Solar radiation–nutrient interaction enhances the resource and predation algal control on bacterioplankton: A short-term experimental study

An in situ experimental assessment was made of the effects of the interaction between spectral composition of solar radiation and the limiting nutrient (phosphorous, P) on the algae–bacteria link, measured over the short term (1.5 h after P enrichment) in an oligotrophic and autotrophic high mountain lake. Variables related to the metabolism of algae (primary production, excretion of photosynthetic carbon) and bacteria (bacterial production, percentages of photosynthetic exudates assimilated and used for bacterial production) were studied. P enrichment suppressed or attenuated the negative effects exerted by ultraviolet radiation on algae when their elemental composition was P deficient. This was reflected in antagonistic interactive effects between P enrichment and solar radiation (P 3 R), which were triggered by a decrease in primary production and an increase in organic carbon excretion due to metabolic adjustments to growth. P enrichment also suppressed or attenuated the main effects exerted by ultraviolet radiation on bacteria. This was reflected in antagonistic P 3 R interactive effects triggered by an enhancement of the dual control (resource and predation) that algae exerted on bacteria and by the growth stimulation of P-deficient bacteria after P enrichment. All of these observed responses contribute to improving the food quality for herbivores and reinforce the flux of carbon and nutrient to the grazing chain, which explains its development in this and other clear-water ecosystems. Solar radiation and nutrient supply are the main ecological factors determining the structure and functioning of ecosystems (e.g., Xenopoulos et al. 2002 and references therein). Hence, any alteration in either or both of these abiotic variables can affect the ecosystem as a whole. It is known that an increase in ultraviolet B (UVB) fluxes derived from stratospheric ozone depletion can affect basal processes in food webs, such as photosynthesis, nutrient uptake, motility, reproduction, or growth of organisms (Helbling and Zagarese 2003 and references therein). In addition, the ecological effects of an increase in nutrient availability on the structure and dynamics of biota are widely known (e.g., Sterner and Elser 2002). Although study of the ecological effects of nutrient availability has traditionally been associated with anthropogenic eutrophication, research on atmospheric pulses of mineral nutrient input is attracting increasing attention in oligotrophic ecosystems such as oceans (Prospero and Lamb 2003) and high-mountain lakes (Villar-Argaiz et al. 2001). Understanding and predicting the effects of these factors 1 Corresponding author ([email protected]).