea is a possible indicator of lake browning. For phytoplankton, variation in species composition complicates the use of chlorophyll a: b, but the chlorophyll b:biomass ratio is a possible indicator.