Possible effects of high concentration of sewage entering the marine environment from recreational craft

There has been little or no research made on the amounts of sewage discharged into port and harbour areas during operational shipping or recreational activities. The major contributor to aquatic pollution in estuary and coastal areas is human sewage from population centres that is discharged from waste treatment plants. The adverse health, environmental and aesthetic impacts from sewage contamination in recreational coastal areas are well documented, and numerous environmental parameters, particularly microbiological, are continually monitored throughout the UK by the environment agencies (EA/SEPA/DOENI) and local authorities.

In accordance with MARPOL regulations, legal discharge of untreated sewage from ships normally occurs over 12 miles from the coast, which is further offshore than from the, so-called, long sea outfalls from land treatment plants. Legal discharges of treated or untreated sewage wastes from ships will not therefore effect marine SACs. Illegal discharges from commercial craft close inshore or in ports where they can be observed are unlikely to take place. Another source of sewage in the marine environment is that from recreational craft that tend to congregate in large numbers close inshore where the effects of uncontrolled discharge are most noticeable. Although increasing numbers of craft are fitted with holding tanks, their effectiveness depends on the availability of onshore waste disposal reception facilities. The provision of such facilities is generally uncommon, but increasing in UK harbours. Sewage may affect the marine environment in three main ways, through oxygen depletion, causing disease, and by nutrient enrichment, which are discussed below.

Oxygen depletion: When sewage decomposes it uses up oxygen from the surrounding water and if the discharged concentration are too great, the amount of oxygen available for fish and other aquatic animals and plants will be insufficient and they may die.

Disease: Sewage can contain disease causing bacteria and viruses which pose a risk to public health for swimmers and those eating contaminated shellfish.

Nutrient enrichment: Sewage discharges also contain nutrients which when elevated slightly may increase algal and plant growth under certain background conditions. However, when present in high concentrations nutrients can be responsible for the formation of algal blooms which reduce light penetration through the water column, may produce toxins and can cause oxygen depletion when decomposition takes place.

Although under certain conditions sewage discharges from recreational craft may disturb marine wildlife, the extent to which this represents a problem in ports and harbours within European marine sites needs to be considered on a site-by-site basis. The amounts of sewage entering the marine environment from recreational craft needs to be considered in perspective with the far greater amounts entering from land-based sources. The impact of sewage from recreational craft in marine SACs will vary depending on the amount of sewage being disposed of into the water, background water quality, temperature, and volume of water and tidal movement. The effect is likely to be the greatest in enclosed areas and shallow water with little or no tidal flow in the summer and autumn when temperatures are at their highest, coinciding with the peak of the boating season.

In addition to the sewage itself, the chemical additives held in portable toilets and holding tanks such as chlorine, ammonium and zinc are toxic to marine life and therefore may potentially affect marine animals and plants. Bearing in mind that many yachtsmen use shore based marina facilities and some use holding tanks, recreational craft may be considered a minor contributor to sewage pollution. However, although boats discharge relatively small amounts of sewage, what is pumped out is often very concentrated and therefore has a high demand on oxygen levels (BMIF 1997) and therefore it may present a localised problem to marine life under certain conditions.

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