Mehler, Knut; Steinmann, Anna; Rick, Johannes J (2025): Abiotic Measurements from a three-month mesocosm experiment on Sylt, Germany, in 2022 [dataset]. PANGAEA, https://doi.org/10.1594/PANGAEA.984270

This dataset comprises abiotic parameters measured during a mesocosm experiment using the AWI Sylt Outdoor Mesocosm (AWISOM) facility (Dummermuth et al., 2023) conducted on the island of Sylt, Germany, in 2022. The experiment spanned approximately three months, beginning on 23 March 2022. A total of 12 mesocosms were deployed, with four mesocosms assigned to each of three temperature treatments: in situ ambient, ambient +1.5 °C, and ambient +3.0 °C. Water samples were collected from each mesocosm at a depth of 0.1 m on 67 discrete sampling days. The sampling frequency varied by parameter, with some variables measured daily and others once or several times per week. Recorded parameters include water temperature, dissolved oxygen, pH (NBS scale), conductivity, chlorophyll a, total alkalinity, and concentrations of dissolved inorganic nutrients (phosphate, nitrate, nitrite, ammonium, and silicate). Measurements were carried out using multiparameter probes and laboratory methods. The dataset provides a multi-parameter time series from a controlled coastal mesocosm experiment designed to investigate the effects of warming on musselbank communities in the Wadden Sea.

Between 23 March and 28 June 2022, twelve mesocosms at the AWI Wadden Sea Station Sylt were operated to investigate temperature effects on benthic communities under controlled, yet environmentally realistic, conditions. The experimental setup comprised three temperature treatments: ambient (following observed field conditions), +1.5 °C, and +3.0 °C, reflecting regional projections based on Representative Concentration Pathway (RCP) climate scenarios. Temperatures were initially matched to field conditions using a LabVIEW-based control system and adjusted manually twice per week to track the seasonal warming from April to June. Diel temperature variability was not actively programmed, but passive day–night fluctuations occurred through natural exposure. Physico-chemical parameters were a primary focus of the monitoring program. Each pair of mesocosms shared a multiparameter measurement system (Hydrolab DS5X) that alternated between tanks, recording water temperature, conductivity, pH, and dissolved oxygen every minute over a 30-minute measurement period per tank. After each period, the system was flushed and switched to the paired mesocosm. These automated data were supplemented and validated by daily manual measurements using handheld sensors for pH (WTW pH330i), conductivity (Knick Portamess 911 Cond), and water temperature. Due to drift observed in some automated pH measurements, only the manually obtained pH values—calibrated weekly against buffer standards—were used for subsequent analyses. Nutrient concentrations were determined from weekly water samples collected from each mesocosm. Analyses included orthophosphate (PO₄³⁻), nitrate (NO₃⁻), and reactive silicate (Si). Chlorophyll a, used as a proxy for phytoplankton biomass and food availability, was measured twice weekly following the method of Jeffrey and Humphrey (1975). Sampling days for the mesocosms were synchronized with the Sylt Roads time series to ensure comparability between experimental and field data. The mesocosm water was supplied continuously with unfiltered seawater pumped from the Wadden Sea (~150 m offshore) into settling tanks before gravity-feeding into each unit, ensuring a steady exchange of approximately 1,800 L per tank per day. This flow-through system maintained nutrient and phytoplankton concentrations close to natural levels while allowing precise control over the temperature treatments and continuous high-resolution monitoring of the physico-chemical environment.