Danish phytoplankton abundance data for QSR23

Danish phytoplankton was collected from the late 1980s to 2022 at more than 20 stations inside the Danish EEZ. The geographical area ranges from south to north, from about 54° N to 57° N, and from west to east, from 8° E to 15° E, around the Baltic island of Bornholm. Samples are collected at the upper 10 meters as pooled samples from discrete depths or by a Lindahl hose. The phytoplankton program continually evolves and has undergone several updates over the years to improve data quality. Some stations have been terminated, while others have been added. The sampling frequencies for phytoplankton include more than 12 yearly samples and, in most cases, more than 20 yearly samples per station. Some of the stations have been monitored for over 25 years. Data parameters include species determination to the lowest possible taxonomical unit, following generally accepted taxonomical keys, abundance, species volume/carbon biomass, and sampling depth. Data are collected following detailed technical guidelines. Subcontractors process the collected samples, adhering to detailed technical guidelines that mirror HELCOM COMBINE in most cases (HELCOM 2022), including initial QA/QC after each sample has been processed. The subcontractor enters the data into a database maintained by the Danish EPA, which is then subjected to final QA/QC by Aarhus University. An overview of biomass calculation and data quality is outlined in Jakobsen et al. (2015). This two-step process ensures the data's accuracy and reliability. The subcontractors also actively participate in inter-calibration workshops led by AU/DCE, further enhancing the consistency of species determinations. HELCOM (2022) Annex C-6: Guidelines concerning phytoplankton species composition, abundance, and biomass. Manual for marine monitoring in the COMBINE program of HELCOM. Jakobsen HH, Carstensen J, Harrison PJ, Zingone A (2015) Estimating time series phytoplankton carbon biomass: Inter-lab comparison of species identification and comparison of volume-to-carbon scaling ratios. Estuarine Coastal Shelf Sci 162:143-150, 10.1016/j.ecss.2015.05.006