Effects of solar radiation, humic substances and nutrients on phytoplankton biomass and distribution in Lake Solumsjö, Sweden

Abstract

Impacts of solar radiation, humic substances and nutrients on phytoplankton abundance at different depths were investigated in a temperate dimictic lake, Lake Solumsjö. Penetration of solar radiation profiles at different depths, represented as light attenuation coefficient (K _d) were examined. Water sampling and downward irradiance of photosynthetically active radiation (PAR), ultraviolet-A (UV-A, 320–400 nm) and ultraviolet-B (UV-B, 280–320 nm) radiation were performed once a week and at three different times of the day (08.00, 12.00 and 16.00 hrs, local time) between September 13 and November 1, 1999. During the period of investigation, solar radiation above the water surface declined from 474 to 94 μmol m⁻² s⁻¹ for PAR, from 1380 to 3.57 W m⁻² for UV-A and from 13.1 to 0.026 W m⁻² for UV-B, respectively. The attenuation coefficient (K _d) for UV-B radiation ranged from 3.7 to 31 m⁻¹ and UV-B radiation could not be detected at depths greater than 0.25 m. Humic substances measured at 440 nm ranged from 35.5 to 57.7 Pt mg l⁻¹. Mean values of biomass, estimated from chlorophyll a, in the whole water column (0–10 m) varied between 2.3 and 5.6 μg l⁻¹ and a diel fluctuation was observed. During stratified conditions, high levels of iron (1.36 mg l⁻¹) and manganese (4.32 mg l⁻¹) were recorded in the hypolimnion, suggesting that the thermocline played a major role in the vertical distribution of phytoplankton communities in Lake Solumsjö. The high levels of iron and manganese stimulated the growth of Trachelomonas volvocinopsis in the hypolimnion at a depth of 10 m. Negative impacts of UV-B radiation on phytoplankton in lake Solumsjö are reduced due to the high levels of humic substances and the high degree of solar zenith angle at the latitude studied.