Cromarty Firth Water Quality

The inshore and deep channel waters of the Cromarty Firth are normally monitored on a 3-year rolling programme. Water samples are taken for chemical analysis and biological samples are taken of the littoral and benthic fauna for examination.

In 1992, the Highland River Purification Board and the Marine Laboratory, Aberdeen, took part in a national survey of nutrient pollution in estuaries. This showed the Cromarty, Dornoch and Inverness firths as being the cleanest of the estuaries on the east coast of Scotland with the lowest nutrient levels. They were the only estuaries on the east coast where the nitrogen in the inflowing freshwater was lower than in the sea water. This is a reflection of the low nitrogen input (from agriculture and sewage) to the main rivers. However, nitrate levels were high in Udale Bay and Nigg Bay as usual due to run-off from agricultural land. Faecal coliform levels all passed the mandatory standard of the Bathing Water Directive.

The SEPA estuary classification scheme is used to classify Scottish estuarine waters. The scheme is applied to all estuaries listed by the Scottish Executive under the EC Urban Waste Water Treatment Directive. It is default based. A relatively small unit of resolution of 0.1 km2 is used and affected areas smaller than 0.01 km2 are ignored. There are four quality classes, A (excellent) through D (seriously polluted). A given area of estuary is classified by allocating it to the highest class to which all of its condition criteria conform.

Over the last 5 years the generally high quality of estuarine waters in the north of Scotland has been maintained. In 2002, the area near Invergordon at the harbour and the pier that served the former Alcan aluminium smelters, improved from class B to class A, because of reductions in tributyl tin (TBT) levels, which are a legacy of activity there.