Response of phytoplankton from the metalimnetic chlorophyll maximum to macro- and micro-nutrients amendments in Lake Geneva

Abstract Despite re-oligotrophication and low trace elements concentrations, direct determination of limiting nutrient(s) of the phytoplankton community in Lake Geneva is still missing. Incubation experiments were carried out with either addition of phosphate (+P), nitrate (+N), reactive silica (+Si), iron (+Fe), nickel (+Ni) and molybdenum (+Mo) to surface water collected between 9 and 12 m, during seasonal fieldtrips in 2015. Macro- and micro-nutrients, dissolved concentrations, temperature and irradiance as well as chlorophyll a (Chl a ) levels, Chl a :POC ratios, POP:POC ratios and Chl a variable fluorescence parameters were quantified. The rapid phosphorus consumption, elevated inorganic nitrogen to phosphorus ratios and increased Chl a , Chl a :POC and POP:POC, in response to P additions, all indicated a phosphorus limitation in phytoplankton. Moreover, none of the measured parameters differed significantly from the control in the other treatments. These results were also confirmed by the response of Chl a variable fluorescence. The effective photosystem II quantum yield in ambient light, the absolute electron transfer rate and the connectivity, under 53 μmol photons/m 2 /s illumination, significantly increased while the non-photochemical quenching decreased in the +P treatment. Phytoplankton cells supplemented with phosphate were thus more efficient to capture photons and to drive them into photochemistry at light levels close to in situ levels. This is the first study to demonstrate directly a phosphorus limitation in Lake Geneva, pointing towards the success of its re-oligotrophication process.