Abstract
Planktonic processes playing a key role in the fate of matter and global biogeochemical cycles, gross primary production (GPP), net community production (NCP) respiration (R), growth (µ) and loss (l) rates of phytoplankton were quantified using high-frequency (HF) data acquired by sensors immersed in in situ mesocosms during experiments simulating warming (Thau Lagoon) and brownification (Hopavågen Bay, Norway). A new method for estimating GPP, R and NCP was established using sensor data measuring dissolved O2 concentration from experiments in Thau lagoon. It was compared with an existing method using HF data and with the classical incubation technique (Winkler). It has the advantage of considering the variability in the coupling between day-night and dissolved O2 cycles and allowed to estimate a daytime respiration on average 41% higher than the nighttime one, in agreement with the positive theoretical effect of light on respiration. Application of this new method in an experiment testing the effects of brownification on planktonic community functioning in Hopavågen Bay revealed a negative effect of brownification of about 30% on GPP and R, associated with significant changes in phytoplankton pigment concentration related to physiological acclimation to low light conditions.The warming scenario for 2100 in the Mediterranean was tested in two in situ mesocosm experiments in spring and autumn 2018 in Thau lagoon. Warming increased phytoplankton’s µ and l, estimated with HF fluorescence data. The estimates of µ and l were compared to the growth and grazing rates obtained via the dilution technique, highlighting a good agreement between the two methods, confirming the robustness of the estimates obtained with the HF data, despite some differences between results due to the fact that l, estimated with the HF data, considers sedimentation, mesozooplankton grazing mortality and viral lysis while the dilutions only allow the estimation of microzooplankton grazing. An experiment simulating a heatwave was also carried out in Thau in spring 2019. The HF data showed a positive effect of the heat wave on GPP, R, µ and l that lasted for several days after the end of the heatwave, except for µ. The heatwave shifted the trophic state of the system towards heterotrophy and favored cyanobacteria at the expense of dinoflagellates, however most of the studied processes showed good resistance and recovery from the heatwave. Warming induced contrasting responses in Thau lagoon planktonic community, shifting the system towards autotrophy or heterotrophy depending on the investigated season. The community from Thau lagoon was more resilient and recovered better from a punctual climate change event than the community from Hopavågen, potentially because the planktonic community from Thau evolved in an environment naturally subject to strong temperature variations and/or because warming is a less drastic disturbance than brownification. The methods established in the thesis represent a novel approach to obtain reliable estimates of planktonic processes highlighting the effects of climate change on the functioning of coastal ecosystems. There are many perspectives to continue this work, using these new methods to study other disturbances in other ecosystems