Size-related photosynthesis of algae in a strongly stratified humic lake

Abstract

We studied the size-specific phytoplankton production and chlorophyll-based biomass in Lake Valkea-Kotinen, where the epilimnetic nutrient concentrations during stratification are low and the phytoplankton community mainly consists of flagellates. We hypothesized that in this humic lake, where large diatoms are few, small species are favoured. For a complete size range, autotrophic picoplankton (APP) were included in the study. We sampled the pristine headwater lake in 2005 and in addition to size-fractionated production and biomass, we also identified the species and calculated the biovolume-based biomass estimates. In contrast to our hypothesis, most of the production was composed of a few large cell size taxa with flagella, e.g. Gonyostomum semen during stratification and Peridinium spp. during overturn periods. The large micro-plankton, mainly G. semen, were the dominant primary producers (46―72% of the total primary production) from late June until the late September. G. semen was also superior in terms of chlorophyll-specific photosynthetic rate, whereas APP were less efficient and thus less successful. Our results indicate that in this lake, the large-sized flagellated phytoplankton could largely overcome nutrient limitation and thus solar irradiance was presumably the main driver for the production of phytoplankton.