Mixotrophic phytoplankton is enhanced by UV radiation in a low altitude, P-limited Mediterranean lake

Abstract

UV radiation promotes harmful effects on phytoplankton populations, but it is influenced by the degree of sensitivity of different populations to the ultraviolet:photosynthetically active radiation ratio (UVR:PAR), part of which is P-dependent. Given the expected increase of UV radiation along with global change, one may ask if phytoplankton populations are able to adapt to the expectedly higher UVR:PAR ratio. If so, how would phytoplankton communities be affected? The main goal of this study is to answer these questions. Field and laboratory experiments were carried out with phytoplankton populations of an oligotrophic, low altitude lake in Central Spain. No changes were observed in abundance of phytoplankton fractions after UVR removal in the lake. However, autotrophic picoplankton underwent lower growth and contribution to total phytoplankton biomass when UVR increased. Phytoplankton biomass under enhanced UVR was one-third lower than the biomass reached under only PAR. UV-related growth changes were species-specific and linked to cell size and metabolism. An UVR increase would then promote phytoplankton assemblages who resulted from a trade-off between competitive advantages of picoplankton in a P-limited system and selected larger algae. Under these circumstances, the mixotrophic character of these larger species happened to be an evolutionary advantage.