Quality control and analyses of the long-term nutrient data from Helgoland Roads, North Sea

The nutrient data set of the Helgoland time series comprises of daily surface water samplings and analyses of nutrients in marine waters at Helgoland Roads from 1962 until the present day.A major issue in producing and using long-time-series data is to assure quality and accuracy of the data, in order to provide homogeneous data sets. In practice, this is frequently overlooked and rigorous data control is rarely in evidence. The objective here is to present the quality control required for this open access data set and thus, evaluate sudden shifts such as observed for silicate in terms of known analytical problems and hydrography. We compare the data with other available data for the North Sea and show that the accuracy of the data over time is such that we can be confident that changes in nutrients, particularly silicate, are probably related to hydrographic shifts and not due to analytical error. With this, the data set is now sufficiently understood and therefore it finally may be used to assess effects of eutrophication and/or de-eutrophication on the pelagic ecosystem represented by Helgoland Roads in the North Sea.