Publication - Consultation paper

Water Environment and Water Services (Scotland) Act 2003 implementation: environmental standards - consultation

Published: 23 Oct 2020
Environment and Forestry Directorate
Part of:
Environment and climate change

Consultation seeking views on our proposals for new or updated environmental standards for Scotland’s water environment, and updates to some of the assessment methods used for deriving such standards.

Water Environment and Water Services (Scotland) Act 2003 implementation: environmental standards - consultation
10. Loch nitrogen

10. Loch nitrogen

We are proposing to introduce new standard for nitrogen in lochs. Nitrogen is a plant nutrient and elevated concentrations can lead to accelerated growth of algae and other plants. The impact on the composition and abundance of plant species can have adverse implications for other aspects of water quality, such as oxygen levels. The various changes can then impact on animals, such as invertebrates and fish. Elevated concentrations result from inputs of nitrogen from a range of sources, including in particular discharges of sewage and various diffuse agricultural sources. The process of nutrient enrichment, accelerated algal and plant growth and associated adverse effects is termed eutrophication.

The Water Framework Directive refers to "nutrient conditions" as one of the general physicochemical elements supporting ecological status. To date, in Scotland phosphorus has been the only nutrient used as a supporting element in freshwaters, primarily because historically it has been considered the most likely to be limiting to plant/algal growth. However, for lochs there is convincing evidence from the recent scientific literature that nitrogen can also play a significant role in the eutrophication process, and that control of both phosphorus and nitrogen concentrations/loadings is desirable where they are both present in excess.

Under the EU WFD Common Implementation Strategy, work was undertaken to review and improve the comparability and consistency of WFD nutrient boundary values across Member States. The outputs of this work have been used to analyse UK lake data to produce the total nitrogen status class boundaries for lochs in Scotland. Further technical detail on the derivation of the proposed standards is available from UKTAG[8]. The proposed nitrogen standards for lochs are set out in Tables 10.1.

Table 10.1 Proposed total nitrogen boundary values (standards) for lochs (mg/l N, annual mean concentration)
Lake type* Status boundary
High/ Good Good/ Moderate Moderate/ Poor Poor/ Bad
Clear, Very Shallow 0.67 1.07 2.13 4.27
Clear, Shallow 0.48 0.77 1.54 3.08
Clear, Deep 0.46 0.74 1.47 2.94
Humic, Very shallow 0.91 1.46 2.92 5.85
Humic, Shallow 0.81 1.30 2.60 5.20
Humic, Deep 0.72 1.16 2.32 4.65

*type as for the UK reporting typology: clear = <30 mg/l Pt, humic > 30 mg/l Pt (includes polyhumic), depth classes (mean depth): very shallow <3m, shallow 3-15m, deep >15m.

Implications of the proposal

Table 10.2 shows the distribution by nitrogen standard class in terms of number of loch waterbodies where monitoring data is available.
Class New proposed loch nitrogen class
High 36
Good 2
Moderate 4
Poor 1
Bad 0

*Relatively few Scottish lochs have been routinely monitored for total nitrogen in recent years.

Out of the five loch waterbodies classed at less than good status for nitrogen, only one would see a downgrade in overall class status from good to moderate. The lochs at less than good status for nitrogen are also impacted by phosphorus and this will remain the primary driver for improvements, so there is unlikely to be any additional regulatory action required as a result of this new standard.