Marine resources, including seaweeds, have always played an important role in the history of the Orkney Islands, particularly over the period of the ‘kelp’ industry between 1719 and the 1930s. Recent studies confirm the presence of major sub-tidal seaweed forests dominated by the brown seaweed Laminaria hyperborea.

In this paper, consideration is given to the possible revitalisation of the seaweed industry in Orkney, involving major developments of both harvesting and processing technology. This is seen as a logical component of an integrated inshore marine resource management strategy for the Islands.

Mass blooms of purple sulphur bacteria belonging to the genera Thiocapsa and Chromatium (> 108 viable cells/g dry wt sediment) develop in localised areas at the sand/air interface on a number of sheltered beaches in Scapa Flow each summer. These anaerobic bacteria grow phototrophically using H2S as e− donor and develop in association with other groups of phototrophic and heterotrophic bacteria. Oxygen consumption by the heterotrophs is greater than O2 production by the algae and diffusion from the surface and this rapidly leads to anaerobic conditions in the sub-surface sediments (0–10 mm depth). Under these conditions, significant populations of sulphate reducing bacteria develop (circa 105 viable cells/g dry wt sediment) and reduce SO2−/4 to S2− during the anaerobic oxidation of low molecular weight organic substrates. Sulphide generated in this process is used as the e− donor in photosynthesis by the purple sulphur bacteria and in turn is re-oxidised to SO2−/4. Sulphate reduction is dependent upon the availability of oxidisable low molecular organic substrates which are produced, as metabolic end-products, during the microbial decomposition of algal detritus. Blooms of Thiocapsa and Chromatium spp. were most pronounced on beaches such as Scapa, Waulkmill and Swanbister, which in localised areas, have high total organic carbon contents (5–12% w/w). The factors regulating the development of these localised mass developments of anaerobic phototrophic bacteria in apparently aerobic environments are discussed.

The physical and biological characteristics of a variety of sandy shores in Orkney are described using transect data from 7 sites within Scapa Flow and 7 sites from the Mainland and North Isles. Shore profiles are described and discussed, shores ranging in length from 80 m to 1200 m. Median grain size of the sediments was generally between 180 and 300 μm (fine to medium grade sands) and the sediments were very clean. The fauna was dominated numerically by polychaetes and amphipods on all shores, Pygospio elegans, Capitella capitata and Bathyporeia spp. being the dominant forms; molluscs were notably insignificant on most shores. Tables giving the maximum densities of every species recorded together with the maximum densities of dominant species at each station on all shores are presentThe results are compared with a range of North European shores which comprise basically similar communities but with much higher population densities in Orkney. A site of unique interest at the Bay of Quoys, Hoy, is described and discussed.

The distribution and relative abundance of various types of coastline are reviewed briefly. The results of a 9 year investigation of selected rocky shore communities are presented. Climatic parameters showed a marked variation during the study but were not correlated with the observed variations in flora and fauna. The main study was based on annual sampling of 8 bedrock shore transects using percentage cover or density for sessile and mobile species respectively. Seasonal fluctuations were studied in fixed areas at 2 sites over a 5 year period. Patterns of seasonal variation in various algal and faunal categories are described and discussed, canopy algae showed summer maxima while understorey species showed winter maxima. Annual studies showed species stability to be site specific: in species which showed marked, apparently cyclic, patterns of change, the changes were often asynchronous both between sites and, to a lesser degree, between stations at a site. The value of obtaining a measure of ‘patchiness’ is discussed.

Fishing has long played an important part in the economy of Orkney. Local vessels are all small, and fish predominantly inshore for shellfish. Demersal and pelagic fish are abundant in the offshore waters around Orkney, but these exposed waters can be fished only by large vessels from more distant ports, so that few fish are landed in the islands.

Gadoids, especially haddock, cod and whiting, form the bulk of the demersal fish caught at Orkney. Stocks of all three are giving cause for concern, with declining spawning stock biomass and reduced recruitment. With the coming of processing, more local landing of demersal fish might occur. The closure of the herring fishery in 1977 was followed by a great expansion of mackerel fishing, but with the lifting of the ban west of 4CW in 1981, herring catches have once again become valuable. Because of fishing pressure, landings of both species are subject to quota restrictions.

In recent years, the traditional fisheries for lobsters and crabs have been supplemented by a new fishery for scallops. Lobster stocks are now under some pressure. Conditions around Orkney appear suited to farming filter-feeding bivalve molluscs, and culture and marketing of Pacific oysters has started.

The size and distribution of Orkney's breeding seabird populations are relatively well known, with cliff-nesting species having been monitored annually since 1976. Conditions for these monitored species appear to be less favourable now than they were at the beginning of the study.

Nationally significant populations of seaducks and waders winter in Orkney. With exceptions, the population size, distribution and ecology of these birds is only poorly understood.

Otters appear to be widespread; however, spraint surveys suggest that these animals may not be as numerous in Orkney as they are in Shetland, the Western Isles and Highland Region.

Grey seals (Halichoerus grypus) and common seals (Phoca vitulina) both occur in Orcadian waters. In previous centuries they formed an important resource for both domestic use and export.

Annual grey seal pup production has more than doubled since 1964. Productivity on individual islands is influenced by human activity. Total population size is estimated to have risen from around 10,000 in 1964 to around 20,000 in 1982 despite a management policy of culling originally intended to stop population growth. Grey seals tagged in Orkney have been recovered around most parts of the North Sea, Norway, the Faeroes and south to Ireland.

Considerably less is known about the biology of common seals in Orkney. A boat survey in 1979 counted 2653 adults and 420 pups.

This paper briefly reviews the legal framework of pollution control in Orkney, and discusses the occurrence of the main categories of contaminants there.

There is no indication of elevated concentrations of organohalogen compounds in Orkney shellfish. The main potential source of hydrocarbon contamination is thoroughly monitored, and no major adverse effect has been noted.

High concentrations (up to 62 μg/g wet wt) of cadmium occur in edible crab hepatopancreas from Orkney, Shetland and surrounding areas. A consideration of cadmium concentrations in a range of species suggests a regional geochemical anomaly for cadmium, affecting biota perhaps through solid phases.