To determine the effect of the rate of temperature increase (acute vs. gradual) and magnitude as well as the timing of nutrient addition on a natural marine phytoplankton community, a bottle incubation experiment has been conducted at the Institute for Chemistry and Biology of the Marine Environment (ICBM) in Wilhelmshaven, Germany. The community was collected at the Helgoland Roads long-term time series site in the German part of the North Sea (https://deims.org/1e96ef9b-0915-4661-849f-b3a72f5aa9b1) on the 6ᵗʰ of March 2022. The surface water containing the phytoplankton community was collected from the RV HEINCKE with a pipe covered with a 200 µm net attached to a diaphragm pump. In the first experimental run, the community was exposed to either gradual or acute temperature increase (from 6 to either 12 or 18°C) with 25 different N:P supply ratios added as a batch at the beginning of the bottle incubation. Simultaneously, the same community was gradually acclimated to their experimental temperatures under ambient nutrients and was used in a second experimental run in which it received the same 25 different N:P supply ratios after temperature acclimation. The light conditions were set to 175 µmol s-1 m-2 and a day-night cycle of 12h:12h which corresponds to the natural conditions at that time of the year. With this, it was possible to test the effect of a gradual vs. acute temperature increase and the timing of nutrient addition i.e., before or after the temperature change. This experimental set-up summed up to 400 units (8 temperature treatments x 5 nitrogen levels x 5 phosphorus levels x 2 replicates). Each experimental run was ended after 12 days. Fluorescence (395/680 Exc./Em.) was measured every second day using a SYNERGY H1 microplate reader (BioTek®) to determine phototrophic growth over time. At the end of each experiment, one replicate was filtered onto pre-combusted acid-washed glass microfiber filters (WHATMAN® GF/C) for intracellular carbon (POC), nitrogen (PON), and phosphorus (POP) content. The POP filters were pre-combusted and then analysed by molybdate reaction after digestion with a potassium peroxydisulfate solution (Wetzel and Likens 2003). The POC and PON filters were dried at 60°C before they were measured in an elemental analyser (Flash EA 1112, Thermo Scientific, Walthman, MA, USA).
To investigate the effect of temperature on a North Sea spring bloom community, we performed an incubation experiment in the mesocosm facility of the Institute for Chemistry and Biology of the Marine Environment (ICBM) in Wilhelmshaven. The plankton community was sampled from the long-term ecological research station Helgoland Roads (https://deims.org/1e96ef9b-0915-4661-849f-b3a72f5aa9b1) on the 6ᵗʰ of March, 2022. Collection of the surface community was conducted from the RV Heincke with a pipe covered with a 200 µm net that was attached to a diaphragm pump. The month-long incubation was started on the 7ᵗʰ of March in twelve indoor mesocosms, the Planktotrons (Gall et al., 2017). We chose three temperatures along the ascending part of the thermal performance curve (TPC) of the in situ community: the minimum temperature for positive growth (6°C, also the field temperature), the middle between the minimum and the optimum temperature (12 °C), and the optimum temperature for growth (18 °C). Ramping up the temperatures was conducted by 1 °C per day until the treatment temperatures were reached, resulting in a ramp phase (first twelve days) and a constant temperature phase. This dataset comprises all data collected within the experiment. Temperature, oxygen, pH, salinity, and in vivo fluorescence were measured daily at 10 am. Samples for dissolved nutrients (nitrate, nitrite, phosphate, silicate), chlorophyll a, DNA, particulate nutrients (biogenic silica, particulate organic carbon/nitrogen/phosphorus), as well as flow cytometric counts of bacteria (stained) and the unstained community were sampled every third day at the same time. The mesocosm water was generally filtered over a 200 µm mesh before sampling to exclude mesozooplankton. However, due to the appearance of large Phaeocystis colonies, additional samples without pre-filtration were taken for particulate organic carbon, nitrogen, phosphorus, and chlorophyll a starting on incubation day 15. PAR, total nitrogen and phosphorus as well as total alkalinity were measured at the start, in the middle, and at the end of the incubation. Samples for Mesozooplankton enumeration were taken and plankton species identified at the end of the experiment. All analysis scripts can be found on github (https://github.com/AntoniaAhme/TopTrons22MesocosmIncubation). The sequence data are available at the European Nucleotide Archive (ENA).