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

CALLIERI, C.; MODENUTTI, BE; QUEIMALINOS, CP; BERTONI, R.; BALSEIRO, E.G

Aquatic Ecology

Springer Verlag

Año: 2007 vol. 41 p. 345 - 362

1386-2588

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 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 data base 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.