Effects of episodic nutrients enrichments on P-limited planktonic communities: Lake Redon ENEX 2013 experiment

Planktonic communities are naturally subjected to episodic nutrient enrichments that may stress or redress the imbalances in limiting nutrients. Human-enhanced atmospheric nitrogen deposition has caused profound N:P imbalance in many remote oligotrophic lakes in which phosphorus has largely become limiting. These lakes offer an opportunity to investigate the planktonic community response to nutrient fluctuations in P-limited conditions. The ENEX experiment in Lake Redon (Pyrenees), performed during August 2013, aimed to investigate the structural and stoichiometric effects of pulse nutrient additions on P-limited planktonic communities. We performed P (PO43-), and N (NH4+ or NO3-) additions to the summer epilimnetic community of the ultraoligotrophic lake using self-filling ~100 L enclosures and analysed the response to varying P availability, N:P imbalance, and N source. The nutrient additions were gradients within the range of values seasonally found in the lake and other oligotrophic lakes of the Pyrenees, with a further P level typical of mesotrophic lakes to provided non-limiting conditions.