Additionally, at four shallow water stations (Booknis Eck, Buelk, Behrensdorf and Katharinenhof) temperature, salinity and dissolved oxygen are continuously logged at 2-3 m depth by self-contained data loggers. These are: (I) MiniDOT loggers (Precision Measurement Engineering; http://pme.com; ±10 µmol L-1 or ±5 % saturation) including copper antifouling option (copper plate and mesh) to measure dissolved oxygen concentration and (II) DST CT salinity & temperature loggers (Star-Oddi; http://star-oddi.com; ±1.5 mS cm-1) to record the conductivity. Both sensor types additionally record water temperature with an accuracy of ± 0.1 °C. The sampling interval was set to 30 minutes for all parameters. In context of the long-term monitoring project RegLocDiv (Regional-Local-Diversity) by M. Wahl (Franz, M. et al. 2019a), another seven stations were equipped with the same two types of sensors at 4-6 m depth to continuously record environmental parameters (again: temperature, salinity, dissolved oxygen) and included into this data set. These stations are at: Falshoeft, Booknis Eck, Schoenberg, Westermarkelsdorf, Staberhuk, Kellenhusen and Salzhaff (abandoned in 2023). Since 2021, in the context of implementing a reef monitoring to fulfil obligations by the EU Habitats Directive, step-by-step, eleven further stations were installed at reefs in the Schleswig-Holstein Baltic Sea. These are at: Platengrund (14 m depth) and Mittelgrund (8 m) (both since 2021), at Walkyriengrund (9 m), Brodtener Ufer (8 m), Außenschlei (11 m), Kalkgrund (8 m), Stollergrund (7.5 m) and Flueggesand (10 m) (all since 2022), as well as at Gabelsflach (10 m), Sagasbank (8.5 m) and Stabehuk (11.5 m) (all since 2023). Again, at all of these 11 stations, temperature, salinity and dissolved oxygen are continuously logged by self-contained data loggers: Conductivity (and temperature) is logged by HOBO® Salt Water Conductivity/Salinity Data Logger (Onset Computer Corporation, Bourne, MA, USA; https://www.onsetcomp.com) using the U2X protective housing to prevent fouling on the sensors. The same MiniDOT loggers (Precision Measurement Engineering) as at the above mentioned more shallow stations (including antifouling copper plate and mesh) are used to measure dissolved oxygen concentration. Dissolved oxygen concentration data measured by the MiniDOT loggers are corrected for a depth of 10 m (or 2,5 m on the shallow stations) using the software provided by the manufacturer. Additionally, a manual compensation for salinity was calculated (see details in Franz, M. et al. 2019b). Quality control was carried out by spike and gradient tests, following recommendations of SeaDataNet quality control procedures (see https://seadatanet.org/Standards/Data-Quality-Control). All data values were flagged according to applied quality checks using the following flags: 1 = Pass, 2 = Suspect, 3 = Fail, 4 = Visually suspect, 5 = Salinity compensation fail (further explanations can be found in Franz, M. et al. 2019b).
In addition to data collection by the logger units, nutrients are measured at 23 locations. Sampling occurs every 14 days at each site. Samples are taken from a water depth of approximately 0.5 meters, filtered (filter size 40 µm), and cooled for further transport. Furthermore, on the sampling day, salinity and temperature are measured at the respective locations using the WTW Cond 3110. After each tour, the water samples are frozen at -20 °C at GEOMAR for later analysis. Since 2020/2021, the analyses have been conducted by Research Area 3: Marine Ecology, FE Experimental Ecology. GEOMAR, Kiel. The samples are analyzed for the concentration of dissolved inorganic nutrients (total oxidized nitrogen, nitrite, ammonium, phosphate and silicate) by UV/VIS spectroscopy using a continuous flow analyzer (type QuAAtro 30; comp. SEAL Analytical, Hamburg, Germany, equipped with a SEAL XY-2 autosampler). Quality control for nutrient measurements is ensured by certified reference material (CRM) by KANSO TECHNOS CO, LTD, Osaka, Japan. Standard analysis methods developed by SEAL Analytical were followed.