The threshold value of the 'Total nitrogen' indicator is an assessment unit specific concentration which is not to be exceeded in order for an assessment unit to be evaluated as having achieved the threshold value indicating good status (Thresholds and Status evaluation figure 1).
Thresholds and Status evaluation figure 1. Schematic representation of the threshold value for the core indictor 'Total nitrogen'. Assessment unit specific threshold values are used (see Thresholds and Status evaluation table 1).
Threshold values for the open sea assessment units have been derived in HELCOM (Thresholds and Status evaluation table 1). For coastal assessment units, national boundaries used for estimating Good Environmental Status under WFD may be used.
Thresholds and Status evaluation table 1. Assessment unit specific threshold values for total nitrogen.
Some of the open-sea indicator threshold values were based on the results obtained in the TARGREV project (HELCOM 2013), also taking advantage of the work carried out during the EUTRO PRO process (HELCOM 2009) and national work for EU WFD implementation. The TARGREV values were derived as geometrical means, thus bearing close resemblance to median values (J. Carstensen, pers. comm.).
However, as Total nitrogen (TN) was not simulated in the TARGREV modelling exercise, only upper limits of annual means of TN derived from estimates of the mean level during 1970-1975 are used as threshold values (see TARGREV report pages 84 and 134). These upper levels might already represent a eutrophied Baltic Sea in the early 1970s, and thus not be in agreement with the threshold value of the other eutrophication indicators with modelled threshold values (e.g. DIN, DIP) or threshold values based on extensive monitoring (e.g. Secchi depth). They are however expected to be in agreement with threshold values based on shorter term monitoring data (e.g. chlorophyll-a).
A new modelling approach has recently provided revised concentrations for German national threshold values of total nutrients in the Kiel Bay, Mecklenburg Bay, Arkona Basin and Bornholm Basin (Hirt et al. 2013; Schernewski et al. 2015; BLANO 2014), taking into account HELCOM, MSFD and WFD requirements for good status. The finally agreed BLANO threshold values represent median values and are included in the Federal Surface Water Ordinance (2016).
Break-point analysis was applied for setting Polish national threshold value in the Gdansk Basin. The results of these exercises were used as additional input in the threshold setting.