The principal activities with the potential to cause a degradation of water quality in European marine sites and their possible mechanisms are listed in the linked table below. The likely mechanisms include those resulting in either a change in the concentration of naturally occurring substances or the introduction of substances not found naturally in the marine environment. These latter substances are termed synthetic or xenobiotic. For the purposes of this report, the term synthetic will be used for these substances and the term non-synthetic will apply to those naturally occurring substances.

Langford (1990) defines biological impacts as:

• lethal effects (direct mortalities);
• controlling effects (on growth, reproduction etc);
• directive effects (behavioural responses);
• indirect effects (through effects on other biota or chemistry).

Any one of the above may be significant for features within a Natura 2000 site. Generally, there is very much less information on indirect or directive effects than on controlled or lethal effects. Available relevant information on all these has been summarised as far as possible in Appendices B and C.

The effects of synthetic substances on marine organisms are generally toxic in that they interfere with one or more essential processes resulting in effects ranging from death of the organism (lethal) to changes in sub-cellular structures (sub-lethal). Non-synthetic (naturally occurring) substances can also be present at concentrations that are toxic to marine organisms (e.g. ammonia and heavy metals). The majority of non-synthetic (naturally occurring) substances result in effects by changing the natural balance of the physico-chemical environment resulting in structural and functional shifts in community composition.

Many thousands of chemicals are released into the environment as a result of human activities. For example, the European Inventory of Existing Commercial Chemical Substances (EINECS) contains more than 100,000 chemicals and a good proportion of these is likely to be released into the environment to some extent. However, only 61 synthetic substances, or groups of substances, have been identified on priority lists for control (see table linked below). These substances have been selected on the basis of persistence, bioaccumulation and toxicity.

The concepts of hazard and risk are widely used in the identification of substances of concern. The hazard associated with a substance is its potential to cause harm and is assessed by collecting data on properties, such as physical-chemical characteristics, mobility and persistence in environmental media, bioaccumulation and acute and chronic toxicity. Risk is the probability that harm will be caused and requires information on likely environmental concentrations of the substance derived from known rates of release and dilution factors in the environment. Reliable information on known release rates of many substances is extremely difficult to find and consequently risk assessments for chemicals already in the environment are not commonly done. Substances on priority lists have been identified primarily in terms of characteristics associated with hazard (persistence, bioaccumlation and toxicity).

For the purposes of this guidance manual, the 61 synthetic substances, or groups of substances, from the priority lists, together with several groups of substances identified by the conservation agencies, have been arranged into groups of toxic and non-toxic substances. For each substance, or group of substances, a profile has been prepared outlining entry to the marine environment, fate and behaviour in the marine environment and main effects on marine organisms. This information has then been used to indicate potential effects on interest features of European marine sites. Information on levels of toxic and non-toxic substances has been collated for most substances and is provided, either in the substance profile or in Appendix D to indicate the levels that might be expected to occur.

The reader is referred to the sections on marine communities which provide an overview of dynamics and sensitivity characteristics for conservation management of marine SACs for the following sub-features:

Zostera Biotopes

Intertidal Sand and Mudflats & Subtidal Mobile Sandbanks

Sea Pens and Burrowing Megafauna

Subtidal Brittlestar Beds

Maerl

Intertidal Reef Biotopes

Infralittoral Reef Biotopes with Kelp Species

Circalittoral Faunal Turfs

Biogenic Reefs

All the above reports include references to impacts of water quality which will serve as a valuable source of further information.

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