Production and biomass of picophytoplankton and larger autotrophs in Andean ultraoligotrophic lakes: differences in light harvesting efficiency in deep layers

We measured biomass and primary production of picophytoplankton (PicoPhy: 0.2–2 μm) and of autotrophic size fraction >2 μm in six deep ultraoligotrophic lakes in the Andean-Patagonian region (around 41°S) during summer stratification. Surface Photosynthetically Active Radiation (PAR) ranged from 1277 to 1849 μmol photons m⁻² s⁻¹, and the euphotic zone, generally deeper than the mixed layer, varied between 28 m and 49 m. We found a strong photoinhibiting effect of high PAR and UV-A at surface levels, whereas UV-B radiation (<320 nm) had low extra contribution in the photosynthesis inhibition. As a consequence, cell numbers, Chl a and primary production rates of both fractions increased towards deep layers in all lakes. The photosynthetic efficiency (Chl-specific production per photon unit) of both fractions increased with depth, although this increase was higher in PicoPhy, indicating a higher fitness to low-light. The per cent contribution of PicoPhy production to total production, showed an inverse significant relation with total dissolved phosphorus (TDP). Moreover our data fitted the existing database showing a significant trend towards a decrease of PicoPhy biomass and an increase of its relative contribution to total biomass with decreasing trophic state. At very low-phosphorus concentration, typical of north Patagonian lakes, we found good evidence of the competitive advantage of PicoPhy. Low-light and low TDP may interact to create the most favourable conditions for the smaller photosynthetic organisms. In conclusion, we found that at low-light and very low nutrient regime PicoPhy achieves higher photosynthetic efficiency than the larger autotrophic organisms.