Experiment of light effects on phytoplankton morphometric traits and nutrient utilization

Near-surface seawater containing the natural mix of phytoplankton, protozoa and bacteria was pumped from Kiel Fjord, Baltic Sea, on 31 October 2016 and distributed through a hose system into 9 mesocosms (volume 1500 L, 1.5m in diameter and 1m high) which were evenly placed in three walk-in climate chambers. Light phase: the mesocosms were exposed to three different constant irradiance levels at a 9:15 hrs light:dark cycle. Light was supplied by computer controlled (GHL, Prometheus) LED light units. Each light treatment was triplicated and evenly distributed among the climate chambers. The three different light intensities were assumed to represent low light stress (low: 31 µmol quanta m-2 s-1), moderately limiting to saturating conditions (medium: 72 µmol quanta m-2 s-1), and saturating to slightly inhibiting conditions (high: 139 µmol quanta m-2 s-1). The experimental temperature was 11°C, representing in situ conditions. Fluorescence measurements were done every day in the lab at the same time using a subsample from the mesocosms. The different light treatments were applied until fluorescence measurements indicated stationary phase of growth (day 7 for the medium and high light treatments, day 11 for the low light treatment). An automatic propeller was used to mix the mesocosms gently, representing natural water waving to avoid sedimentation and achieve homogeneity of the plankton community.