Danish chlorophyll biomass data for QSR23

Danish chlorophyll was collected from the late 1980s to 2022 at 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. The chlorophyll program continually evolves and has undergone several updates to improve data quality. Some stations have been terminated, others have been added, and there are 2023 about 50 active stations. Samples are collected at the upper 10 meters as pooled samples from discrete depths or by a Lindahl hose. Data are collected following detailed technical guidelines by the Danish EPA; after the collection, samples are processed adhering to detailed technical guidelines that mirror HELCOM COMBINE in most cases. (HELCOM 2022), including initial QA/QC. Briefly, Chlorophyll was determined according to the method described by (Strickland & Parsons 1972). Briefly, seawater was GF/F filtered, and chlorophyll was extracted from the filter into ethanol and measured with a spectrophotometer (Holm-Hansen & Riemann 1978). When more than one chlorophyll sample was collected from the upper 10 meters, measurement values were averaged. The sampling frequencies for chlorophyll 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 30 years. The chlorophyll measurements are stored in a national database maintained by the Danish EPA, which is then subjected to final QA/QC by Aarhus University. This two-step process ensures the accuracy and reliability of the data. HELCOM (2022) Annex C-4: Guidelines for monitoring of chlorophyll a. Manual for marine monitoring in the COMBINE program of HELCOM. Holm-Hansen O, Riemann B (1978) Chlorophyll a determination - Improvements in methodology. Oikos 30:438-447., Strickland JD, Parsons TR (eds) (1972) A practical handbook of seawater analysis. Ottawa, Fisheries Research Board of Canada, 1972., Ottawa.